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Re: VOP_GETATTR: locking protocol change proposal
On Fri, Nov 18, 2011 at 06:31:21AM +0000, YAMAMOTO Takashi wrote:
> >> postgresql assumes instant lseek(SEEK_END) to get the size of
> >> their heap files.
> >> http://rhaas.blogspot.com/2011/11/linux-lseek-scalability.html
> >> as fsync etc keeps the vnode lock during i/o, it might cause severe
> >> performance regression.
> > I wonder if it is worth having a separate VOP for that, which would
> > retrieve a subset of vattr without lock held. There are potentially
> > more uses in the tree.
> while it's good to have VOP_GETATTR take a mask to specify a set of
> requested attributes, i don't think it's worth to have a serparete VOP
> for this. (PR/30250 is related)
> IMO we should make unlocked VOP calls safe (against revoke and umount -f)
> and put VOP_GETATTR locking back into filesystem internal.
ad looked into that and concluded it was prohibitively expensive; too
much contention on every entry/exit.
Also, retrieving data protected by a particular lock without holding
it is inherently wrong; if we're going to move the file length out
from under the main vnode lock we'd better do it explicitly.
David A. Holland
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