From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bryn M. Reeves Date: Fri, 20 Mar 2009 17:37:20 +0000 Subject: lvcreate -s - why specifying size for snapshot ? In-Reply-To: <37d33d830903201038v6ceba2cdxf37e3ee686025aee@mail.gmail.com> References: <37d33d830903200847w45cee0b1xc973299627dead2f@mail.gmail.com> <49C3BFDF.1090900@redhat.com> <37d33d830903200940t753c52few9ec36b3e75d68a6f@mail.gmail.com> <49C3CD3D.3000002@redhat.com> <37d33d830903201038v6ceba2cdxf37e3ee686025aee@mail.gmail.com> Message-ID: <49C3D450.8090001@redhat.com> List-Id: To: lvm-devel@redhat.com MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Sandeep K Sinha wrote: > On Fri, Mar 20, 2009 at 10:37 PM, Bryn M. Reeves wrote: > Sandeep K Sinha wrote: >>>> On Fri, Mar 20, 2009 at 9:48 PM, Jonathan Brassow wrote: >>>>> On Mar 20, 2009, at 11:10 AM, Bryn M. Reeves wrote: >>>>> >>>>>> -----BEGIN PGP SIGNED MESSAGE----- >>>>>> Hash: SHA1 >>>>>> >>>>>> Sandeep K Sinha wrote: >>>>>>> Hi all, >>>>>>> Disk >>>>>>> After hunting for relevant documents and failing to find one. >>>>>>> I would like to know if its not true that the size of the snapshot >>>>>>> should be equal to the original volume? >>>>>>> If so, then why do we accept size for a snapshot? >>>>>>> >>>>>>> Or Am I missing something somewhere else? >>>>>>> >>>>>> The size of the snapshot governs the amount of space set aside for >>>>>> storing the changes to the origin volume. E.g. if you made a snapshot >>>>>> and then completely overwrote the origin the snapshot would have to be >>>>>> at least as big as the origin volume to hold the changes. >>>>>> >>>> Agreed to all the explainations, but then I would like to know why am >>>> I able to succeed in creating a snapshot > original volume. >>>> LVM should puke an error for it right. > Not at all - going back to the original example: suppose you created a > snapshot and then completely overwrite the origin volume *twice*. Now > the snapshot must be twice the size of the origin - one to hold the data > that was originally on the origin and one to hold the data from the > first overwrite. > > >> I am getting slightly confused here, >> do you mean to say that we keep incremental backup of the original LV >> blocks in the snapshots. No, ignore that. I was being dumb. >> What jonathan said, that if you maintain several point in time >> snapshot in the same cow device then that makes sense to me that >> atleast we would be able to revert back to those point in time states. Right. Cheers, Bryn. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org iEYEARECAAYFAknD1FAACgkQ6YSQoMYUY97migCgv18SSfqZ4fnF9Y6YmglwOfC5 8xYAoLkowBhDEHa16NrZhsCBJw/QZWfD =iqC5 -----END PGP SIGNATURE-----