[linux-lvm] snapshot questions

[Andreas Dilger]

On Nov 02, 2001  13:41 -0800, Kyle Hayes wrote:
> I am confused by the claim (see below) that you need the same amount
> of space for the snapshot partition as you had on the original LV.

They are wrong.

> Under what circumstances do snapshots work?  I am confused by the
> claims that a snapshot partition is going to be "corrupt" as far as
> the OS is concerned.

You need to ensure that the the data/filesystem is "synced" at the
time that you create the snapshot.  With journaling filesystems,
you need to use the VFS locking.  With other filesystems, you
should be OK with a "sync" (which is done for you in the kernel).
For oracle, you need to put your tablespaces into hot backup mode.

> 1) lock all tables against writes.

Is this the same as hot backup?  At a company I used to work for, we
did live backups without any kind of outage.  Oracle would commit
the writes into a journal (or whatever, don't know the correct term)
and then commit them once the table was out of hot backup mode.  No
need to lock all writes.

Depending on your I/O and response requirements, you can do the backups
while your tables are in hot backup mode, without using snapshots at
all.  

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/