Xen users - Aug 2007 - using lvm snapshots for taking domU backups

Hallo!

I guess this is a lot discussed topic of how to use lvm snapshots to get
domU backups and i browsed lists archive, too. I still could be very
thankful if anyone could comment on this.

I consider setup where in dom0 are defined LVM volumes like this

/dev/volume/domU_1_os
/dev/volume/domU_1_mysql_data
/dev/volume/domU_1_postgresql_data

/dev/volume/domU_2_os
...

and domU''s run from these volumes. Now, if i just take lvm snapshots in
dom0 while domU is running i most probably get the resulting backups
which after some self-recovery in filesystem and database datafile level
run but this is dirty. So, what i consider doing is to

1. say to mysql and to postgresql respectively ''flush tables with read
lock;'' and ''select 
pg_start_backup(''backup'');'' to make data filesystems
settle down and be consistent
2. take in dom0 snapshots on data volumes
3. tell mysql and pgsql respectively ''unlock tables''; and
select
''pg_stop_backup()'';
4. copy data from snapshots for backup or use these filesystem snapshots
some other way
5. destroy snapshots

This should sound more or less like a plan but i have doubts if it
really works like this since i settle down filesystem access in domU and
take snapshots in dom0. It means, should i rather set up things so that
i can take snapshots too being in domU? Of course it means that there
should be defined LVM''s in domU too and so to say there are two layers
of LVM all together, one on top of the other which i''d rahter avoid
unless necessary.


Imre

PS Backup for os partition is taken less frequently and when domU is
taken down, for example.

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