mirror of
https://github.com/schoebel/mars
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user-manual: rework emergency mode
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
2ea8d33599
commit
5ff5c1de3f
@ -6912,6 +6912,13 @@ restarting
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\begin_layout Paragraph
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Retaining a Split Brain Version (optionally, typically not needed, may be
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skipped)
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\begin_inset CommandInset label
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LatexCommand label
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name "par:Retaining-a-Split"
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\end_inset
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\end_layout
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\begin_layout Standard
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@ -7687,23 +7694,63 @@ This leads to a potential risk from the perspective of a sysadmin: what
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\end_layout
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\begin_layout Standard
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No risk, no fun.
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If you want a system which survives long-lasting network outages while
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keeping your replicas always consistent (anytime consistency), you
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In practice, no harm will occur to your data.
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MARS will automatically go into the so-called emergency mode.
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Resolution of emergency mode is very similar to resolution of split brain
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(section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:Resolution-of-Split"
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plural "false"
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caps "false"
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noprefix "false"
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\end_inset
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): at all of your secondaries, type (repeatedly)
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\begin_inset Newline newline
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\end_inset
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\family typewriter
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marsadm invalidate all
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\end_layout
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\begin_layout Standard
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This is all you need to know.
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If you are impatient, you may now skip the rest of this section.
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\end_layout
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\begin_layout Standard
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For some background explanations, keep reading on.
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\end_layout
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\begin_layout Standard
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Overflow and its treatment is
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\emph on
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unavoidable
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\emph default
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for long-distance replication.
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If you want a system which can survive long-lasting network outages, while
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keeping your replicas consistent as long as possible (called
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\series bold
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anytime consistency
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\series default
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), then you
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\emph on
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need
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\emph default
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dynamic memory for that.
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dynamic storage.
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It is
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\emph on
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impossible
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\emph default
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to solve that problem using static memory
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to solve with static pre-allocated memory
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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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The bitmaps used by DRBD don't preserve the
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The bitmaps used by DRBD cannot preserve the
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\emph on
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order
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\emph default
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@ -7737,20 +7784,82 @@ facts
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\end_inset
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.
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A true solution would need
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\emph on
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infinite memory
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\emph default
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.
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But suchalike does not exist on earth.
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\end_layout
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\begin_layout Standard
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Therefore, DRBD and MARS have different application areas.
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It would be an even worse idea to statically pre-allocate a lot of space
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for
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\emph on
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each
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\emph default
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of your resources.
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The latter would waste a lot of space, because some resources will likely
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fill much more quickly than others.
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MARS deals with this by using a
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\emph on
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common
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\emph default
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filesystem /mars which is
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\emph on
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shared
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\emph default
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by the transaction logs of
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\emph on
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all
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\emph default
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of your resources.
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\end_layout
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\begin_layout Standard
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Although the size of /mars is statically allocated at cluster generation
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time, there is a workaround for the problem.
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When
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\family typewriter
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/mars
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\family default
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fills up during a network outage, and you have some spare space on your
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VG, and when the network outage will be repaired shortly, you may decide
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to dynamically extend /mars during operation.
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\end_layout
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\begin_layout Standard
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Because of these fundamental differences, DRBD and MARS have different applicati
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on areas.
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If you just want a simple system for mirroring your data over short distances
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like a crossover cable, DRBD will be a suitable choice.
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via passive
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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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Notice: newer generation 10GBit technologies like SFP+ are no longer passive.
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They involve some active chips, which may fail independently from your
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servers.
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In case of a failure, the CAP theorem property P is violated, and you only
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have the choice between C and A.
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For details, see
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\family typewriter
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mars-architecture-guide.pdf
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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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crossover cable, and when failures of the crossover cables are very unlikely,
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DRBD will be a suitable choice.
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However, if you need to replicate over longer distances, or if you need
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higher levels of reliability even when multiple failures may accumulate
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(such as network loss during a
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\emph on
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re
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\emph default
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sync of DRBD), the transaction logs of MARS can solve that, but at some
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sync of DRBD), the transaction logs of MARS can solve it, but at some
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\emph on
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cost
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\emph default
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@ -7758,7 +7867,7 @@ cost
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\end_layout
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\begin_layout Subsection
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Countermeasures
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Countermeasures against overflow
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\end_layout
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\begin_layout Subsubsection
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@ -7780,7 +7889,7 @@ The first (and most important) measure against overflow of
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/mars/
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\family default
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is simply to dimension it large enough to survive longer-lasting problems,
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at least one weekend.
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preferably one weekend.
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\end_layout
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\begin_layout Standard
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@ -7954,11 +8063,7 @@ disconnect
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.
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Therefore, a simple
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\family typewriter
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marsadm connect-global all
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\family default
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followed by
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\family typewriter
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marsadm resume-replay-global all
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marsadm up all
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\family default
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may also work miracles (if you didn't want to freeze some mirror deliberately).
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\begin_inset Newline newline
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@ -7991,12 +8096,6 @@ marsadm leave-resource $res
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exactly that(!)
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\emph default
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secondary site where the mirror is frozen, can also work miracles.
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If you want to automate this in unserspace, be careful.
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It is easy to get unintended effects when choosing the wrong site for
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\family typewriter
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leave-resource
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\family default
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.
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\begin_inset Newline newline
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\end_inset
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@ -8298,10 +8397,23 @@ status open
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\begin_layout Subsubsection
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Throttling
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\begin_inset CommandInset label
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LatexCommand label
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name "subsec:Throttling"
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\end_inset
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\end_layout
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\begin_layout Standard
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The last measure for defense of overflow is
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This not generally recommended.
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It may harm the IO performance from the viewpoint of your customers.
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Thus use it only as a
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\emph on
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desperate
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\emph default
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defense against overflow, by
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\series bold
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throttling your performance pigs
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\series default
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@ -8318,30 +8430,38 @@ ssh
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very
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\emph default
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silly things.
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For example, some of them are creating their own backups via user-cron
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jobs, and they do it every 5 minutes.
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For example,
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\end_layout
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\begin_layout Itemize
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some users are creating their own backups via user-cron jobs, and they do
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it every 5 minutes.
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Some example guy created a zip archive (almost 1GB) by regularly copying
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his old zip archive into a new one, then appending deltas to the new one,
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and finally deleting the old archive.
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Every 5 minutes.
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Yes, every 5 minutes, although almost never any new files were added to
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the archive.
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Although almost never any new files were added to the archive.
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Essentially, he copied over his archive, for nothing.
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This led to massive bulk write requests, for ridiculous reasons.
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\end_layout
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\begin_layout Itemize
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another user wrote his own shell script for his own private backup of his
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website, although there already is a daily system backup.
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He regularly made a complete copy of his entire webspace (more than 60GiB)
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via
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\family typewriter
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cp -a
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\family default
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, then created a tarball out of the copy, uploaded it into the cloud, finally
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removed both the tarball and the complete filesystem copy.
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Each time, about 100GB was temporarily allocated (and replicated via MARS).
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\end_layout
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\begin_layout Standard
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In general, your hard disks (or even RAID systems) allow much higher write
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IO rates than you can ever transport over a standard TCP network from your
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primary site to your secondary, at least over longer distances (see use
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cases for MARS in chapter
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "chap:Use-Cases-for"
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\end_inset
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).
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Typically, your hard disks / RAID systems allow much higher write IO rates
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than you can ever transport over a standard TCP network from your primary
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site to your secondary, at least over longer distances.
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Therefore, it is easy to create a such a high write load that it will be
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\emph on
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@ -8355,12 +8475,9 @@ by construction
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\end_layout
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\begin_layout Standard
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Therefore, we
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\emph on
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need
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\emph default
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some mechanism for throttling bulk writers whenever the network is weaker
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than your IO subsystem.
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MARS has some mechanism for throttling bulk writers whenever the network
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is weaker than your IO subsystem.
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It is off by default.
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\end_layout
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\begin_layout Standard
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@ -8395,7 +8512,7 @@ reference "subsec:Dimensioning-of-/mars/"
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\end_inset
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), MARS will start to throttle your application writes.
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), MARS may be used for throttling your application writes.
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\end_layout
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\begin_layout Standard
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@ -8421,7 +8538,16 @@ write_throttle_start_percent
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slowly
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\emph default
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.
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Typical values for this are 60%.
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Defaul value is 0, which means
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\begin_inset Quotes eld
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\end_inset
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off
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\begin_inset Quotes erd
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\end_inset
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.
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Practical values for this coule be around 80%.
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\end_layout
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\begin_layout Description
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@ -8431,7 +8557,7 @@ write_throttle_end_percent
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\family default
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Maximum throttling will occur once this space threshold is reached, i.e.
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the throttling is now at its maximum effect.
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Typical values for this are 90%.
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A practical value is 90%, which is the default.
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When the actual space in
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\family typewriter
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/mars/
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@ -8500,7 +8626,7 @@ In case of lighter throttling, the input flow into
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/mars/
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\family default
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.
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The default value is 5.000 KB/s.
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The default value is 10.000 KB/s.
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Please adjust this value to your application needs and to your environment.
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\end_layout
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@ -8600,7 +8726,7 @@ incorrect
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.
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\end_layout
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\begin_layout Subsection
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\begin_layout Section
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Emergency Mode and its Resolution
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\begin_inset CommandInset label
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LatexCommand label
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@ -8611,6 +8737,11 @@ name "subsec:Emergency-Mode"
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\end_layout
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\begin_layout Standard
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This section explains some implementation details.
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You may skip it.
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\end_layout
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\begin_layout Standard
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When
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\family typewriter
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@ -8653,7 +8784,7 @@ new
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\end_inset
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logfile is left empty, i.e.
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no data ist written to it (for now).
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no data is written to it (for now).
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The hole in the numbering will prevent any secondaries from replaying any
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logfiles behind the hole (should they ever contain some data, e.g.
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because the emergency mode has been left again).
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@ -8714,7 +8845,8 @@ Free enough space.
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\end_layout
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\begin_layout Enumerate
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If
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The following control is intended for testing.
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If
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\family typewriter
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\begin_inset Flex URL
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@ -8729,8 +8861,8 @@ status open
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\family default
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is not set, now it is time to set it.
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Normally, it should be already set.
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is off, now is the time to set it.
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By default, it should be already set.
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\end_layout
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\begin_layout Enumerate
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@ -8775,7 +8907,7 @@ marsadm invalidate $res
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\begin_layout Enumerate
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On the primary:
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\family typewriter
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marsadm log-delete-all all
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marsadm cron
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\end_layout
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\begin_layout Enumerate
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@ -8796,136 +8928,89 @@ Orphan
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\end_layout
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\begin_layout Standard
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Alternatively, there is another method by roughly following the instructions
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from appendix
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Alternatively, there is a more complicated method, which keeps more intermediate
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emergency backup replicas:
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\end_layout
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\begin_layout Enumerate
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On
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\emph on
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all
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\emph default
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of your secondaries hostX:
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\begin_inset Newline newline
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\end_inset
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\family typewriter
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marsadm leave-resource mydata
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\end_layout
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\begin_layout Enumerate
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At the primary hostA:
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\begin_inset Newline newline
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\end_inset
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\family typewriter
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marsadm cron
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\end_layout
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\begin_layout Enumerate
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Wait until
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\family typewriter
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df /mars
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\family default
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shows no longer an overflow.
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\end_layout
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\begin_layout Enumerate
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On the first secondary hostB:
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\begin_inset Newline newline
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\end_inset
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\family typewriter
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marsadm join-resource mydata /dev/lv/mydata
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\end_layout
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\begin_layout Enumerate
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Wait until sync has finished at hostB.
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\end_layout
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\begin_layout Enumerate
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If you have more than 2 replicas in total: proceed with step 4 at hostC,
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and so on.
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This time, you could join multipe resources in parallel, because you already
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have a life replica at hostB.
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\end_layout
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\begin_layout Standard
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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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|
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Expert advice, if you have only 2 replicas, and provided you have enough
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VG space: analogously to paragraph
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\begin_inset CommandInset ref
|
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LatexCommand ref
|
||||
reference "chap:Alternative-Methods-for"
|
||||
LatexCommand vref
|
||||
reference "par:Retaining-a-Split"
|
||||
plural "false"
|
||||
caps "false"
|
||||
noprefix "false"
|
||||
|
||||
\end_inset
|
||||
|
||||
, but in a slightly different order.
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||||
In this case, do
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you may use
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\family typewriter
|
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leave-resource
|
||||
lvrename
|
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\family default
|
||||
everywhere on
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||||
\emph on
|
||||
all
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||||
\emph default
|
||||
secondaries, but
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||||
\emph on
|
||||
don't
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||||
\emph default
|
||||
start the
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||||
\family typewriter
|
||||
join-resource
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\family default
|
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phase
|
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\emph on
|
||||
for now
|
||||
\emph default
|
||||
.
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||||
Then cleanup all your secondaries via
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||||
\family typewriter
|
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log-purge-all
|
||||
\family default
|
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, and finally
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\family typewriter
|
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log-delete-all all
|
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\family default
|
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at the primary, and wait until the emergency has vanished everywhere.
|
||||
Only after that, re-
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||||
\family typewriter
|
||||
join-resource
|
||||
\family default
|
||||
your secondaries.
|
||||
\begin_inset Newline newline
|
||||
\end_inset
|
||||
|
||||
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
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lyxscale 12
|
||||
scale 7
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||||
|
||||
\end_inset
|
||||
|
||||
Expert advice for
|
||||
\begin_inset Formula $k=2$
|
||||
\end_inset
|
||||
|
||||
replicas: this means you had only 1 mirror per resource before the overflow
|
||||
happened.
|
||||
Provided that you have enough space on your LVMs and on
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||||
\family typewriter
|
||||
/mars/
|
||||
\family default
|
||||
, and provided that transaction logging has automatically restarted after
|
||||
|
||||
\family typewriter
|
||||
leave-resource
|
||||
\family default
|
||||
and
|
||||
\family typewriter
|
||||
log-purge-all
|
||||
\family default
|
||||
, you can recover redundancy by creating a
|
||||
\emph on
|
||||
new
|
||||
\emph default
|
||||
replica via
|
||||
\family typewriter
|
||||
marsadm join-resource $res
|
||||
\family default
|
||||
on a
|
||||
\emph on
|
||||
third
|
||||
\emph default
|
||||
node.
|
||||
Only after the initial full sync has finished there, run
|
||||
\family typewriter
|
||||
join-resource
|
||||
\family default
|
||||
at your original mirror.
|
||||
This way, you will always retain at least one
|
||||
\series bold
|
||||
consistent mirror
|
||||
\series default
|
||||
somewhere.
|
||||
After all is up-to-date, you can delete the superfluous mirror by
|
||||
\family typewriter
|
||||
marsadm leave-resource $res
|
||||
\family default
|
||||
and reclaim the disk space from its underlying LVM disk.
|
||||
\begin_inset Newline newline
|
||||
\end_inset
|
||||
|
||||
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
If you already have
|
||||
\begin_inset Formula $k>2$
|
||||
\end_inset
|
||||
|
||||
replicas in total, it may be a wise idea to prefer the
|
||||
\family typewriter
|
||||
leave-resource ; log-purge-all ; join-resource
|
||||
\family default
|
||||
method in front of
|
||||
\family typewriter
|
||||
invalidate
|
||||
\family default
|
||||
because it does not invalidate
|
||||
\emph on
|
||||
all
|
||||
\emph default
|
||||
your replicas at the same time (when handled properly in the right order).
|
||||
for keeping an outdated emergency backup before creating a new LV with
|
||||
the old name, and before re-joining the latter.
|
||||
Don't forget to remove your backup LV after sync has finished!
|
||||
\end_layout
|
||||
|
||||
\begin_layout Chapter
|
||||
|
||||
Loading…
Reference in New Issue
Block a user