[linux-lvm] snapshots data

[Ming Zhang]

COW = copy on write

so once a original block is going to be overwritten, its data is copied
out and put into snapshot.


ming

On Sat, 2005-08-13 at 16:07 +0300, Imre Gergely wrote:
> hi
> 
> i created a snapshot of a lvm, like this:
> 
> lvcreate -s -L +50G -p r -n backup /dev/mail/mail /dev/sda2
> 
> that is, a 50gb, read-only snapshot for /dev/mail/mail to the PV sda2.
> the i did a lvdisplay -v
> 
>   --- Logical volume ---
>   LV Name                /dev/mail/mail
>   VG Name                mail
>   LV UUID                jhjDsx-FvKT-zJer-uSc3-2rrb-eHaC-Af7jBq
>   LV Write Access        read/write
>   LV snapshot status     source of
>                          /dev/mail/backup [active]
>   LV Status              available
>   # open                 1
>   LV Size                150.22 GB
>   Current LE             4807
>   Segments               1
>   Allocation             next free (default)
>   Read ahead sectors     0
>   Block device           254:3
> 
>   --- Logical volume ---
>   LV Name                /dev/mail/backup
>   VG Name                mail
>   LV UUID                nQrrrx-cTN8-4lqX-op9F-1vDO-VJ1V-hbQlTU
>   LV Write Access        read only
>   LV snapshot status     active destination for /dev/mail/mail
>   LV Status              available
>   # open                 0
>   LV Size                150.22 GB
>   Current LE             4807
>   Segments               1
>   Snapshot chunk size    8.00 KB
>   Allocated to snapshot  0.13%
>   Allocation             next free (default)
>   Read ahead sectors     0
>   Block device           254:6
> 
> how can i tell, how big is that snapshot i created? i know that it's
> 50gb, but where can i see this information. and what is "Allocated to
> snapshot"? that percentage is from the 50gb a allocated for the
> snapshot? or from the origin LV ?
> 
> lvm> version
>   LVM version:     2.00.15 (2004-04-19)
>   Library version: 1.00.14-ioctl (2004-04-06)
>   Driver version:  4.4.0
> 
> 
> i installed lvm from CVS too, and i noticed the output of lvdisplay is a
> bit different there. in fact, lvdisplay does not even show the snapshot
> LV i created... if i do a vgdisplay -v, only then appears:
> 
>   --- Logical volume ---
>   LV Name                /dev/test/root
>   VG Name                test
>   LV UUID                wqoG5m-X3Fn-Gsaj-8P9t-cSwS-AXi4-IJ5j2P
>   LV Write Access        read/write
>   LV snapshot status     source of
>                          /dev/test/backup_root [active]
>   LV Status              available
>   # open                 1
>   LV Size                976.00 MB
>   Current LE             244
>   Segments               1
>   Allocation             inherit
>   Read ahead sectors     0
>   Block device           253:3
> 
>   --- Logical volume ---
>   LV Name                /dev/test/backup_root
>   VG Name                test
>   LV UUID                cHs3wn-EUJl-f1wq-R8SJ-kpxZ-u9CG-L9Qhy1
>   LV Write Access        read only
>   LV snapshot status     active destination for /dev/test/root
>   LV Status              available
>   # open                 0
>   LV Size                976.00 MB
>   Current LE             244
>   COW-table size         1016.00 MB
>   COW-table LE           254
>   Allocated to snapshot  2.05%
>   Snapshot chunk size    8.00 KB
>   Segments               1
>   Allocation             inherit
>   Read ahead sectors     0
>   Block device           253:2
> 
> lvm> version
>   LVM version:     2.01.13-cvs (2005-06-14)
>   Library version: 1.01.01 (2005-03-29)
>   Driver version:  4.4.0
> 
> 
> what's COW? :)
> 
> thx.
> 
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