RAID5 for Linux

[bob at sinister.com (Bob Keyes)]

On Wed, 28 Apr 2004, Rich Braun wrote:

> Bob Keyes wrote:
> > At my new job, we have a NAS "Snap Appliance", which is pretty
> > horrible and I want to get rid of it. So I am going to push for
> > a Linux solution. I'd want RAID level 5, and Serial ATA drives,
> > and of course linux niceness. I was looking at the Adaptec
> > 2810SA, but it's got 8 ports..I though you needed 9 for RAID5
> > (one for each bit plus one parity) or am I misunderstanding
> > this? I'd also want hot-swappable. Anyone have any suggestions?
>
> I've read through some of the responses and have a couple things to add:
>
> - I think the Snap product *is* Linux-based.  What you're wanting, of course,
> is control over the O/S environment in your server.

True enough.

> - You don't say explain what features of the NAS box make it horrible in your
> situation, or what level of performance you're lokoing for.

No support for Group IDs in file permissions. No portability of symbolic
links between CIFS and NFS users. Extremely limited authentication
options, just local users or NTLM (not even NTLMv2, never mind kerberos)
so there's no encrypted authentication.

> - The stated requirements of serial ATA and hot-swap imply high-end hardware
> that will come at a hefty price.

Serial ATA is not a real neccessity. I just thought it would be higher
performance that SCSI without the cost.

>  I'm frugal, personally, and generally
> question whether high-priced features really fit the bill.  For example,
> hot-swap can be justified on a mail server at an ISP but not at most 9-to-5
> corporate sites where it's easy enough to shut the system off for 5 minutes at
> the end of the workday.

The SNAP appliance has hot-swap, so its replacement should, I was
thinking. But you may be right, if the cost is too high it may not be
worth it.

>  Serial ATA can be justified if you need 15,000rpm
> drives because the system is handling 1000 transactions per second in a
> banking app, but not if you're running a Clearcase server for a half-dozen VB
> programmers.
> - A $75 motherboard with a $85 CPU has enough horsepower and IDE ports to run
> software RAID1 or RAID5 on four drives in 98% of the environments that I've
> seen.

Now that the reason for the number of drives was explained to me, it makes
sense. I may be able to use software RAID on a low-end system after all.
But for some reason, SCSI doesn't have a lot of respect around this
office.

Yesterday, after I got home, my neighbor gave me a pile of stuff (he's
moving) without any knowledge of my RAID troubles. This included a nice
(but older) DPT raid controller. Hrm...well, I am not going to give it
away to my workplace ;)

> - You don't get any penalty for running RAID1 in software, and you can't get a
> performance boost running RAID1 in hardware, on a 2-drive system.  You would
> get a performance boost running hardware RAID5 vs. software RAID5, but the
> boost may not be measurable if your application is not I/O-bound.
>
> Advances in hardware performance have outraced application requirements in
> most situations for a few years now.  Just because a high-end product is
> available does not mean it's worth paying the premium.
>
> > For backup, I am thinking the Sony AIT-2 changer, LIB-D81/A2. Don't
> > know if it's compatible with Bacula though...
>
> Thumbs up to AIT-2.  I use a Cybernetics/Spectra changer for my AIT-2 drive at
> home.  The SCSI standard for changers is widespread:  any brand should work
> with any Linux or Windows software.  I use amanda for backups and am
> more-or-less happy with it, though I wish it were more popular and were
> getting more new features and bug-fixes.

That's good news, because I can't get any solid info out of Sony or the
vendor regarding compatibility. We'd have to pay $333 (15%) for restocking
if we're wrong, so I want to make sure that management understands the
risks we'd be taking.