Hi,

Adam Goryachev wrote on 2008-12-12 11:09:14 +1100 [Re: [BackupPC-users] Backing 
up the backup to an external USB drive]:
> Rodrigo Real wrote:
> > [...]
> > Adam Goryachev wrote:
> > The only problem I see is that I don't have disk space to make a
> > snapshot and after it to dd the file to the usb disk, I never used this
> > snapshot feature, I will see if I can redirect the file as a pipe
> > directly to the usb drive.
>
> You could always reduce your FS size, to make space for the
> snapshots,.... depending on your pool size/etc....

I think the point to point out is that making an LVM snapshot does not involve
copying the whole partition. You need a limited amount of free space in the VG
- enough to hold the changes to the LV during the life time of the snapshot.
LVM presents to you the (unchanging) snapshot on the one hand and the normal
(read-write) LV on the other hand, both as block devices. You do *not* get a
file containing an image of your LV (well, you can create that yourself, but
there is no reason to do that). The difficult part is to figure out beforehand
*how much* space you will need for the snapshot. That depends on how long you
need the snapshot and how much write activity on your LV will occur during this
time. If you can do all of it outside of your backup window, you will need
virtually no space for the snapshot, but then again, you wouldn't really need
a snapshot in this case :).

That said, you *will* need *some* space in your VG, so if it's all allocated
right now, you will need to reduce the size of your root or backup volumes.
On the bright side, that would mean you have some spare space on your USB disk
for a live root partition.

Regards,
Holger

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