[linux-lvm] lvremove: 'snapshot': can't remove 'active'? , non-mounted file system

[Luca Berra]

On Tue, Jun 29, 2010 at 07:07:29AM -0700, Linda A. Walsh wrote:
> lvremove -v /dev/Home/2010.06.27-06.18.55
>    Using logical volume(s) on command line
>  Can't remove open logical volume "2010.06.27-06.18.55"
>
> But it isn't mounted.
>
> So why is it saying it is 'open':
something is using it
try looking in /sys/block/dm-4/holders, fuser, lsof ...

> ll /dev/mapper shows:
>
>
> crw-rw---- 1  10, 58 2010-02-02 21:08 control
> brw-r----- 1 252,  4 2010-02-02 21:08 Home-2010.06.27--06.18.55
> brw-r----- 1 252,  3 2010-02-02 21:08 Home-2010.06.27--06.18.55-cow
> brw-r----- 1 252,  2 2010-02-02 21:08 Home-Home
> brw-r----- 1 252,  1 2010-02-02 21:08 Home-Home-real
> brw-r----- 1 252,  0 2010-02-02 21:08 Home-Share
> brw-r----- 1 252,  5 2010-02-02 21:08 Space-Backup
> brw-r----- 1 252, 11 2010-06-29 06:40 Space-boot
> brw-r----- 1 252,  6 2010-06-29 06:40 Space-root
> brw-r----- 1 252,  7 2010-06-28 17:45 Space-Torrents
> brw-r----- 1 252,  8 2010-06-29 06:40 Space-usr
> brw-r----- 1 252,  9 2010-06-29 06:40 Space-var
> brw-r----- 1 252, 10 2010-06-29 06:40 Space-var_cache
>
> So 252 is the mapper device?

252 is the dynamic major assigned to device-mapper on your system

> Is my real Home partition on 252,1?
before you created the snapshot your Home device was
/dev/mapper/Home-Home (aka dm-2)
in order to create a snapshot lvm will use 4 devices
1) Home-Home-real (linear) which has the same mapping as Home-Home before the
    snapshot took place
2) Home-2010.06.27--06.18.55-cow (linear) which maps the space that will
    contains the COW data
3) Home-2010.06.27--06.18.55 (snapshot) this is a combination of (1) and
    (2) in order to show the visible snapshot
4) Home-Home (snapshot-origin) this device replaces your Home device
    (keeping the same minor number)

so after taking the snapshot your Home device is still
/dev/mapper/Home-Home, which maps to /dev/mapper/Home-Home-real, but
before any write to /dev/mapper/Home-Home-real the original data is
saved to Home-2010.06.27--06.18.55-cow, so Home-2010.06.27--06.18.55
(which is based on Home-Home-real and Home-2010.06.27--06.18.55-cow) will
not change

please look also at linux/Documentation/device-mapper/snapshot.txt

> I just got a backup error when I tried to backup Home,
> which would correspond to 252,2 above.  It thinks it
> is no longer the same file system as the previous backup,
details please

> Should I be backing up Home-Home-real?  That doesn't
> seem right...
usually one backs up the snapshot, which is supposed to be a
point-in-time copy of the original, so i think you should backup
Home-2010.06.27--06.18.55

L.

-- 
Luca Berra -- bluca at comedia.it
         Communication Media & Services S.r.l.
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