Re: Filesystem snapshots

[John <john@commonpeople.uk>]

Thank you Graham and Florian,

I'd have thought my problem was commonplace. The machine is a server. It 
has to remain available continuously and can be updated at any moment. 
It handles, for example, SQL updates and email, I can't just switch it 
to read-only for the duration of the backup.

The LVM process allocates a bit of scratch disk elsewhere to be 
read-write, marks the whole of my normally-read-write partition as 
read-only, and intercepts all that partition's write activity and puts 
it onto the scratch space. Any subsequent reading from that partition 
takes account of those updates, but the partition mount from which I'm 
backing up remains read-only until I've finished - because all the 
writes have been diverted.

When I've finished, the partition goes back to read-write after the 
scratch area updates are merged back into it, so it ends up just how it 
would have been if I'd not gone near it. The effect is as though it had 
remained read-write all the time, but the backup was taken from a stable 
read-only condition.
When Graham writes "Shouldn't you be making the backup from a read-only 
copy of your filesystem", yes, definitely. That's why I don't understand 
how anyone can archive from a home directory on a desktop without, for 
instance, closing X and logging out the user. Nothing short of that is 
going to allow the home directory to be made read-only. Unless a 
snapshot is taken at some level, and the snapshot used for the backup.

I will, as both of you agree, move the cache elsewhere. I hadn't thought 
of that. Even so, I still need to keep the SQL and email-server process 
writeable throughout the backup.

John Harris