On Thu, Nov 10, 2011 at 14:12, Tomas Forsman <st...@acc.umu.se> wrote:
> On 10 November, 2011 - Bob Friesenhahn sent me these 1,6K bytes:
>> On Wed, 9 Nov 2011, Tomas Forsman wrote:
>>>> At all times, if there's a server crash, ZFS will come back along at next
>>>> boot or mount, and the filesystem will be in a consistent state, that was
>>>> indeed a valid state which the filesystem actually passed through at some
>>>> moment in time.  So as long as all the applications you're running can
>>>> accept the possibility of "going back in time" as much as 30 sec, following
>>>> an ungraceful ZFS crash, then it's safe to disable ZIL (set sync=disabled).
>>> Client writes block 0, server says OK and writes it to disk.
>>> Client writes block 1, server says OK and crashes before it's on disk.
>>> Client writes block 2.. waaiits.. waiits.. server comes up and, server
>>> says OK and writes it to disk.
> When a client writes something, and something else ends up on disk - I
> call that corruption. Doesn't matter whose fault it is and technical
> details, the wrong data was stored despite the client being careful when
> writing.
If the hardware is behaving itself (actually doing a cache flush when
ZFS asks it to, for example) the server won't say OK for block 1 until
it's actually on disk.  This behavior is what makes NFS over ZFS slow
without a slog: NFS does everything O_SYNC by default, so ZFS runs
around syncing all the disks all the time.  Therefore, you won't lose
data in this circumstance.

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