[Lustre-discuss] MDS

[Brian J. Murrell]

On Thu, 2008-08-07 at 10:51 -0700, Cliff White wrote:
> 
> Just to be clear, there is a potential data loss issue due to the time 
> delta between the backup and the live system. Any transactions in play
> that miss the snapshot could result in lost data, as the MDS will replay 
> transaction logs and delete orphans on startup. So testing on your live 
> system definately is for the brave.

Indeed.  There are a couple of alternatives to consider.  I know your
production MO will be to take an LVM snapshot of the running MDT and
back that up, but if the MDT (i.e. filesystem) were shut down prior to
the backup, what you restore should be an identical MDT which you could
then start the filesystem against without the risks of in-play
transactions and orphan deletion.  But indeed it is not a 100%
reproduction of what would happen restoring from an in-production
backup.

Alternatively, rather than trying to start the OSTs against the restored
MDT you could simply do a filesystem level (i.e. ldiskfs) comparison of
the restored MDT against the production MDT.

Indeed, there are other variations that you could use to satisfy
yourself that the restore worked.

I would highly suggest you do any of this testing either on a testbed
(which you could build with a VirtualBox virtual cluster) or on your
production system before you put production data on it.  It is good
system deployment policy to have fully tested backup and restore
policies before going live anyway.

b.

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