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PV_FLUSH and ext3 (was Re: max mount count)

Stephen writes:
> However, I'm still nervous: journaling can keep buffers around for
> quite a while for various reasons.  WHY does LVM want to invalidate
> buffers?  I'm scared that by "fixing" this we actually mask a deeper
> problem.  By causing journaled buffers to remain in the system after
> an invalidate, are we going to violate some assumption that LVM is
> making about its ability to flush things from the kernel?

I'm not sure exactly why this is done.  There are some issues w.r.t. the
fact that the LVM user tools write metadata directly into the device,
but for snapshots the I/O is done from the kernel via kiobufs.

There is no other LVM metadata I/O from the kernel, AFAIK.

The PV_FLUSH code was added to LVM a while ago, so it is probably not
possible to find the reasons in the CVS changelogs or the mailing list

I've cc'd Heinz, maybe he can tell us why PV_FLUSH is called on all PVs
when doing pvscan/vgscan/etc?

Cheers, Andreas
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert

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