[CentOS] Some basic LVM questions

Curt Mills hacker at fluke.com
Mon Nov 9 19:19:05 UTC 2009

On Mon, 9 Nov 2009, Monte Milanuk wrote:

> This is where I run up against a pre-conceived notion, which may or may not
> be correct.  I had been thinking in terms of putting the stored data under
> /srv on the larger drive(s), and if I were going to use RAID, probably
> mirror those drives so if one starts acting sickly, I would have a bit of a
> buffer in terms of being able to pull one drive, replace it, etc.  What
> you're describing almost sounds like just having two drives period, both
> 500GB (for example), with /boot, /, /var, /etc, /usr, i.e. everything on
> there along with the stored data, in a mirrored RAID 1 configuration.  For
> whatever reason, just using RAID 1 with 'only' two drives of the same size
> never occured to me... guess I thought it wasn't 'proper' or that you
> needed a separate non-RAID drive for some of the system stuff like /boot...
> but now that I think on it... I'm not sure why it *wouldn't* work...?

It works.  I have a software RAID mirrored configuration on two
750GB SATA drives running now to the left of me, doing backups to
those two hard drives for several other machines.  I'm using Amanda
currently but am thinking of switching to backuppc or duplicity soon
in order to retain backups for a longer period.  I'm also backing
the same systems up to tape.

The OS is mirrored on the same two drives.  I also have a bootable
OS DVD that I can use for rescue, and know how to use it.

Be sure you know how to patch up your system for booting from the
2nd drive in case the first drive fails, and how to
identify/replace/rebuild a new drive when one fails.

One think to think about when running RAID is automated notification
when a drive is about to go out, plus automated notification when
one has gone bye-bye.  RAID likes to just keep running, so unless
you've set up notifications you might lose a 2nd or a 3rd drive,
taking down your filesystem for good, before you'd notice it
otherwise.  This might seem obvious, but to some people it's not.

Also:  I've used RAID5 in a system and had two drives go out in the
same weekend (before I could replace the first failed drive), taking
down the system.  This can happen with mirrored or RAID 1 + 0 as
well, but it's less likely that two drives in the same mirror will
go out.  With RAID5, ANY two drives going out means you lose it all.
I'm not a big fan of RAID5 at the moment.  Yes, I had good backups!

Curt Mills, WE7U                             hacker at fluke dot com
Senior Methods Engineer/SysAdmin
   "Lotto:  A tax on people who are bad at math."     -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"

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