Re: common layout xattr

[Andreas Dilger <adilger@sun.com>]

On Jul 15, 2009  14:24 -0700, Sage Weil wrote:
> A few Ceph ioctls for getting and setting file layout (striping)
> parameters.
> +
> +#define CEPH_IOC_GET_LAYOUT _IOR(CEPH_IOCTL_MAGIC, 1, struct ceph_file_layout)
> +#define CEPH_IOC_SET_LAYOUT _IOW(CEPH_IOCTL_MAGIC, 2, struct ceph_file_layout)

I've been trying to get some steam behind a more common interface for
backing up and restoring file layout information, using virtual xattrs.

This will allow tar/rsync/etc to back up the layout xattr and then
restore it (preferrably before the data is written) to the target,
so that any user-specified file layout is preserved.

I'm thinking of using simple ASCII key=value pairs to store basic
layout information like chunk size, stripe count, mirror count,
RAID type, etc.  Some of them may not be applicable/usable by all
filesystems, but having a handful of "well known" keys and values
for a common xattr name would at least be better than what we have
now (which is nothing).

Something like (not necessarily a firm proposal yet):

trusted.common_layout:
chunk_bytes=65536
stripe_count=32
mirror_count=3
raid_type=1+0

Is this something you would be interested to pursue?  I've also discussed
this with Panasas, and they had some interest in this as well.  Any GPFS
developers watching?

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