On Oct 11, 2007, at 7:16 PM, William L. Maltby wrote:
> On Thu, 2007-10-11 at 15:47 -0400, Tony Schreiner wrote:
>> I've recently begun to try out LVM snapshots and I have 2 things I'm
>> wondering about
>>
>> My intention is to run backups off the snapshot. As a first
>> iteration, I left 15% of the disk for the snapshot LV.
>>
>> 1.
>> The HOWTO instructions suggest that the snapshot is in effect as long
>> as the snapshot LV exists, and use lvremove to disable the snapshot.
>> Is this the proper way?
>
> I reviewd some code I wrote and tested during 8/06 and I did not
> remove
> the logical volume. I just unmounted it. ISTR that my reading of stuff
> at that time did *not* require the snapshot volume be removed.
>
>>
>> 2.
>> I chose 15% because the disk is not that active and I think it will
>> be plenty for the duration of the backup I'm doing. Is there a way to
>> track how much space the snapshot is actually using?
>
> My code seems to indicate that once the snapshot voluke is mounted,
> the
> usual "df <options of your choice>" work just as on any other logical
> volume.
>
> BTW, this *may* have been LVM 1? Not sure now and haven't researched
> since. But it did work great and the snapshot volume (IIRC) retains
> its
> contents. This, I'm sure can be useful. Haven't really though about it
> though.
>
>>
>> Cheers,
>>
>> Tony Schreiner
>> <snip sig stuff>
>
> HTH (and that I'm correct)
> --
> Bill
When the snap shot volume is mounted, yes you can do df on it. But
that shows you the state of the volume you take the snapshot of at
the time of the snapshot. That's not what I want to know.
For example in my case
# vgdisplay
--- Volume group ---
VG Name vg_data
.... snip ...
VG Size 956.34 GB
PE Size 4.00 MB
Total PE 244824
Alloc PE / Size 244824 / 956.34 GB
Free PE / Size 0 / 0
# lvdisplay vg_data
--- Logical volume ---
LV Name /dev/vg_data/lv_data
VG Name vg_data
... snip...
LV Size 812.89 GB
Current LE 208100
... snip ...
--- Logical volume ---
LV Name /dev/vg_data/lv_backup
VG Name vg_data
,.. snip ...
LV snapshot status active destination for /dev/vg_data/lv_data
LV Status available
... snip ...
LV Size 812.89 GB
Current LE 208100
COW-table size 143.45 GB
COW-table LE 36724
... snip ...
the original volume has 208100 LEs, and the snapshot was created
with 36724 LEs ( 85 % , 15 % ratio ).
mount the snapshot and do df
# df /data /data/backup
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_data-lv_data
801G 289G 471G 39% /data
/dev/mapper/vg_data-lv_backup
801G 289G 471G 39% /data/backup
the snapshot on /data/backup is showing the disk free of the original.
If I'm understanding LVM snapshots, initially no blocks are changed
on the snapshot volume, they get changed as the original volume
changes until you disable the snapshot, and eventually you will run
out of space to store that. That is what I'm trying to track.
Tony Schreiner