mirror of https://github.com/schoebel/mars
user-manual: move pseudo shapshots to appendix
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
2cde907480
commit
de9ba2cda3
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@ -32217,215 +32217,6 @@ marsadm
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Tips and Tricks
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\end_layout
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\begin_layout Section
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Creating Backups via Pseudo Snapshots
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\end_layout
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\begin_layout Standard
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When all your secondaries are all homogenously located in a standby datacenter,
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they will be almost idle all the time.
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This is a waste of computing resources.
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\end_layout
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\begin_layout Standard
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Since MARS is no substitute for a full-fledged backup system, and since
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backups may put high system load onto your active side, you may want to
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utilize your passive hardware resources in a better way.
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\end_layout
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\begin_layout Standard
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MARS supports this thanks to its ability to switch the
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\family typewriter
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pause-replay
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\family default
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\emph on
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independently
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\emph default
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from
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\family typewriter
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pause-fetch
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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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The basic idea is simple: just use
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\family typewriter
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pause-replay
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\family default
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at your secondary site, but leave the replication of transaction logfiles
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intact by deliberately
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\emph on
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not
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\emph default
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saying
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\family typewriter
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pause-fetch
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\family default
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.
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This way, your secondary replica (block device) will stay frozen for a
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limited time, without loosing your redundancy: since the transaction logs
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will continue to replicate in the meantime, you can start
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\family typewriter
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resume-replay
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\family default
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at any time, in particular when a primary-side incident should happen unexpecte
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dly.
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The former secondary will just catch up by replaying the outstanding parts
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of the transaction logs in order to become recent.
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\end_layout
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\begin_layout Standard
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However, some
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\emph on
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details
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\emph default
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have to be obeyed.
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In particular, the current version of MARS needs an additional
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\family typewriter
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detach
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\family default
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operation, in order to release exclusive access to the underlying disk
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\family typewriter
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/dev/lv/$res
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\family default
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.
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Future versions of MARS are planned to support this more directly, without
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need for an intermediate
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\family typewriter
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detach
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\family default
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operation.
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\end_layout
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\begin_layout Standard
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\begin_inset Graphics
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filename images/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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Beware:
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\family typewriter
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mount -o ro /dev/vg/$res
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\family default
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can lead to
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\series bold
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unnoticed write operations
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\series default
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if you are not careful! Some journalling filesystems like
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\family typewriter
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xfs
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\family default
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or
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\family typewriter
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ext4
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\family default
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may replay their journals onto the disk, leading to
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\emph on
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binary
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\emph default
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differences and thus
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\series bold
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destroying your consistency
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\series default
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later when you re-enable
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\family typewriter
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resume-replay
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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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\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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Therefore, you may use small LVM snapshots (only in such cases).
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Typically,
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\family typewriter
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xfs
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\family default
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journal replay will require only a few megabytes.
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Therefore you typically don't need much temporary space for this.
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Here is a more detailed description of steps:
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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marsadm pause-replay $res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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marsadm detach $res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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lvcreate --size 100m --snapshot --name ro-$res /dev/vg/$res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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mount -o ro /dev/vg/ro-$res /mnt/tmp
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\end_layout
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\begin_layout Enumerate
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Now draw your backup from
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\family typewriter
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/mnt/tmp/
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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umount /mnt/tmp
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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lvremove -f /dev/vg/ro-$res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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marsadm up $res
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\end_layout
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\begin_layout Standard
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Hint: during the backup, the transaction logs will accumulate on
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\family typewriter
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/mars/
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\family default
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.
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In order to avoid overflow of
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\family typewriter
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/mars/
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\family default
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(c.f.
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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:Defending-Overflow"
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\end_inset
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), don't unnecessarily prolong the backup duration.
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\end_layout
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\begin_layout Chapter
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\start_of_appendix
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Technical Data MARS
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@ -34620,6 +34411,222 @@ reference "sec:Scripting-HOWTO"
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.
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\end_layout
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\begin_layout Chapter
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Creating Backups via Pseudo Snapshots
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\begin_inset CommandInset label
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LatexCommand label
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name "chap:Creating-Backups-via"
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\end_inset
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\end_layout
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\begin_layout Standard
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When all your secondaries are all homogenously located in a standby datacenter,
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they will be almost idle all the time.
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This is a waste of computing resources.
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\end_layout
|
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\begin_layout Standard
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Since MARS is no substitute for a full-fledged backup system, and since
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backups may put high system load onto your active side, you may want to
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utilize your passive hardware resources in a better way.
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\end_layout
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\begin_layout Standard
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MARS supports this thanks to its ability to switch the
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\family typewriter
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pause-replay
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\family default
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\emph on
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independently
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\emph default
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from
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\family typewriter
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pause-fetch
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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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The basic idea is simple: just use
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\family typewriter
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pause-replay
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\family default
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at your secondary site, but leave the replication of transaction logfiles
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intact by deliberately
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\emph on
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not
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\emph default
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saying
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\family typewriter
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pause-fetch
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\family default
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.
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This way, your secondary replica (block device) will stay frozen for a
|
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limited time, without loosing your redundancy: since the transaction logs
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will continue to replicate in the meantime, you can start
|
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\family typewriter
|
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resume-replay
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\family default
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at any time, in particular when a primary-side incident should happen unexpecte
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dly.
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The former secondary will just catch up by replaying the outstanding parts
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of the transaction logs in order to become recent.
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\end_layout
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\begin_layout Standard
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However, some
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\emph on
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details
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\emph default
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have to be obeyed.
|
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In particular, the current version of MARS needs an additional
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\family typewriter
|
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detach
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\family default
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operation, in order to release exclusive access to the underlying disk
|
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|
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\family typewriter
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/dev/lv/$res
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\family default
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.
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Future versions of MARS are planned to support this more directly, without
|
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need for an intermediate
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\family typewriter
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detach
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\family default
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operation.
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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/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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Beware:
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\family typewriter
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mount -o ro /dev/vg/$res
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\family default
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can lead to
|
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\series bold
|
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unnoticed write operations
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\series default
|
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if you are not careful! Some journalling filesystems like
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\family typewriter
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xfs
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\family default
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or
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\family typewriter
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ext4
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\family default
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may replay their journals onto the disk, leading to
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\emph on
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binary
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\emph default
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differences and thus
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\series bold
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destroying your consistency
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\series default
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later when you re-enable
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\family typewriter
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resume-replay
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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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\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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Therefore, you may use small LVM snapshots (only in such cases).
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Typically,
|
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\family typewriter
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xfs
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\family default
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journal replay will require only a few megabytes.
|
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Therefore you typically don't need much temporary space for this.
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||||
Here is a more detailed description of steps:
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||||
\end_layout
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\begin_layout Enumerate
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\family typewriter
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marsadm pause-replay $res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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marsadm detach $res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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lvcreate --size 100m --snapshot --name ro-$res /dev/vg/$res
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\end_layout
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\begin_layout Enumerate
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|
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\family typewriter
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mount -o ro /dev/vg/ro-$res /mnt/tmp
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\end_layout
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|
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\begin_layout Enumerate
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Now draw your backup from
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\family typewriter
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/mnt/tmp/
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\end_layout
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|
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\begin_layout Enumerate
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\family typewriter
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umount /mnt/tmp
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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lvremove -f /dev/vg/ro-$res
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\end_layout
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\begin_layout Enumerate
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\family typewriter
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marsadm up $res
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\end_layout
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|
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\begin_layout Standard
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Hint: during the backup, the transaction logs will accumulate on
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\family typewriter
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/mars/
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\family default
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.
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In order to avoid overflow of
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||||
\family typewriter
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||||
/mars/
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||||
\family default
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(c.f.
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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:Defending-Overflow"
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||||
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||||
\end_inset
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||||
|
||||
), don't unnecessarily prolong the backup duration.
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\end_layout
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||||
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\begin_layout Chapter
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Command Documentation for Userspace Tools
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||||
\begin_inset CommandInset label
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||||
|
|
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