Hi,
I'm hoping I can start a brief thread discussing the potential risks involved
with backing up live mounted (RW) ext2/3/4 filesystems using dump(8). Here are
the reasons I ask this:
1. My understanding is that it is safest to dump unmounted filesystems to
ensure all buffers are flushed and all files on the device are consistent and
up-to-date. However...
2. Performing filesystem dumps can be a time consuming process and therefore
taking the extra time to boot to level 1 and mount / RO to access utilities
just adds additional work. Booting to read-only media with utilities is just
as time consuming.
3. If / is mounted RO, it is not possible to write records to /etc/dumpdates
as would occur with /sbin/dump -u. Obviously one can mount / RW to dump other
filesystems, but it still seems awkward and time consuming to have to drop to
level 1 anyway, which may be necessary to unmount and dump /home say.
4. Obviously dropping the runlevel to 1 or booting to RO media such as Fedora
Live also prevents anyone other than root from using the system.
Clearly dumping backups of live mounted RW filesystems will not guarantee that
file data written between dump passes are completely consistent, but I am
looking to better understand the risks. Clearly databases on filesystems being
dumped should be closed and unmounted due to the extra software-level buffering
that many databases perform, but if the mounted filesystems are generally idle,
are there any gotchas one can expect when restoring such backups. Also, my
filesystems are all ext4 on Fedora Core 12. Any additional protection that the
journaling and journal checksum features can provide in this regard?
Cheers,
-J
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