VXFS Snapshot Restore

[Steve Bonds]

Does HP-UX support a fast restore from an OnlineJFS snapshot, or is the only way to copy all the files in from the snapshot?

(We have a 'test' Oracle database that regularly gets messed up and restored from a known good backup, but I'm looking for some way to reduce the hassle of constant restores without resorting to a complete mirror.)

[A. Clay Stephenson · Accepted solution]

Hi Steve:

The 'restore' is simply a file by file copy. As far as you are concerned, the snapshot is simply a read-only filesystem. Anything you can do with a read-only filesystem, you can do with the snapshot. However, there is a gotcha.
Since you are copying from the snapshot back to the original, I think that every block changed in the original filesystem for the FIRST time has to be written to the snapshot buffer first. Typically these buffers are only about 10-15% od the original so that you would quickly file up the snapshot buffer. This is not really the way vxfs snapshots were intended to work. They were really intended to allow backups while the filesystem stays up but not to directly restore to the original filesystem. Your better option is to mirror and split off a mirror. You can then very quickly restore the data.

Regards, Clay

[Roger Baptiste]

<>

Did you mean restoring from
the JFS snapshot copy to
the original?? No, that''s
not possible. Remember the
JFS snapshot is actually
a "'delta"" filesystem which
only contains the original
copy of the changes being made
on the actual filesystem.
JFS snapshot is not a exact
mirror of a filesystem.

<>

If you have taken a backup
of the snapshot , it is
as good as a regular filesystem
backup and the tapes of this
backup can be used for restore.
But, there is no way you can restore from the JFS snapshot itself.


<<(We have a 'test' Oracle database that regularly gets messed up and restored from a known good backup, but I'm looking for some way to reduce the hassle of constant restores without resorting to a complete mirror.)>>

You can approach this
problem from the database level, by using archivelog mode and taking regular export/hot backup. that way, you save the chore of restoring the whole database. Instead, you canrestore archives based on db recommendation .
Here''s an extract from JFS snapshot description:
****
A snapshot looks like an ordinary file system to the user, except that
snapshots are always mounted read-only. That is, none of the directories or
files within a snapshot can be modified. Internally, however, something very
different is going on. The device that ``contains'' a snapshot is actually
only used to hold blocks that have changed on the primary file system
since the snapshot was created. The rest of the blocks--the ones that
have not changed--can be found on the device that contains the primary
file system, so there is no need for a copy. All this is done transparently
within the kernel.
****

HTH
raj

[Steve Bonds]

Thanks for the replies. It's technically possible to restore the original filesystem back to the contents of the snapshot, just by restoring all the copied-on-write data. I believe this is how some dedicated NFS server/appliances restore back from a snapshot.

I was hoping that there might be a way to do the same thing with vxfs. Oh, well.