Hi there, I was thinking about using snapshots to build a backup system when I came across this: http://www.tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html Full snapshot are automatically disabled If the snapshot logical volume becomes full it will be dropped (become unusable) so it is vitally important to allocate enough space. The amount of space necessary is dependent on the usage of the snapshot, so there is no set recipe to follow for this. **If the snapshot size equals the origin size, it will never overflow.** (** emphasis mine) When I read that, I set up a testbed and did the following: root@eden:~# lvcreate --name testing --size 4M vm Logical volume "testing" created root@eden:~# lvcreate --snapshot --name testing-snapshot --size 4M vm/testing Logical volume "testing-snapshot" created root@eden:~# lvs --units b /dev/vm/testing* LV VG Attr LSize Origin Snap% Move Log Copy% testing vm owi-a- 4194304B testing-snapshot vm swi-a- 4194304B testing 0.39 I thought it was very weird that just by creating the snapshot, I would be using 0.39% of it :-? That would mean that I could only snapshot 99.61% of the actual LV! I thought, "Nonsense, the HOWTO clearly says that by creating a snapshot the size of the LV, things cannot go wrong". So I tried, root@eden:~# dd if=/dev/zero of=/dev/vm/testing dd: writing to `/dev/vm/testing': No space left on device 8193+0 records in 8192+0 records out 4194304 bytes (4.2 MB) copied, 0.518928 s, 8.1 MB/s root@eden:~# lvs --units b /dev/vm/testing* /dev/vm/testing-snapshot: read failed after 0 of 4096 at 4128768: Input/output error /dev/vm/testing-snapshot: read failed after 0 of 4096 at 0: Input/output error LV VG Attr LSize Origin Snap% Move Log Copy% testing vm owi-a- 4194304B testing-snapshot vm Swi-I- 4194304B testing 100.00 So, it actually broke :-( I wanted to see what the real usage was, because percents are not really useful when I want to see what is going on. So I tweaked the lvm tools, just adding one line to the percent calculation: --- 8< ----------------------------------------------------------------------- --- dev_manager.c 2007-06-22 13:19:15.000000000 +0200 +++ dev_manager.jjmaestro.c 2008-11-22 20:31:59.000000000 +0100 @@ -391,6 +391,7 @@ else if (*percent < 0) *percent = 100; + log_verbose("LV raw extent usage: %" PRIu64 " of %" PRIu64 " used", total_numerator, total_denominator); log_debug("LV percent: %f", *percent); r = 1; --- 8< ----------------------------------------------------------------------- Now, when doing an lvdisplay -v I could see that right after creating a snapshot, total_numerator was always 32, that in my case is 32*512 = 16384 or 2*8KB. That is, 2 chunks of the snapshot were always used just by creating it! Thus, the HOWTO is **very** wrong and the only way to be sure that a snapshot wont overflow is making it bigger than the LV, in fact, I believe 2 chunks bigger! This is impossible, since apparently in my case I can only create LVs in 4M chunks. This is not too bad, since it is marginal as the LV size is bigger, but it is certainly annoying having to check what size the LV is and sum 4M to be sure that nothing will ever break... In any case, why do snapshots use those 8K upfront? (my guess is that it holds a pointer translation table or something like that) Could they not keep whatever info stored in those 8K somewhere else? Why the lvcreate --snapshot sentence when missing the --size flag does not create a snapshot "big enough" to avoid overflow? Is this an easy patch to work on? Thanks in advanced, Cheers, -- .''`. : :' : J. Javier Maestro `. `'` <jjmaestro@ieee.org> `- _______________________________________________ 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/
