OnlineJFS snapshot question.

[Gino Castoldi_2]

Hi,

Is there a way that we can estimate how long it would take to create snapshots of our filesystems using OnlineJFS?

I would like to calculate how long it would take to backup
(snapshot) our entire application (OVO) using this feature.

(We don't have OnlineJFS yet but if I can prove its worth
then hopefully they will purchase it).

10 points to any good answer.
Thank you Gino.

[S.K. Chan · Accepted solution]

Haven't really got around measuring how long it took to create the snapshot vxfs, as I can recalled it's quite fast but then again I'm doing it on a 4GB mounted vxfs filesystem with only about 200MB of utilized space. Backing up the snapshot filesystem with your backup tools is probably the one that's going to take time, not creation of the snapshot vxfs. I did a quick test .. (/dev/vg02/lvol1 - 4GB) ..
# mkdir /mysnap
# lvcreate -L 400 -n snap /dev/vg02
==> I'm assuming 10% changes.
# mount -F vxfs -o snapof=/dev/vg02/lvol1 /dev/vg02/snap /mysnap
==> This creates the snapshot fs and it's instantaneous.
Now /mysnap is ready to be backed up.

[A. Clay Stephenson]

It's essentially instantaneous - about 10 seconds is as long as I've ever seen. You see they don't work like you think they do. All that happens is that a bitmap is created of blocks that have changed (only the first time a block was changed is of interest). In the original filesystem just before a block is changed, a copy is written to the "snapshot buffer" and that block is recorded in the bitmap. The "snapshot" is a composite of the unchanged blocks in the original filesystem and the changed blocks in the snapshot buffer.

As to how long the actual tape backup of the snapshot(s) would take - who knows? who cares? You can do them at leisure while the application continues to run.