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: 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: 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 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- _______________________________________________ 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/
