[linux-lvm] read hangs when snapshot gets full

[linux-lvm] read hangs when snapshot gets full

[Martin Petermann]

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 to
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 is
not completely avoiding the problem.

Is there any fix in a newer kernel version?

Thanks for any ideas!

Martin

Re: [linux-lvm] read hangs when snapshot gets full

[Patrick Caulfield]

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 to
> 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 is
> 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 2.4.16

patrick

Re: [linux-lvm] read hangs when snapshot gets full

[Martin Petermann]

> 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
to
> > 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
is
> > 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
2.4.16
>
> patrick

Hi,

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
   "/dev/develop/snapshot"
   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!

Martin

Re: [linux-lvm] read hangs when snapshot gets full

[Patrick Caulfield]

On Thu, Dec 13, 2001 at 12:02:41PM +0100, Martin Petermann wrote:
> Hi,
> 
> on my machine I installed kernel 2.4.16 and lvm 1.0.1. I've created a 400M
> filesystem and a 20M snapshot:
 
> Now I open the snapdevice for reading and copy a 10M file into the orig fs:

> Now the snapshot always gets inactivated:

Check dmesg to see why but I suspect it's just getting full. 10Meg is not very much
you know! You need to take into account the size of the COW table and things such as
the journal as well as the data that is copied.

patrick