Il 26-04-2017 16:33 Zdenek Kabelac ha scritto:
But you get correct 'write' error - so from application POV - you get
failing
transaction update/write - so app knows 'data' were lost and should
not proceed with next transaction - so it's in line with 'no data is
lost' and filesystem is not damaged and is in correct state
(mountable).
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.
--
Danti Gionatan
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Assyoma S.r.l. - www.assyoma.it
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