backups

[rex007can]

Is there a reason why I can't backup straight to tape using something like tapeware or backup-exec or something like that?

All the backup procedures I see involve LVM and snapshots and other extra steps. I have tapeware enterprise with the Linux client installed. Why can't I just run backups with those every night?

[florian]

well, you certainly can; as all Scalix data is stored in the Filesystem, you can just run your backup tool to backup the /var/opt/scalix directory tree.

Problem is consistency though. Everything is fine when you shut down Scalix using the omshut command before the backup. If you leave it running, files will change during the time it takes to make the backup, so some files will have an older state than others, your backup will be inconsistent and when you restore in the event of data loss, you don't really know where you are and might experience some message store corruption.,

This is where snapshots come in; a snapshot will be a frozen image of your volume at any point in time, so all the data in the Snapshot will be consistent. To even ensure that during the very short time (<1s) it takes to make the snapshot, no inconsistencies are generated, we recommend freezing Scalix disk activity using the omsuspend command.

Bottom ...
... offline backup: no snapshots needed.
... online backup: snapshot heavily recommended; if backing up without snapshot, data consistency cannot be guaranteed and message store might be corrupted after restore. Therefore, online backup without snapshot facility is not supported!

If you need online backup or not depends very much on the availability needs of your user community. The assumption is that most corporate/enterprise email operations are now 24x7, so the downtime involved with an offline backup is not acceptable for those organisations.

Hope this helps, cheers,
Florian.

[rex007can]

There's something I don't understand about snapshots. You say it takes a second to take it. But what exactly does it contain? How can a snapshot possibly contain all 20 gigs of data on my mail server in a second? And if it doesn't contain all my data, how could I use it to restore a lost/destroyed/failed mail server?

[florian]

well, snapshots used to be what you describe - full copies of all your data; actually in those days, you would use software RAID-1 (mirroring) to keep two copies of your data the same all the time, the just split off one mirror and do your backup and then, during ongoing operations, do the resync.

In the meantime, all implementations of snapshots are actually what we call virtual snapshots with copy-on-write behaviour, i.e. the snapshot just remembers the time when it was taken; from that moment on, whenevery any app writes to the original disk, the block being overwritten is backed up to the snapshot area. when you access the data through the snapshot, you get a mixture of blocks from the main storage and from the backed up blocks in the snapshot area. Therefore, the actual snapshot operation takes no time (as it only records the timestamp and prepares the snapshot area) and typically space required is app. 10% of the actual data space (the space actually depends on the amount of data in the master volume being changed during the existence of the snapshot, but 10% is quite typical).

Snapshots are available on the OS level (through LVM or EVMS) and also on many better hardware-raid disk arrays inside the storage.

They are a favourite backup strategy for many applications and online backup today.

Cheers,
Florian.

[rex007can]

I have the server all ready with a 404 lvm drive mounted and ready to accept data. I'm just not familiar with the process and I'm always reluctant to implement procedures I don't understand on my servers...

Thank's for the info. I'll do some more digging.

[rex007can]

I have the server all ready with a 404 lvm drive .

I ment 40 gig, not 404...

[florian]

i believe if you search for "snapshot" on this forum, you'll see some more exmaples and information.

also, generic information about LVM snapshots for online backup should be available on the net through google.

in worst case, you can always play it save and fall back to offline backup with scalix shut down; especially for a new system, this should suffice for a while.

cheers,
Florian.