Recurring problem: processes block accessing UFS file system
kris at obsecurity.org
Tue Nov 22 02:12:26 GMT 2005
On Mon, Nov 21, 2005 at 05:54:09PM -0600, Greg Rivers wrote:
> I've recently put up three busy email relay hosts running 6.0-STABLE.
> Performance is excellent except for a nagging critical issue that keeps
> cropping up.
> /var/spool is its own file system mounted on a geom stripe of four BSD
> partitions (details below). Once every two or three days all the
> processes accessing /var/spool block forever in disk wait. All three
> machines suffer this problem. No diagnostic messages are generated and
> the machines continue running fine otherwise, but a reboot is required to
> clear the condition. This problem occurs during normal operation, but is
> particularly likely to occur during a backup when dump makes a snapshot.
> There doesn't appear to be a problem with gstripe, as gstripe status is
> "UP" and I can read the raw device just fine while processes continue to
> block on the file system. I tried running a kernel with WITNESS and
> DIAGNOSTIC, but these options shed no light.
> If I catch the problem early enough I can break successfully into kdb;
> otherwise, if too many processes stack up, the machine hangs going into
> kdb and must be power-cycled.
Make sure you have KDB_STOP_NMI in your kernel.
> I obtained the following process listing and traces from kdb. I traced
> mksnap_ffs which was blocked in "ufs", and two random sendmail processes
> that were blocked in "ufs" and "suspfs" respectively.
Looks like a UFS snapshot deadlock. Are you running something like
dump -L on this filesystem, or making other use of snapshots? fsck -B
also uses them, but shouldn't be running except at boot time.
You should take this up with Kirk McKusick <freebsd at McKusick.COM> - in
the meantime you can work around it by not making use of UFS
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