On October 17, 2008 10:51 am Kevin Fox wrote:
> On Fri, 2008-10-17 at 10:41 -0700, Javier Guerra wrote:
> > On Fri, Oct 17, 2008 at 12:29 PM, Kevin Fox <Kevin.Fox@xxxxxxx> wrote:
> > > If you were running a single OS with lvm snapshots (IE, no xen),
> > > would the snapshots be consistent? IE, no fsck needed ever for the
> > > snapshots?
> >
> > > If so, then there must be a mechanism for LVM snapshotting to tell
> > > the file system to coalesce to disk before the snapshot.
> >
> > No.
Not directly, but some FS, like XFS, have features that make it more
amenable to LVM-style snapshots. xfs_freeze can be used to temporarily
put all I/O on "pause", and flush data to the FS. Then you can take an
LVM snapshot and get a consistent snapshot. After the snapshot is
created, you unfreeze the filesystem. If you script it, you get an I/O
interruption of only a couple of seconds at most.
However, this wouldn't help in the "snapshot LVM from dom0" situation, as
you couldn't run xfs_freeze directly from the dom0. But, a little SSH
and sudo trickery could allow you to do it.
In the domU, configure a backups user, add them to sudoers with the
ability to run xfs_freeze without a password. Then, from the dom0, you
could run something like:
#!/bin/sh
ssh -l backups <domU> "sudo xfs_freeze -f /mountpoint"
lvcreate -s -n domU-snapshot /path/to/domU/lv
ssh -l backups <domU> "sudo xfs_freeze -u /mountpoint"
<do your backups using the domU-snapshot>
> Strange that the file system and LVM have enough knowledge of each
> other to do online resizing, but not snapshotting.
That's the nature of LVM-style snapshots. The snapshots are done below
the filesystem layer, where the filesystem has no knowledge of what's
going on. As far as the FS is concerned, it's running on a harddrive.
What I really dislike about LVM-style snapshots is that the snapshot is
outside of the FS, and you have to plan ahead for how much space you
think you'll need for each snapshot, and you can't keep a bunch of them
around for very long.
This is one area where in-FS snapshots can work better, depending on how
they are done. For example, UFS2 snapshots in FreeBSD can take up to a
minute to create, and don't scale beyond a handful of snapshots. But ZFS
snapshots in FreeBSD (I don't have access to Solaris) are virtually
instantaneous, and (so far in our testing) scale above 64 simultaneous
snapshots without any issues. Plus, you can stream snapshots between
servers, making remote backups a breeze.
But, that's all kind of beside the point here.
I find doing backups from within the domU (same as is done on physical
servers) to be the easiest.
--
Freddie Cash
fjwcash@xxxxxxxxx
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