[linux-lvm] quiescing physical volumes

[linux-lvm] quiescing physical volumes

[m.klaeschen]

Hallo

we would like to back up LVM managed volumes without using snapshot
features. Background is that we have quite a couple of disk which we do not
want to double to have a snapshot device.

We run Linux for zSeries next to some z/OS systems on IBM 2064 machines in
LPAR mode. Linux data is stored on 3390-9 DASD in CDL Format (i.e. ECKD).
We use LVM to build logical volumes from DASD devices. In that logical
volumes ReiserFS is established as filesystem.

The DASD volumes for Linux are also accessable from other LPARs, so we are
(at least in a technical way) able to back up Linux data onto tape
catridges using z/OS ADRDSSU. Main problem is that ADRDSSU does not take
care about delayed writes to physical volumes. So filesystem are to be
mounted read only to back it up properly.

A way z/OS Unix filesystem zFS deals with this issue is called quiescing
r/w mounted filesystems. Before an ADRDSSU full volume dump is processed,
all activity on the zFS VSAM linar data sets (which might be compared to as
a physical volume) is going to be drained and new requests will be
suspended. Next step after ADRDSSU dump is to unquiesce the zFS filesystem,
i.e. I/O operation to VSAM will continue.

Main benefit is that all filesystem operations from applications can
continue since quiescing is invisible for users - apart from some delay of
response time.

Is there a way to use such a proceeding with Linux LVM managed DASD?

Kind regards
Michael Kl�schen
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Re: [linux-lvm] quiescing physical volumes

[Joe Thornber]

On Thu, Mar 18, 2004 at 11:33:36AM +0100, m.klaeschen@provinzial.de wrote:
> features. Background is that we have quite a couple of disk which we do not
> want to double to have a snapshot device.

The snapshot devices are generally much smaller than the origin
devices since they only store blocks that have changed.

- Joe

Re: [linux-lvm] quiescing physical volumes

[Craig Ringer]

On Thu, 2004-03-18 at 18:33, m.klaeschen@provinzial.de wrote:

> Is there a way to use such a proceeding with Linux LVM managed DASD?

I think it can be done at the filesystem level if you're using XFS (look
into "xfs_freeze"), but don't believe it can be done for other
filesystems. 

OTOH, as Joe Thornber just mentioned, LVM snapshots only need to store
the amount of data that changes over the lifetime of the snapshot, so
they do not need to be anywhere near as big as the original volumes.
Perhaps they're a better approach for backing up your ReiserFS volumes?

Craig Ringer