[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM

Gionatan Danti g.danti at assyoma.it
Tue May 2 11:00:37 UTC 2017

On 26/04/2017 18:37, Gionatan Danti wrote:
> True, but the case exists that, even on a full pool, an application with
> multiple outstanding writes will have some of them completed/commited
> while other get I/O error, as writes to already allocated space are
> permitted while writes to non-allocated space are failed. If, for
> example, I overwrite some already-allocated files, writes will be
> committed even if the pool is completely full.
> In past discussion, I had the impression that the only filesystem you
> feel safe with thinpool is ext4 + remount-ro, on the assumption that
> *any* failed writes will trigger the read-only mode. But from my test it
> seems that only *failed metadata updates* trigger the read-only mode. If
> this is really the case, remount-ro really is a mandatory option.
> However, as metadata can reside on alredy-allocated blocks, even of a
> full pool they have a chance to be committed, without triggering the
> remount-ro.
> At the same time, I thought that you consider the thinpool + xfs combo
> somewhat "risky", as xfs does not have a remount-ro option. Actually,
> xfs seems to *always* shutdown the filesystem in case of failed metadata
> update.
> Maybe I misunderstood some yours message; in this case, sorry for that.
> Anyway, I think (and maybe I am wrong...) that the better solution is to
> fail *all* writes to a full pool, even the ones directed to allocated
> space. This will effectively "freeze" the pool and avoid any
> long-standing inconsistencies.
> Thanks.

Hi Zdeneck, I would *really* to hear back you on these questions.
Can we consider thinlvm + xfs as safe as thinlvm + ext4 ?


Danti Gionatan
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Assyoma S.r.l. - www.assyoma.it
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