[TriLUG] Re: RAID questions

[Chip Turner]

Tanner Lovelace <clubjuggler at gmail.com> writes:

> On Tue, 30 Nov 2004 13:18:00 -0500, Aaron S. Joyner <aaron at joyner.ws> wrote:
>> First sentence, untrue.  The proper way to state this would be that most
>> on-board IDE controllers are not capable of hot swapping drives.  And
>> since most boxen these days use the on-board controller, most internally
>> mounted IDE drives can not be swapped.  On the other hand, most on-board
>> RAID controllers, or almost all PCI RAID controllers, are capable of
>> swapping in and out drives.  Most can even disconnect one drive on a
>> channel, while leaving the other drive on the channel sufficiently
>> intact.  I have done this numerous times in the BSD world, with generic
>> PC hardware, so I know the hardware is capable of it.  It's been a while
>> since I've done it in the Linux world, so I can't describe the process
>> off the top of my head, but I imagine there are folks on the list who
>> can.  In the case of internal drives used for backups, you're often
>> going to need to add a controller -- make it a $20 IDE RAID controller,
>> and you can easily swap in and out the disks.  Even if they're in a
>> drive enclosure which is attached to said controller.  :)
>
> While we're talking about RAID what version of RAID would people 
> suggest?  A couple of weeks ago I had my 160GB /home drive
> die on me and I'm thinking of replacing it with a RAID setup.  The 
> idea here isn't to replace regular backups, but rather to make it
> so that I'm not completely screwed if a drive decides to die on me.
> Should I go with two large drives and mirror it or would it be
> better to get 3 slightly smaller drives and do something like RAID 5?
> What are the advantages/disadvantages of each setup?
>
> Also, should I consider getting a "$20 IDE RAID controller" or just
> use software RAID?

Software RAID probably is what you want (cheap IDE raid cards tend to
not be as good, but 3wave makes a good one, apparently).  Don't count
on hot swap, though, on SCSI, as there are race conditions that tend
to cause hangs on busy servers (and work fine on others).  If you need
hotswap, go with more expensive SCSI raid, otherwise cheap linux
software raid works fine.

However... I've been steering away from RAID for data integrity and
redundancy at home.  RAID doesn't protect you from an overzealous 'rm'
or a need to check a file's state on previous days.  Unless you can't
afford any downtime at all, it is worth considering using the separate
drive like a big tape -- in other words, for backups.  I personally
use rsync and it works fine.  You can even use rsync in a certain
incremental mode[*] that lets you use separate dirs for different
days, letting you go back in time.  The downside is you still might
lose a bit of data if a drive dies, but you protect yourself from
common mistakes (though remember, rm -Rf / will follow mounted
partitions, so unmount it after backups are complete!).

[*] - http://www.mikerubel.org/computers/rsync_snapshots/ as well as
the --backup-dir option to rsync itself

Chip

-- 
Chip Turner                   cturner at pattern.net