Re: [PATCHES] Fix mdsync never-ending loop problem

ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
> Here is another patch to fix never-ending loop in mdsync. I introduced
> a mdsync counter (cycle id) and cancel flags to fix the problem.

> The mdsync counter is incremented at the every beginning of mdsync().
> Each pending entry has a field assigned from the counter when it is
> newly inserted to pendingOpsTable. Only entries that have smaller counter
> values than the mdsync counter are fsync-ed in mdsync().

> Another change is to add a cancel flag in each pending entry. When a
> relation is dropped and bgwriter receives a forget-request, the corresponding
> entry is marked as dropped but we don't delete it at that time. Actual
> deletion is performed in the next fsync loop. We don't have to retry after
> AbsorbFsyncRequests() because entries are not removed outside of seqscan.

This patch looks fairly sane to me; I have a few small gripes about
coding style but that can be fixed while applying.  Heikki, you were
concerned about the cycle-ID idea; do you have any objection to this
patch?

> This patch can be applied to HEAD, 8.2 and 8.1 with a few hunks.

I don't think we should back-patch something that's a performance fix
for an extreme case, especially not when it's not been through any
extensive testing yet ...
        regards, tom lane