[linux-lvm] Page cache corruption when creating a snapshot

[Greg Hudson]

On Fri, 2008-02-29 at 19:10 +0000, Bryn M. Reeves wrote:
> Greg Hudson wrote:
> > Nope, that never made it go away.  I'm not sure in what situations
> > flushing write buffers would have any effect.  If I had a way to throw
> > away the read-only page cache and force a file reload from disk, I would
> > expect that to eliminate the visible effect of the corruption; at the
> > moment the only reliable way I know how to do that is to reboot.

> To free pagecache:
>         echo 1 > /proc/sys/vm/drop_caches

Thank you!  Verified that freeing the page cache causes the
apparently-corrupted file's md5sum to revert to the correct value:

[Abbreviated diff output:]
--- /tmp/sum.pre        2008-02-29 14:30:29.000000000 -0500
+++ /tmp/sum.post       2008-02-29 14:30:32.000000000 -0500
-3e497fef20ce899c5621dcfbcbfec9a3  /tmp/test/krb5-1.6.dfsg/src/windows/cns/kerbnet.hlp
+e9725c38b630515dc304a26bde28fa51  /tmp/test/krb5-1.6.dfsg/src/windows/cns/kerbnet.hlp

root at linux-build-10:~# md5sum /tmp/test/krb5-1.6.dfsg/src/windows/cns/kerbnet.hlp
e9725c38b630515dc304a26bde28fa51  /tmp/test/krb5-1.6.dfsg/src/windows/cns/kerbnet.hlp
root at linux-build-10:~# echo 1 > /proc/sys/vm/drop_caches
root at linux-build-10:~# md5sum /tmp/test/krb5-1.6.dfsg/src/windows/cns/kerbnet.hlp
3e497fef20ce899c5621dcfbcbfec9a3  /tmp/test/krb5-1.6.dfsg/src/windows/cns/kerbnet.hlp