Hi Jeff!
On Tue, Jun 7, 2022 at 1:21 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
I'm taking a stab at converting ceph to use the netfs write helpers. One
thing I'm seeing though is that the kclient takes great care to write
back data to the OSDs in order of snap context age, such that an older
snapc will get written back before a newer one. With the netfs helpers,
that may not happen quite as naturally. We may end up with it trying to
flush data for a newer snapc ahead of the older one.
My question is: is that necessarily a problem? We'd be sending along the
correct info from the capsnap for each snapc, which seems like it should
be sufficient to ensure that the writes get applied correctly. If we
were to send these out of order, what would break and how?
Writing back snapshotted data out of order would corrupt the snapshot content. The OSD can only create clone objects (snapshotted data) from the live/head/most recent version of the object it has, which means that a newer write followed by an older one will apply the old data to the newer object. Exactly how that shakes out depends on where the writes are relative to object boundaries (the cloning on the OSD is by object) and how the writes were ordered relative to the snapshots, but in any case the end result will not be correct.
:( Hopefully that doesn't make the netfs transition unworkable!
sage
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