zfs discuss - Feb 2011 - Question regarding "zfs snapshot -r"

Hello All,

I have two questions related to "zfs snapshot -r"

1. When "zfs snapshot -r tank at today" command is issued, does it
creates
snapshots for all the descendent file systems at the same moment? I mean to
say if the command is issued at 10:20:35 PM, does the creation time of all
the snapshots for descendent file systems are same?

2. Say, tank has around 5000 descendent file systems and "zfs snapshot -r
tank at today" takes around 10 seconds to complete. If there is a new file
systems created under tank within that 10 seconds period, does that snapshot
process includes the new file system created within that 10 seconds?

OR it will exclude that newly created filesystem?

Thanks,

-- Rahul
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On Tue, Feb  1 at 10:54, Rahul Deb wrote:
>   Hello All,
>   I have two questions related to "zfs snapshot -r"
>   1. When "zfs snapshot -r tank at today" command is issued, does
it creates
>   snapshots for all the?descendent file systems at the same moment? I mean
>   to say if the command is issued at 10:20:35 PM, does the creation time of
>   all the snapshots for descendent file systems are same?
>   2. Say, tank has around 5000 descendent file systems and "zfs
snapshot -r
>   tank at today" takes around 10 seconds to complete. If there is a
new file
>   systems created under tank within that 10 seconds period, does that
>   snapshot process includes the new file system created within that 10
>   seconds?
>   OR it will exclude that newly created filesystem?
>   Thanks,
>   -- Rahul
I believe the contract is that the content of all recursive snapshots
are consistent with the instant in time at which the snapshot command
was executed.

Quoting from the ZFS Administration Guide:

  Recursive ZFS snapshots are created quickly as one atomic
  operation. The snapshots are created together (all at once) or not
  created at all. The benefit of such an operation is that the snapshot
  data is always taken at one consistent time, even across descendent
  file systems.

Therefore, in #2 above, the snapshot wouldn''t include the new file in
the descendent file system, because it was created after the moment in
time when the snapshot was initiated.

In #1 above, I would guess the snapshot time is the time of the
initial command across all filesystems in the tree, even if it takes
10 seconds to actually complete the command.  However, I have no such
system where I can prove this guess as correct or not.

--eric

-- 
Eric D. Mudama
edmudama at mail.bounceswoosh.org