Re: enlarging snapshot volume automatically?

On Tue, Apr 29, 2003 at 08:01:57PM +0900, Kwon SoonSon wrote:
> > Write a sh script that checks how much free space
> > there is every 5
> > minutes and extends if neccessary ?
> 
> Thanks very much for your quick reply.
> That sounds a very interesting idea but I think if
> many
> blocks are overwritten exceeding the remaining free
> space of a specific snapshot within 5 minutes,
> then the snapshot would be gone.

There are a couple of arguments that you can give to your script that
will allow the sys admin to control it as he likes:

i) Poll period, eg, if 5 minutes is too infrequent, set it to 1 minute, 30 
seconds etc.

ii) The threshold at which the script decides to extend the volume. eg 80% full.

> Isn't there any other solution that always guarantees
> a safer copy-on-write as long as there are unused
> blocks
> in the volume group?

The kernel knows nothing about extents or volume groups, so no.

> Is it impossible to run a
> user-level
> command(lvextend?) in kernel so that it can get more
> space for that snapshot volume as soon as it detects
> the space shortage?

We've been trying hard to get rid of complex behaviour like this from
the kernel, it belongs in userland, this is _policy_ not mechanism.

Why not just allocate more space to the snapshot to start with ?

- Joe