[linux-lvm] Difference in LVM and LVM2 and their strength/weakness
Heinz Mauelshagen
mauelshagen at redhat.com
Mon Jan 23 21:23:39 UTC 2006
On Mon, Jan 23, 2006 at 11:23:11AM -0800, Alex Chen wrote:
> Heinz,
> Thank you so much for the valuable information.
>
> My last questions, I hope, are
>
> 1. What commands are need to perform the snapshot and back up?
> I assume I need to create a snapshot first, (lvmcreate?)
> Mount that snapshot (mount?) and make back up from it (tar or cpio?)
> A simple example will be great.
See lvcreate manual (-s option), mount and use whatever backup program
of your choice.
Say your existing logical volume is called /dev/VG/LV and is 100GB large,
make a snapshot with a 6GB exception store (5-10% is ususally a good size):
# lvcreate -s -L6G -n SNAP /dev/VG/LV
# mkdir /mnt/SNAP
# mount -r /dev/VG/SNAP /mnt/SNAP
# cd /mnt/SNAP
# find .|cpio -o ...
# cd
# umount /mnt/SNAP
# lvremove /dev/VG/SNAP
>
> 2. Can I do this with Red Hat 9 if I enable LVM1 there?
Yes.
Regards,
Heinz
>
> Thanks
>
> Alex
>
> -----Original Message-----
> From: Heinz Mauelshagen [mailto:mauelshagen at redhat.com]
> Sent: Monday, January 23, 2006 1:41 AM
> To: Alex Chen
> Cc: mauelshagen at redhat.com; LVM general discussion and development
> Subject: Re: [linux-lvm] Difference in LVM and LVM2 and their
> strength/weakness
>
> On Sun, Jan 22, 2006 at 06:16:33PM -0800, Alex Chen wrote:
> > Heinz, thanks for the information/
> >
> > The main intention of our interest of LIME to use it for snapshots.
> > I am told that it is a very quick way to make backups. Is that true?
> Is there any measuring number for this?
>
> Hi Alex,
>
> yes, using logical volume snapshots for backups is one of their main
> purposes.
>
> The creation of a snapshot happens instantly (ie. just some allocation
> for
> snapshot internal store and some lvm metadata updates which is *fast*).
>
> The performance of accessing logical voluems (aka origins) with
> snapshots
> largely depends on your io patterns to the origin (ie. heavy writes to
> the
> origin slow access to the snapshot down). See remarks below as well.
>
>
> >
> > >From the recent discussion on snapshots, it seems there are still
> some issues, and even a kernel patch.
> > Does RHEL 4 contain such patch? Or is there a downloadable patch if
> we buy one?
>
> We're working on fixes and they'll show up ASAP.
>
> >
> > >From what I've read from the Web regarding LIME is that LVM1 is a
> read/only while LVM2 is read/write.
>
> Correct.
>
> > The read/only mode, if I understand correctly, is that the PE where
> the data resides is first copied to an exception table, (is this the COW
> table?)
> > before the destination PE is changed. Is this correct? If so, does
> the copy involves actual data movement or only a manipulation of
> > of the FAT table?
>
> The exception table holds the information about which chunks (smaller
> than PEs)
> have been read off the origin and stored in the exception store before a
> write
> to the origin was allowed.
>
> That happens at the block devive level and therefore below any
> filesystem
> or arbitrary application on top.
>
> > The LVM2's read/write mode seems to simply mark the PE in the snapshot
> but the change is not copied from the changed PE to the snapshot, right?
>
> No, any change to the origin leads to a copy *before* the change
> happens.
> That's why lots of writes to the origin lead to lots of exceptions and
> slow
> down access.
> If a change to the snapshot is carried out, no copy from the origin is
> needed,
> just the expcetion table is updated.
>
> >
> > In either way, the snapshot seems to be keeping tracks of only the
> changes, i.e. delta.
>
> Yes.
>
> > Do we need to first create a full copy of the file system as the base,
> or we only need to keep the deltas, if we want to use snapshot for
> system backups?
>
> The snapshot covers that for you. No user initated copy or anything
> needed,
> just create a snapshot, mount (or let your arbitrary application access)
> it and run your backup on it.
>
> >
> > Sorry of all the questions, some of them may be pretty basic or
> ignorant. If you feel I should get the answers from other sources, I
> would really apprecaite it if you can give me the pointer.
>
> NP.
>
> Heinz
>
> >
> > Alex
> >
> > ________________________________
> >
> > From: linux-lvm-bounces at redhat.com on behalf of Heinz Mauelshagen
> > Sent: Sat 1/21/2006 09:08
> > To: LVM general discussion and development
> > Subject: Re: [linux-lvm] Difference in LVM and LVM2 and their
> strength/weakness
> >
> >
> >
> > On Fri, Jan 20, 2006 at 05:54:12PM -0800, Alex Chen wrote:
> > > Greetings,
> >
> > Hi.
> >
> > >
> > > I am tasked to investigate the differences between LVM and LVM2.
> I am
> > > new to LVM so it would be greatly appreciated if someone can explain
> or
> > > point me to the right direction for the following questions:
> > >
> > >
> > >
> > > 1. What does LVM2 have that is not available in LVM?
> >
> > It has command line extensions beyond lvm (eg, vgs, lvs, pvs
> commands).
> >
> > It's configurable in various regards (eg, device name filters)
> >
> > The limits of 256 grand total logical volumes and ~64k extents per
> logical
> > volume are gone.
> >
> > Logical volume snapshots are asynchronous rather than synchronuous
> > and gain better performance.
> >
> > pvmove (online data relocation) is based on temporary, restartable
> mirrors
> > which wasn't the case in LVM1.
> >
> >
> > > Or is it a
> > > only better architectured implementation of LVM?
> >
> > I'ld not say 'only', but yes, it is :)
> >
> >
> > > 2. Which version of Red Hat enterprise system uses which LVM?
> And
> > > does the user need to do any configuration during installation?
> >
> > RHEL3 comes with LVM, RHEL4 with LVM2.
> >
> > > 3. If LVM2 is definitely better than LVM, how stable is it?
> >
> > It is stable with the exception of rare snapshot issues,
> > we're fixing right now.
> >
> > >
> > >
> > >
> > > Thanks.
> > >
> > > Alex
> > >
> >
> > > _______________________________________________
> > > linux-lvm mailing list
> > > linux-lvm at redhat.com
> > > https://www.redhat.com/mailman/listinfo/linux-lvm
> > > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> >
> > --
> >
> > Regards,
> > Heinz -- The LVM Guy --
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Heinz Mauelshagen Red Hat GmbH
Consulting Development Engineer Am Sonnenhang 11
Cluster and Storage Development 56242 Marienrachdorf
Germany
Mauelshagen at RedHat.com +49 2626 141200
FAX 924446
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