Re: EVMS 2.5.4 snapshot on Linux 2.6.20

Hi all,

We have resolved the issue and just thought of documenting the reason
here for folks who hit the same issue in the future....

It looks like that post-Linux 2.6.16 kernel, there was a patch that
was applied in the kernel device mapper code to remove the chunk size
argument when activating the snapshot - the reasoning behind that
after creating the snapshot the chunk size cannot be changed. This
patch is available at :
http://kernel.org/pub/linux/kernel/people/agk/patches/2.6/2.6.19/dm-snapshot-allow-zero-chunk_size.patch

While EVMS still seems to pass the chunk size while activating the
snapshot, device mapper refuses to accept it owing to the above patch
being applied on all kernels starting from 2.6.17.

On revoking the patch on our kernel tree, the snapshots have started
working properly. Am not sure of whether there has to be a patch on
EVMS side to not to send the chunk size for activating the snapshot.

Hope it helps,

Srini


On 2/6/07, V VS <[email protected]> wrote:
> Hi all,
>
> We are  using EVMS 2.5.4 version on Linux kernel version 2.6.16 along. It
> was working fine. For some of the other performance reasons, we had to move
> to Linux 2.6.20 version - specifically we are currently on 2.6.20-rc6
> version.
>
> We are observing that the snapshots are not getting created successfully in
> this kernel version. Have anyone successfully created a snapshot in this
> kernel version yet? We also tried to backtrack the kernel version to
> 2.6.19.1 but in vain - snapshots are not getting created but the rest of the
> EVMS is working fine.
>
> When we create the snapshots, we observe that the snapshot object is getting
> successfully created (when we do a query:output) but the corresponding node
> is not getting created in (/dev/evms/.nodes) that we want to mount for
> accessing the snapshot contents.
>
> We tried to do the following couple of items after going through *somewhat
> related issues* reported in the EVMS mail archives but we did not observe
> any change in the behavior:
>
> a) Going by (
> http://marc.theaimsgroup.com/?l=evms-devel&m=112699668724855&w=2)
> as suggested by Kevin, we modified the drivers in the kernel source tree
> drivers/dm/dm-snap.c and changed the value of SNAPSHOT_PAGES to be that of
> 64 instead of the currently 256.
> b) Going by
> (http://marc.theaimsgroup.com/?l=evms-devel&m=114536935000036&w=2
> ), we removed the O_DIRECT flag in plugins/disk/localdskmgr.c while opening
> the node.
>
> Both these changes in the 2.5.4 code base refused to make any changes in the
> observed behavior. This is what we attempted to do for creating the
> snapshots. We have also attached a detailed ('debug' log) for the EVMS
> commands that we used.
>
> We would really appreciate if someone can give us a few thoughts on what we
> are missing to do as we have been raking our brains on this for quite some
> time now....
>

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