Raid 5 (was: Mounting of Linux volumes)

Michael Torrie torriem at chem.byu.edu
Fri Dec 2 11:12:03 MST 2005


On Fri, 2005-12-02 at 10:58 -0700, Ross Werner wrote:
> I think we've established that mail files are ok because if you rsync 
> while it's being written to, you'll simply get a backup of the old version 
> of the file (not an empty or deleted file). But are there other programs 
> where you *will* get data loss in this sort of scenario?

SQL will have problems for sure.
Apache will be fine
OpenLDAP may have problems, so you'd want to restore from ldif
sendmail should be fine, not sure about any other MTA
Squid would be fine, but I'd delete the entire cache before bring it
online anyway

I'm sure there are other examples.  Treat the rsync image is if it was a
dirty shutdown (which is was really).  you may have to scrub
various /var/run lockfiles.

Rsyncing and RAID10 (or RAID1) do serve different purposes.  I'm going
for a type of backup rather than redundancy, although rsync can provide
you with a certain amount of redundancy.  RAID is not backup.  If you
hose disk a with random data, it's mirror is hosed too.  So I'm not
suggesting rsync over RAID1 if what you really want and need is RAID1.
As others have pointed out a complete FS snapshot is really better (at
least as far as possible data corruption goes).  So ideally I suppose
you should snapshot your file system, then rsync the snapshot for backup
purposes.  That would even probably work for SQL, since we hope SQL
commits are atomic.

having a mirror system and automatic failover is another story all
together. (very cool stuff)

Michael



> 
>  	~ Ross
> 
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Michael Torrie <torriem at chem.byu.edu>




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