[linux-lvm] read hangs when snapshot gets full

Martin Petermann martin.petermann at de.ibm.com
Thu Dec 13 05:05:01 UTC 2001

> On Mon, Dec 10, 2001 at 06:18:15PM +0100, Martin Petermann wrote:
> > Hi all,
> >
> > I'm using a SuSE Linux (V7.2) with a 2.4.4 kernel. If I start a program
> > which reads from a snapshot (e.g. cat /dev/vg/snapshot > /tmp/file)
with a
> > low priority (nice -20), sometimes the program hangs within the
> > read() call if the snapshot gets full. If this is the case I'm not able
> > kill the process of the application (even SIGKILL). In my application I
> > reduced the number of blocks in each read() call reads and I stop
> > reading after the snapshot is full of more then 90%. But this solution
> > not completely avoiding the problem.
> >
> > Is there any fix in a newer kernel version?
> I think this is fixed in 1.0.1 - it certainly works for me on Linux
> patrick


on my machine I installed kernel 2.4.16 and lvm 1.0.1. I've created a 400M
filesystem and a 20M snapshot:

   >lvcreate -L 20M -n snapshot -s /dev/develop/400M
   lvcreate -- WARNING: the snapshot will be automatically disabled once it
   gets full
   lvcreate -- INFO: using default snapshot chunk size of 64.00 KB for
   lvcreate -- doing automatic backup of "develop"
   lvcreate -- logical volume "/dev/develop/snapshot" successfully created

Now I open the snapdevice for reading and copy a 10M file into the orig fs:

   seska:root /home/peterman
   >nice -20 cat /dev/develop/snapshot > /BIGFS/file
   cat: /dev/develop/snapshot: Eingabe-/Ausgabefehler
   seska:root /home/peterman
   >cp tf0 /400M
   seska:root /BIGFS2

Now the snapshot always gets inactivated:

   >lvdisplay /dev/develop/snapshot
   --- Logical volume ---
   LV Name                /dev/develop/snapshot
   VG Name                develop
   LV Write Access        read only
   LV snapshot status     INACTIVE destination for /dev/develop/400M
   LV Status              NOT available
   LV #                   6
   # open                 0
   LV Size                400.00 MB
   Current LE             100
   Allocated LE           100
   snapshot chunk size    64.00 KB
   Allocated to snapshot  0.00% [0/19.69 MB]
   Allocated to COW-table 320.00 KB
   Allocation             next free
   Read ahead sectors     120
   Block device           58:12

Thanks for any help!


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