[Lustre-discuss] backup MDT question

[Andreas Dilger]

On Apr 02, 2009  21:09 +0200, Johann Lombardi wrote:
> On Thu, Apr 02, 2009 at 01:27:52PM -0500, Nirmal Seenu wrote:
> > To reduce the size of the backup file use the "--sparse" option in your 
> > tar command. This option only reduces the backup file size but not the 
> > time taken to tar up your backup as tar keeps reading a whole bunch of 
> > zeros from /dev/zero before it determines that the file is sparse.
> 
> In this particular case, no blocks are allocated, so tar actually does not
> need to read the file content. This is something we can implement in our
> lustre-aware tar (bug 15524). I'm going to file a bug for this.

You're right.  A filesystem-aware backup tool like "dump" would hopefully
not back up all of this data.  Similarly, if tar were to use the "FIEMAP"
feature present in ldiskfs for 1.6.7+ and 2.6.28+ upstream kernels it
could also avoid reading any of the file blocks.

I would recommend that we also make a /proc tunable that can enable
or disable the MDS file size data, for sites that are not using e2scan,
but are doing MDS backups with tar instead of dump/LVM/raw.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.