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user-manual: move transaction logger
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
455e392775
commit
1d529623e0
@ -299,6 +299,446 @@ LatexCommand tableofcontents
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Briefing: how MARS works
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\end_layout
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\begin_layout Section
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The Transaction Logger
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:The-Transaction-Logger"
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\end_inset
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\end_layout
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\begin_layout Standard
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\noindent
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\align center
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\begin_inset Graphics
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filename images/MARS_Data_Flow.pdf
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lyxscale 60
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width 100text%
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\end_inset
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\end_layout
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\begin_layout Standard
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\noindent
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The basic idea of MARS is to record all changes made to your block device
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in a so-called
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\series bold
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transaction logfile
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\series default
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.
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\emph on
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Any
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\emph default
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write reqeuest is treated like a transaction which changes the contents
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of your block device.
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\end_layout
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\begin_layout Standard
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This is similar in concept to some database systems, but there exists no
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separate
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\begin_inset Quotes eld
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\end_inset
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commit
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\begin_inset Quotes erd
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\end_inset
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operation:
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\emph on
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any
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\emph default
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write request is acting like a commit.
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\end_layout
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\begin_layout Standard
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The picture shows the flow of write requests.
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Let's start with the primary node.
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\end_layout
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\begin_layout Standard
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Upon submission of a write request on
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\family typewriter
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/dev/mars/mydata
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\family default
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, it is first buffered in a
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\emph on
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temporary
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\emph default
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memory buffer.
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\end_layout
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\begin_layout Standard
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The temporary memory buffer serves multiple purposes:
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\end_layout
|
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|
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\begin_layout Itemize
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It keeps track of the order of write operations.
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\end_layout
|
||||
|
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\begin_layout Itemize
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Additionally, it keeps track of the positions in the underlying disk
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\family typewriter
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/dev/lv-x/mydata
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\family default
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.
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In particular, it detects when the same block is overwritten multiple times.
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\end_layout
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\begin_layout Itemize
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During pending write operation, any concurrent reads are served from the
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memory buffer.
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\end_layout
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\begin_layout Standard
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After the write has been buffered in the temporary memory buffer, the main
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logger thread of the transaction logger creates a so-called
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\emph on
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log entry
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\emph default
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and starts an
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\begin_inset Quotes eld
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\end_inset
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append
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\begin_inset Quotes erd
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\end_inset
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operation on the transaction logfile.
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The log entry contains vital information such as the logical block number
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in the underlying disk, the length of the data, a timestamp, some header
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magic in order to detect corruption, the log entry sequence number, of
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course the data itself, and optional information like a checksum or compression
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information.
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\end_layout
|
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\begin_layout Standard
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Once the log entry has been written through to the
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\family typewriter
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/mars/
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\family default
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filesystem via fsync(), the application waiting for the write operation
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at
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\family typewriter
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/dev/mars/mydata
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\family default
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is signalled that the write was successful.
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\end_layout
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\begin_layout Standard
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This may happen even
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\emph on
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before
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\emph default
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the writeback to the underlying disk
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\family typewriter
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/dev/lv-x/mydata
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\family default
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has started.
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Even when you power off the system right now, the information is not lost:
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it is present in the logfile, and can be reconstructed from there.
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\end_layout
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\begin_layout Standard
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Notice that the order of log records present in the transaction log defines
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a total order among the write requests which is
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\emph on
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compatible
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\emph default
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to the partial order of write requests issued on
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\family typewriter
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/dev/mars/mydata
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\family default
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.
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\end_layout
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\begin_layout Standard
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Also notice that despite its sequential nature, the transaction logfile
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is typically
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\emph on
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not
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\emph default
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the performance bottleneck of the system: since appending to a logfile
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is almost purely sequential IO, it runs much faster than random IO on typical
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datacenter workloads.
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\end_layout
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\begin_layout Standard
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In order to reclaim the temporary memory buffer, its content must be written
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back to the underlying disk
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\family typewriter
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/dev/lv-x/mydat
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\family default
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a somewhen.
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After writeback, the temporary space is freed.
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The writeback can do the following optimizations:
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\end_layout
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\begin_layout Enumerate
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writeback may be in
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\emph on
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any
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\emph default
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order; in particular, it may be
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\emph on
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sorted
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\emph default
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according to ascending sector ´numbers.
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This will reduce the average seek distances of magnetic disks in general.
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\end_layout
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\begin_layout Enumerate
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when the same sector is overwritten multiple times, only the
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\begin_inset Quotes eld
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\end_inset
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last
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\begin_inset Quotes erd
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\end_inset
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version need to be written back, skipping some intermediate versions.
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\end_layout
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\begin_layout Standard
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In case the primary node crashes during writeback, it suffices to replay
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the log entries from some point in the past until the end of the transaction
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logfile.
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It does no harm if you accidentally replay some log entries twice or even
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more often: since the replay is in the original total order, any temporary
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inconsistency is
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\emph on
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healed
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\emph default
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by the logfile application.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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\end_inset
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In mathematics, the property that you can apply your logfile twice to your
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data (or even as often as you want), is called
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\series bold
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idempotence
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\series default
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.
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This is a very desirable property: it ensures that nothing goes wrong when
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replaying
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\begin_inset Quotes eld
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\end_inset
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too much
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\begin_inset Quotes erd
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\end_inset
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/ starting your replay
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\begin_inset Quotes eld
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\end_inset
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too early
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\begin_inset Quotes erd
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\end_inset
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.
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Idempotence is even more beneficial: in case anything should go wrong with
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your data on your disk (e.g.
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IO errors), replaying your logfile once more often may
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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Miracles cannot be guaranteed, but
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\emph on
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higher chances
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\emph default
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and
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\emph on
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improvements
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\emph default
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can be expected (e.g.
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better chances for
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\family typewriter
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fsck
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\family default
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).
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\end_layout
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\end_inset
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even
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\series bold
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heal
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\series default
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some defects.
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Good news for desperate sysadmins forced to work with flaky hardware!
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\end_layout
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\begin_layout Standard
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The basic idea of the asynchronous replication of MARS is rather simple:
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just transfer the logfiles to your secondary nodes, and replay them onto
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their copy of the disk data (also called
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\emph on
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mirror
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\emph default
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) in the same order as the total order defined by the primary.
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\end_layout
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\begin_layout Standard
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Therefore, a mirror of your data on any secondary may be outdated, but it
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always corresponds to some version which was valid in the past.
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This property is called
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\series bold
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anytime consistency
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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Your secondary nodes are always consistent in themselves.
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Notice that this kind of consistency is a
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\emph on
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local
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\emph default
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consistency model.
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There exists no global consistency in MARS.
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Global consistency would be practically impossible in long-distance replication
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where Einstein's law of the speed of light is limiting global consistency.
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The front-cover pictures showing the planets Earth and Mars tries to lead
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your imagination away from global consistency models as used in
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\begin_inset Quotes eld
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\end_inset
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DRBD Think(tm)
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\begin_inset Quotes erd
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\end_inset
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, and try to prepare you mentally for local consistency as in
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\begin_inset Quotes eld
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\end_inset
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|
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MARS Think(tm)
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\begin_inset Quotes erd
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\end_inset
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.
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\end_layout
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|
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\end_inset
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.
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\end_layout
|
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|
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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|
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\end_inset
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As you can see in the picture, the process of transfering the logfiles is
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\emph on
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independent
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\emph default
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from the process which replays the logfiles onto the data at some secondary
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site.
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Both processes can be switched on / off separately (see commands
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\family typewriter
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marsadm {dis,}connect
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\family default
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and
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\family typewriter
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marsadm {pause,resume}-replay
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\family default
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in section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "subsec:Operation-of-the"
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\end_inset
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).
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This may be
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\emph on
|
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exploited
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\emph default
|
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: for example, you may replicate your logfiles as soon as possible (to protect
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against catastrophic failures), but deliberately wait one hour until it
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is replayed (under regular circumstances).
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If your data inside your filesystem
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\family typewriter
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/mydata/
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\family default
|
||||
at the primary site is accidentally destroyed by
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\family typewriter
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rm -rf /mydata/
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\family default
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, you have an old copy at the secondary site.
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This way, you can substitute
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\emph on
|
||||
some parts
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\begin_inset Foot
|
||||
status open
|
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|
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\begin_layout Plain Layout
|
||||
Please note that MARS cannot
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\emph on
|
||||
fully
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\emph default
|
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substitute a backup system, because it can keep only
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\emph on
|
||||
physical
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\emph default
|
||||
copies, and does not create logical copies.
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\end_layout
|
||||
|
||||
\end_inset
|
||||
|
||||
|
||||
\emph default
|
||||
of conventional backup functionality by MARS.
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In case you need the actual version, just replay in
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
fast-forward
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||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
mode (similar to old-fashioned video tapes).
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
Future versions of MARS Full are planned to also allow
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
fast-backward
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
rewinding, of course at some cost.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Chapter
|
||||
HOWTO setup MARS
|
||||
\end_layout
|
||||
@ -4552,446 +4992,6 @@ When taking this naïvely, you could easily step into some trivial pitfalls,
|
||||
MARS.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Section
|
||||
The Transaction Logger
|
||||
\begin_inset CommandInset label
|
||||
LatexCommand label
|
||||
name "sec:The-Transaction-Logger"
|
||||
|
||||
\end_inset
|
||||
|
||||
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\align center
|
||||
\begin_inset Graphics
|
||||
filename images/MARS_Data_Flow.pdf
|
||||
lyxscale 60
|
||||
width 100text%
|
||||
|
||||
\end_inset
|
||||
|
||||
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
The basic idea of MARS is to record all changes made to your block device
|
||||
in a so-called
|
||||
\series bold
|
||||
transaction logfile
|
||||
\series default
|
||||
.
|
||||
|
||||
\emph on
|
||||
Any
|
||||
\emph default
|
||||
write reqeuest is treated like a transaction which changes the contents
|
||||
of your block device.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
This is similar in concept to some database systems, but there exists no
|
||||
separate
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
commit
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
operation:
|
||||
\emph on
|
||||
any
|
||||
\emph default
|
||||
write request is acting like a commit.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
The picture shows the flow of write requests.
|
||||
Let's start with the primary node.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Upon submission of a write request on
|
||||
\family typewriter
|
||||
/dev/mars/mydata
|
||||
\family default
|
||||
, it is first buffered in a
|
||||
\emph on
|
||||
temporary
|
||||
\emph default
|
||||
memory buffer.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
The temporary memory buffer serves multiple purposes:
|
||||
\end_layout
|
||||
|
||||
\begin_layout Itemize
|
||||
It keeps track of the order of write operations.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Itemize
|
||||
Additionally, it keeps track of the positions in the underlying disk
|
||||
\family typewriter
|
||||
/dev/lv-x/mydata
|
||||
\family default
|
||||
.
|
||||
In particular, it detects when the same block is overwritten multiple times.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Itemize
|
||||
During pending write operation, any concurrent reads are served from the
|
||||
memory buffer.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
After the write has been buffered in the temporary memory buffer, the main
|
||||
logger thread of the transaction logger creates a so-called
|
||||
\emph on
|
||||
log entry
|
||||
\emph default
|
||||
and starts an
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
append
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
operation on the transaction logfile.
|
||||
The log entry contains vital information such as the logical block number
|
||||
in the underlying disk, the length of the data, a timestamp, some header
|
||||
magic in order to detect corruption, the log entry sequence number, of
|
||||
course the data itself, and optional information like a checksum or compression
|
||||
information.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Once the log entry has been written through to the
|
||||
\family typewriter
|
||||
/mars/
|
||||
\family default
|
||||
filesystem via fsync(), the application waiting for the write operation
|
||||
at
|
||||
\family typewriter
|
||||
/dev/mars/mydata
|
||||
\family default
|
||||
is signalled that the write was successful.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
This may happen even
|
||||
\emph on
|
||||
before
|
||||
\emph default
|
||||
the writeback to the underlying disk
|
||||
\family typewriter
|
||||
/dev/lv-x/mydata
|
||||
\family default
|
||||
has started.
|
||||
Even when you power off the system right now, the information is not lost:
|
||||
it is present in the logfile, and can be reconstructed from there.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Notice that the order of log records present in the transaction log defines
|
||||
a total order among the write requests which is
|
||||
\emph on
|
||||
compatible
|
||||
\emph default
|
||||
to the partial order of write requests issued on
|
||||
\family typewriter
|
||||
/dev/mars/mydata
|
||||
\family default
|
||||
.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Also notice that despite its sequential nature, the transaction logfile
|
||||
is typically
|
||||
\emph on
|
||||
not
|
||||
\emph default
|
||||
the performance bottleneck of the system: since appending to a logfile
|
||||
is almost purely sequential IO, it runs much faster than random IO on typical
|
||||
datacenter workloads.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
In order to reclaim the temporary memory buffer, its content must be written
|
||||
back to the underlying disk
|
||||
\family typewriter
|
||||
/dev/lv-x/mydat
|
||||
\family default
|
||||
a somewhen.
|
||||
After writeback, the temporary space is freed.
|
||||
The writeback can do the following optimizations:
|
||||
\end_layout
|
||||
|
||||
\begin_layout Enumerate
|
||||
writeback may be in
|
||||
\emph on
|
||||
any
|
||||
\emph default
|
||||
order; in particular, it may be
|
||||
\emph on
|
||||
sorted
|
||||
\emph default
|
||||
according to ascending sector ´numbers.
|
||||
This will reduce the average seek distances of magnetic disks in general.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Enumerate
|
||||
when the same sector is overwritten multiple times, only the
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
last
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
version need to be written back, skipping some intermediate versions.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
In case the primary node crashes during writeback, it suffices to replay
|
||||
the log entries from some point in the past until the end of the transaction
|
||||
logfile.
|
||||
It does no harm if you accidentally replay some log entries twice or even
|
||||
more often: since the replay is in the original total order, any temporary
|
||||
inconsistency is
|
||||
\emph on
|
||||
healed
|
||||
\emph default
|
||||
by the logfile application.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
In mathematics, the property that you can apply your logfile twice to your
|
||||
data (or even as often as you want), is called
|
||||
\series bold
|
||||
idempotence
|
||||
\series default
|
||||
.
|
||||
This is a very desirable property: it ensures that nothing goes wrong when
|
||||
replaying
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
too much
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
/ starting your replay
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
too early
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
.
|
||||
Idempotence is even more beneficial: in case anything should go wrong with
|
||||
your data on your disk (e.g.
|
||||
IO errors), replaying your logfile once more often may
|
||||
\begin_inset Foot
|
||||
status open
|
||||
|
||||
\begin_layout Plain Layout
|
||||
Miracles cannot be guaranteed, but
|
||||
\emph on
|
||||
higher chances
|
||||
\emph default
|
||||
and
|
||||
\emph on
|
||||
improvements
|
||||
\emph default
|
||||
can be expected (e.g.
|
||||
better chances for
|
||||
\family typewriter
|
||||
fsck
|
||||
\family default
|
||||
).
|
||||
\end_layout
|
||||
|
||||
\end_inset
|
||||
|
||||
even
|
||||
\series bold
|
||||
heal
|
||||
\series default
|
||||
some defects.
|
||||
Good news for desperate sysadmins forced to work with flaky hardware!
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
The basic idea of the asynchronous replication of MARS is rather simple:
|
||||
just transfer the logfiles to your secondary nodes, and replay them onto
|
||||
their copy of the disk data (also called
|
||||
\emph on
|
||||
mirror
|
||||
\emph default
|
||||
) in the same order as the total order defined by the primary.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Therefore, a mirror of your data on any secondary may be outdated, but it
|
||||
always corresponds to some version which was valid in the past.
|
||||
This property is called
|
||||
\series bold
|
||||
anytime consistency
|
||||
\begin_inset Foot
|
||||
status open
|
||||
|
||||
\begin_layout Plain Layout
|
||||
Your secondary nodes are always consistent in themselves.
|
||||
Notice that this kind of consistency is a
|
||||
\emph on
|
||||
local
|
||||
\emph default
|
||||
consistency model.
|
||||
There exists no global consistency in MARS.
|
||||
Global consistency would be practically impossible in long-distance replication
|
||||
where Einstein's law of the speed of light is limiting global consistency.
|
||||
The front-cover pictures showing the planets Earth and Mars tries to lead
|
||||
your imagination away from global consistency models as used in
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
DRBD Think(tm)
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
, and try to prepare you mentally for local consistency as in
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
MARS Think(tm)
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
.
|
||||
\end_layout
|
||||
|
||||
\end_inset
|
||||
|
||||
.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
As you can see in the picture, the process of transfering the logfiles is
|
||||
|
||||
\emph on
|
||||
independent
|
||||
\emph default
|
||||
from the process which replays the logfiles onto the data at some secondary
|
||||
site.
|
||||
Both processes can be switched on / off separately (see commands
|
||||
\family typewriter
|
||||
marsadm {dis,}connect
|
||||
\family default
|
||||
and
|
||||
\family typewriter
|
||||
marsadm {pause,resume}-replay
|
||||
\family default
|
||||
in section
|
||||
\begin_inset CommandInset ref
|
||||
LatexCommand ref
|
||||
reference "subsec:Operation-of-the"
|
||||
|
||||
\end_inset
|
||||
|
||||
).
|
||||
This may be
|
||||
\emph on
|
||||
exploited
|
||||
\emph default
|
||||
: for example, you may replicate your logfiles as soon as possible (to protect
|
||||
against catastrophic failures), but deliberately wait one hour until it
|
||||
is replayed (under regular circumstances).
|
||||
If your data inside your filesystem
|
||||
\family typewriter
|
||||
/mydata/
|
||||
\family default
|
||||
at the primary site is accidentally destroyed by
|
||||
\family typewriter
|
||||
rm -rf /mydata/
|
||||
\family default
|
||||
, you have an old copy at the secondary site.
|
||||
This way, you can substitute
|
||||
\emph on
|
||||
some parts
|
||||
\begin_inset Foot
|
||||
status open
|
||||
|
||||
\begin_layout Plain Layout
|
||||
Please note that MARS cannot
|
||||
\emph on
|
||||
fully
|
||||
\emph default
|
||||
substitute a backup system, because it can keep only
|
||||
\emph on
|
||||
physical
|
||||
\emph default
|
||||
copies, and does not create logical copies.
|
||||
\end_layout
|
||||
|
||||
\end_inset
|
||||
|
||||
|
||||
\emph default
|
||||
of conventional backup functionality by MARS.
|
||||
In case you need the actual version, just replay in
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
fast-forward
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
mode (similar to old-fashioned video tapes).
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
Future versions of MARS Full are planned to also allow
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
fast-backward
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
rewinding, of course at some cost.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Section
|
||||
Defending Overflow of
|
||||
\family typewriter
|
||||
|
||||
Loading…
Reference in New Issue
Block a user