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Graham Perrin
Asked: 2012-12-09 01:13:07 +0800 CST

How to check that all ZFS snapshots within a pool are without holds before destroying that pool

  • 772

Question

Already I can check each snapshot of a filesystem individually, manually.

I would prefer to check all at once (all with a single command or script). Please:

  • can that be done with a script?

The answer should be good for file systems with a space within the name.

Background

From the man page for zfs(8):

zfs holds [-H] [-r] snapshot…

… -r Specifies that a hold with the given tag is applied recursively to the snapshots of all descendent file systems.

I wondered whether recent snapshots are treated as descendants of an older snapshot. No: 

Last login: Sat Dec  8 09:02:26 on ttys003
macbookpro08-centrim:~ gjp22$ zfs holds -r gjp22@2012-12-08-081957
NAME                     TAG  TIMESTAMP
macbookpro08-centrim:~ gjp22$ zfs holds -r gjp22@2012-10-28-212255
NAME                     TAG                                           TIMESTAMP
gjp22@2012-10-28-212255  problem with LocalStorage for WOT for Safari  Mon Oct 29  6:44 2012
macbookpro08-centrim:~ gjp22$ zfs hold experiment gjp22@2012-12-08-081957
macbookpro08-centrim:~ gjp22$ zfs holds -r gjp22@2012-10-28-212255
NAME                     TAG                                           TIMESTAMP
gjp22@2012-10-28-212255  problem with LocalStorage for WOT for Safari  Mon Oct 29  6:44 2012
macbookpro08-centrim:~ gjp22$ zfs holds -r gjp22@2012-12-08-081957
NAME                     TAG         TIMESTAMP
gjp22@2012-12-08-081957  experiment  Sat Dec  8  9:04 2012
macbookpro08-centrim:~ gjp22$ zfs holds -r gjp22@2012-10-28-212255
NAME                     TAG                                           TIMESTAMP
gjp22@2012-10-28-212255  problem with LocalStorage for WOT for Safari  Mon Oct 29  6:44 2012
macbookpro08-centrim:~ gjp22$

Clarification

I do not plan to combine both checks and destruction in a single command or script. This question is essentially about the checks.

scripting

4 Answers

  1. Best Answer

    Not sure about how this looked back in 2012 but now you can check the userrefs property:

    zfs get userrefs

    To list all holds in all pools:

    zfs get -Ht snapshot userrefs | grep -v $'\t'0 | cut -d $'\t' -f 1 | tr '\n' '\0' | xargs -0 zfs holds

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  2. For a pool with multiple file systems

    zfs list -H -r -d 1 -t snapshot -o name nameoffilesystematroot | xargs -n1 zfs holds -H -r

    Credit to calmh in irc://irc.freenode.net/#zfs but I'm not sure whether the syntax needs a little more work.

    For me, with a simple file system hierarchy (only one child) and relatively few snapshots (currently seventeen of the child), the command seems to not reach a conclusion. Example: 

    macbookpro08-centrim:~ gjp22$ zfs list -H -r -d 1 -t snapshot -o name gjp22 | xargs zfs holds -r
load: 4.94  cmd: zfs 39152 running 17.80u 112.52s
load: 3.73  cmd: zfs 39152 running 55.01u 349.29s
load: 3.15  cmd: zfs 39152 running 167.48u 1061.47s
load: 4.59  cmd: zfs 39152 running 267.57u 1697.49s
load: 5.19  cmd: zfs 39152 running 372.19u 2355.99s
load: 5.29  cmd: zfs 39152 running 432.89u 2736.79s

    Borrowing from the other answer, with attention to the child file system alone: 

    macbookpro08-centrim:~ gjp22$ zfs list -H -r -d 1 -t snapshot -o name gjp22/intrigue | xargs zfs holds
NAME                              TAG  TIMESTAMP
macbookpro08-centrim:~ gjp22$

    – and that output is almost immediate.

    ZFS here is ZEVO Community Edition 1.1.1.

    • 1

  3. For a pool with a single file system

    zfs list -H -r -d 1 -t snapshot -o name nameoffilesystem | xargs zfs holds

    – that is, without -r recursion to the right of the pipe.

    Credit to calmh in irc://irc.freenode.net/#zfs

    Working example

    For a file system with no space in its name: 

    macbookpro08-centrim:~ gjp22$ zfs list -H -r -d 1 -t snapshot -o name gjp22 | xargs zfs holds
load: 4.82  cmd: zfs 43038 running 0.59u 3.28s
NAME                     TAG                                           TIMESTAMP
gjp22@2012-10-28-212255  problem with LocalStorage for WOT for Safari  Mon Oct 29  6:44 2012
gjp22@2012-12-08-081957  experiment                                    Sat Dec  8  9:04 2012

    There was one Control-T to see how things were running.

    For completeness, I should state that there is a child of gjp22. But I guess that this example (without attention to descendants) does prove the effectiveness of the command.

    Non-working examples

    For a file system named Pocket Time Machine (spaces within its name), neither of the following commands succeeds: 

    zfs list -H -r -d 1 -t snapshot -o name "tall/backups/zhandy/Pocket Time Machine" | xargs zfs holds

zfs list -H -r -d 1 -t snapshot -o name tall/backups/zhandy/Pocket\ Time\ Machine | xargs zfs holds

    Output: 

    'tall/backups/zhandy/Pocket' is not a snapshot
'Time' is not a snapshot
cannot open 'tall/backups/zhandy/Pocket': dataset does not exist
cannot open 'Time': dataset does not exist
cannot open 'Machine@2012-09-18-210251': dataset does not exist
cannot open 'tall/backups/zhandy/Pocket': dataset does not exist
cannot open 'Time': dataset does not exist
cannot open 'Machine@2012-09-23-0330': dataset does not exist

    … and so on.

    This is implicitly a question within an answer, sorry … someone with good command line knowledge (not me) might be able to smarten this answer without me spinning off to a separate question. I'll seek advice in chat.

    • 1

  4. The following command will show all snapshots of [pool] (<-replace this with your pool name) that have holds

    zfs list -H -o name -t snapshot -r pool | xargs -n1 zfs holds -H

    the properties will be listed as property:stuff

    with that information we can free the snapshots..

    zfs list -H -o name -t snapshot -r pool | xargs -n1 zfs holds -H | awk '{print $1}' | xargs -n1 zfs release property:stuff

    (replace 'property:stuff' with whatever is holding your dataset)

    ..and finally delete them

    zfs destroy -r [pool]/[dataset][@snapshot]
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