[linux-lvm] Re: [PATCH] [CFT] - LVM snapshotting support for XFS

Wed May 16 19:14:07 UTC 2001

Steve,

thanks a lot for your patch which I've added to our feature patch queue.

I'ld like to add it to post 1.0 LVM.

Regards,
Heinz    -- The LVM Guy --

On Mon, May 14, 2001 at 12:52:32PM -0500, Steve Lord wrote:
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> > I'll be glad to give this a try.  I've been using the attached patches (in a
> > tgz) to use snapshots with xfs in the past.  Basically they are some short
> > LVM patches to allow writable snapshots.  Using this I was able to take a
> > snapshot and mount the snapshot read-write, allowing XFS to straighten
> > things out (without the benefit of an xfs_freeze-like command, it was like
> > turning the power off on the original of the system).  Then I would unmount
> > the snapshot, change it back to read-only, and remount it for normal
> > snapshot use.
> > 
> > I did have one other problem using XFS snapshots.  As long as the original
> > was mounted, I couldn't mount the snapshot as well because it had the same
> > uuid.  So before I could mount the snapshot read-write I had to change the
> > UUID using xfs_db (which also required writeable snapshots).  Actually, I
> > had to change it twice.  My usual sequence would be something like this:
> 
> I forgot about the uuid change step in there, yes this will need changing
> to make the filesystem mountable - I will think about this. The reason for
> the uuid code in XFS is to deal with hardware which has alternate paths to
> disks - it prevents you from mounting the same filesystem from what appear
> to be two devices, but are actually alternate routes to the same disks. It
> may be reasonable to override it for readonly mounts - although that too
> is really dangerous in the alternate path case.
> 
> 
> > 
> > # sync
> > # lvcreate --size 100 --chunksize 64  --snapshot --name snap0
> > /dev/volgr1/lvol0
> > # lvchange -p rw /dev/volgr1/snap0
> > # xfs_db -x -p xfs_admin -c 'uuid generate'
> > # xfs_db -x -p xfs_admin -c 'uuid generate'
> > # mount /dev/volgr1/snap0 /hd/vol_mnt1
> > # umount /dev/volgr1/snap0
> > # lvchange -p r /dev/volgr1/snap0
> > # mount /dev/volgr1/snap0 /hd/vol_mnt1
> > 
> > With a way to freeze the filesystem, I should be safer than just calling
> > sync and I don't think I'll need to do the read-write mount to replay.  Is
> > there a way around changing the UUID?
> 
> Using this could you should be able to do a readonly mount of the xfs
> snapshotted filesystem, modulo the uuid issue.
> 
> Steve
> 
> > 
> > CCed to linux-lvm, since somebody over there may be interested in testing
> > XFS snapshots.
> > 
> > Dale Stephenson
> > steph at connex.com
> > 
> > > -----Original Message-----
> > > From: Steve Lord [mailto:lord at sgi.com]
> > > Sent: Monday, May 14, 2001 9:50 AM
> > > To: linux-xfs at oss.sgi.com
> > > Subject: [PATCH] [CFT] - LVM snapshotting support for XFS
> > > 
> > > 
> > > 
> > > Here is some experimental code for XFS which should allow use 
> > > of the LVM
> > > snapshotting capability. There are two attachments to this message, a
> > > kernel patch which will apply to the current development tree 
> > > (2.4.4 base)
> > > and a tar file which adds a new command to the xfs commands. 
> > > You will also
> > > need to apply the VFS-lock.patch from the lvm sources to 
> > > enable snapshotting.
> > > 
> > > The new command added is xfs_freeze which can be used to stop 
> > > and flush
> > > an xfs filesystem and then restart it. If you apply the VFS-lock patch
> > > from the lvm sources then you will not need this.
> > > 
> > > I am looking for people to test this code, previous 
> > > experience with lvm 
> > > and its snapshotting facility would probably be a pre-requisit.
> > > 
> > > Steve
> > > 
> > > 
> > > 
> > 
> > 
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> 
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Senior Consultant/Developer                       Am Sonnenhang 11
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