[Lustre-discuss] redistribute used space to other OSTs (free space)

[Andreas Dilger]

On Nov 04, 2008  11:49 -0500, Brian J. Murrell wrote:
> > [root at rs1 ~]# lfs getstripe /mnt/lustre/test3.img
> > OBDS:
> > 0: testfs-OST0000_UUID ACTIVE
> > 1: testfs-OST0001_UUID ACTIVE
> > 2: testfs-OST0002_UUID ACTIVE
> > 3: testfs-OST0003_UUID ACTIVE
> > /mnt/lustre/test3.img
> >         obdidx           objid          objid            group
> >              0               7            0x7                0
> >              2               6            0x6                0
> >              3              70           0x46                0
> >              1              69           0x45                0
> > 
> > [root at rs1 ~]#
> > 
> > So, how can i move objid=7 from OST:0 to OST:2, keeping 
> > intact /mnt/lustre/test3.img file?
> 
> You can't work with that much precision.  You first deactivate the OST
> on the MDS, then when you cp the file, the destination file will be
> restriped, but will not get striped to the deactivated OST.

As an FYI, being able to migrate data between OSTs arbitrarily is a
project we are working on, but it won't be ready for some time.

> You must note that the copied file does not inherit the striping policy of
> the original automatically.  If the files you want to move are just using
> the filesystem default striping then this is not a problem.  However if
> the striping for a directory is different than the filesystem, you will
> want to create your target directory first and set the stripe policy and
> then cp/rm the files.

If the target file is created in the same directory as the source, then
it will get the same directory default striping, unless a specific
striping was specified.  It is easier to cp/rm the files one at a time
in the same directory tree than to move the whole tree.

> I said earlier that you could use mv.  I'm not positive that will always
> have the desired results so likely you should stick with cp/rm.

Using "mv" will only rename the inode on the MDS, it doesn't change any
of the data.  That is true for local filesystems as well as Lustre.

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