We have been using ADSM and TSM to back up a number of OpenMail servers
running under HP-UX. OpenMail stores messages in a database implemented as a
large collection of small Unix files. A sound backup requires a
self-consistent snapshot of the entire collection of files. We have been able
to stop OpenMail briefly, obtain snapshots of the file systems containing the
message store, and start OpenMail again. OpenMail could then run while *SM was
backing up the snapshots.

We have used two methods of obtaining the file system snapshots. Some of the
file systems are on mirrored disks, with the mirroring implemented by the
Logical Volume Manager. In these cases we use the lvsplit command to sever the
connection between the two disks, mount the former mirror, and have *SM back
it up. When the backup was done, we unmounted the former mirror and used the
lvmerge command to return it to its normal role as a mirror. More recently
some of the message store file systems have used RAID-5 implemented within the
disk subsystem controller. In these cases we mount a relatively small logical
volume using the snapof option to make it a read-only snapshot of the message
store file system. The space in the snapshot logical volume is used to record
the old versions of blocks changed by writes to the message store. A read from
the snapshot will obtain data from the message store logical volume if the
disk block involved has not changed since the snapshot was created. Otherwise
the read will obtain the saved version of the block from the snapshot logical
volume.

We are now considering running an OpenMail server under Linux. Does Linux have
anything analogous to either of the mechanisms described in the previous
paragraph?