On Oct 11, 2007, at 7:16 PM, William L. Maltby wrote:

On Thu, 2007-10-11 at 15:47 -0400, Tony Schreiner wrote:

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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.

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.

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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.

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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

----- "Tony Schreiner" schreian@bc.edu escreveu:

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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.

Sorry to answer my own question, but I just learned, lvdisplay shows the amount currently used by the snapshot

#lvdisplay /dev/vg_data/lv_backup

Hi Tony,

You can use "lvs" too:

#lvs LV VG Attr LSize Origin Snap% Move Log Copy% home sys -wi-ao 2.00G opt sys owi-ao 250.00G root sys -wi-ao 1.50G usr sys -wi-ao 1.50G var sys -wi-ao 2.00G zcssnap sys swi-ao 150.00G opt 0.24 zimbrabkp sys -wi-ao 150.00G

"zcssnap" is a snapshot of "opt", and during the time spent to sync it with "zimbrabkp" the "difference" between "opt" and it's snapshot was 0.24%, I think this is what you are looking for.

Antonio.