Raid 5 (was: Mounting of Linux volumes)

Michael Torrie torriem at chem.byu.edu
Fri Dec 2 09:17:13 MST 2005


On Fri, 2005-12-02 at 08:52 -0700, Nicholas Leippe wrote:
> On Friday 02 December 2005 07:54 am, Ross Werner wrote:
> > I can understand databases, but any application that produces temporary
> > files ... well, won't that just mean you won't copy all the temporary
> > files with 100% accuracy? I mean, they're *temporary* ... in the event of
> > a complete data loss and restore from backups, won't restarting the
> > service or whatever application it is flush those temporary files, making
> > losing temporary files of little concern?
> 
> It depends on their lifespan.  Temporary doesn't necessarily imply not
> being used to store state between invocations.

Well if you need to switch to you backup disk, you are most likely going
to have to experience some down time anyway, so the server processes
will have to restart, thus the temporary files won't mean a whole lot.

A catastrophic disk failure will also likely bring the whole machine
down uncleanly anyway.  Most server apps can deal with an unclean
recovery.

> 
> > I'm just wondering if there's anything in a typical Linux server
> > installation that won't typically survive a 2 AM rsync.
> 
> What about mail files and their corresponding index files?

Not a problem.  They are locked on write-access anyway (fairly coarse
granularity too with mbox).  Of course if the file changes while it is
being backed up, you just get a snaphot of the file before the change
happened. On any system that allows multiple sources to read and write
same file (say the mail spool), access is always controlled by locking.
Otherwise the imap daemon would clobber the file if the MTA tried to
deliver a message at the same time someone was checking their e-mail.
Even in a worse-case scenerio, a slightly corrupted mbox (which can be
fixed with a text editor) can be dealt with.  I recommend converting to
Maildir, however, and you'll never have to deal with that type of
corruption.

The only things that can't be backed up live are databases, sql, bdb,
etc.  For example, I backup ldap by dumping an ldif file and backing
that up.  Mail spoools. home directories, the web server's config and
webroot, the name server, etc, all can be backed up live just fine.

I don't see any problems backing up a live system in the middle of the
night.

> 
-- 
Michael Torrie <torriem at chem.byu.edu>




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