On 7/24/06, Dominic Coulombe <[EMAIL PROTECTED]> wrote:
in *theory* you can do live backup of machines with journaled filesystems (at least ext3) without any problem.
Stop dreaming. Not even in theory - at least not my theory.
Linux will repair filesystems from journal after recovering your backups.
Well sort of. It will recover the *directory* information and make sure that you don't end up with cross-linked files or inconsistent information about which blocks on disk are free or in-use. But if there was data still to be written to disk when you did your backup, it will just not be there (unless you enable journaling of file data as well which will slow you down seriously). And that's only the risks when you suddenly pull the plug. With a physical backup you sweeps over the disk copying track by track in a way that is not at all coordinated with how Linux writes out data to disk. You may end up with additional inconsistency because the journal and the directories it journals are not being backed up at the same time. With flashcopy you can at least assure that the backup is an atomic operation.
You need to be sure that there is no activity on the Linux machine, or you might end with corrupted data. As an example, you can stop your Oracle databases, backup, then restart the databases.
It's typically hard to predict whether activity will occur during your backup. The only reliable approach is to stop your applications, so in many cases it may be as attractive to just stop the entire system and not risk the other data either. Which is why we have said so all the time... I remember VM:Backup used a neat trick when doing a physical backup of a CMS disk. After the backup of that mini disk was done, it would check whether the disk was modified since the start of the backup. If so, it would do another backup and repeat this several times if needed to get a clean backup. Something like that might make sense for Linux as well. Rob ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
