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Re: [linux-lvm] Difference in LVM and LVM2 and their strength/weakness



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 redhat com] 
> Sent: Monday, January 23, 2006 1:41 AM
> To: Alex Chen
> Cc: mauelshagen 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 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 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 RedHat com                            +49 2626 141200
                                                       FAX 924446
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