On Thu, Jun 24, 2010 at 07:31:57AM +1000, Neil Brown wrote: > On Wed, 23 Jun 2010 14:28:38 -0400 > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > On Thu, Jun 17, 2010 at 02:54:01PM +1000, Neil Brown wrote: > > > > > > If you export two subvolumes of a btrfs filesystem, they will both be > > > given the same uuid so lookups will be confused. > > > blkid cannot differentiate the two, so we must use the fsid from > > > statfs64 to identify the filesystem. > > > > > > We cannot tell if blkid or statfs is best without knowing internal > > > details of the filesystem in question, so we need to encode specific > > > knowledge of btrfs in mountd. This is unfortunate. > > > > > > To ensure smooth handling of this and possible future changes in uuid > > > generation, we add infrastructure for multiple different uuids to be > > > recognised on old filehandles, but only the preferred on is used on > > > new filehandles. > > > > Could you just contatenate the two (or hash them somehow)? Or does that > > just use up too much space in the filehandle? > > I did consider xoring them together but came to the conclusion that would > actually be a regression. > If you look down at the comment that I included in uuid_by_path, you will see > that some filesystems (e.g. VFAT) just use the major/minor device number for > the f_fsid from statfs. As you know that is not necessarily stable over > reboots, while the UUID that blkid gives is. > So if we always adding the two uuids somehow, it would be an improvement for > btrfs, no change for e.g. ext3/XFS, and a regression for VFAT (and others I > think). Not good. OK, got it. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
