Re: [Q] How to flush disk cache w/read-only filesystem w/o unmount&remount? (shared SAN filesystem)

[Andreas Dilger <adilger@clusterfs.com>]

On Nov 05, 2002  10:18 -0500, Jim Lawson wrote:
> I'm looking for a way to flush or invalidate the cache on the block
> device/filesystem, so that the system is forced to go all the way to the
> disk.  Unmounting and remounting would accomplish this, of course, but
> that's tough to do in production.
> 
> I've tried blockdev --flushbufs, which appears to do a BLKFLSBUF, but that
> seems to be equivalent to "sync" - just pushes the dirty buffers to disk,
> which doesn't help me.
> 
> Looking in fs/dcache.c, I see shrink_dcache_sb(struct super_block *),
> which *might* do what I want, but there doesn't seem to be any way to call
> that from user-land.  And that probably just updates the dentries - I
> don't know if that has any effect on the file data.

You may be in luck - we likely need to have an ioctl to do this from
e2fsck because the ext3 htree repacking of bad directories is causing
a bunch of problems.  In theory, the startup scripts should reboot the
system in this case, but there have also been cases reported where
people ran "e2fsck -D" on an ro-mounted root and then read-write mounted
it again.

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