2 basic LVM questions with RAMDISKs

Hank heskin at gmail.com
Sat Feb 5 13:50:57 CST 2011


Hello all,

I have a PE2650 running CentOS 3 with an LVM  VG full with one LV.    I
wanted to create a snapshot to backup this LV, but then realized that the VG
has no free space (the LV takes up all that VG space).

 I found this page that talks about extending the VG with a ramdisk in order
to provide some free space for the snapshot.  :

http://linuxsoftware.co.nz/blog/2008/03/11/lvm-snapshot-with-no-free-diskspace

Two questions:

1. When I try to use the "pvcreate /dev/ram1" command, I get an error:
"pvcreate -- invalid physical volume "/dev/ram0" which I have no idea how to
resolve (yes, a listing of /dev/ lists 20 ramdisks ram0 through ram19)

2.  I just want to confirm that when the snapshot is created using the
freespace of the ramdisk, that any changed LV snapshot blocks are written to
the ramdisk and that the current (new) data is written to the original part
of the LV.... so when I'm done with the snapshot and "vgreduce" the VG to
remove the ramdisk, that the current (i.e. changed) blocks are not removed
with it.

Or if anyone has any other suggestions on how to create a short-term
snapshot of a LV in a full VG, that also would be most helpful.

Thanks,

-Hank
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