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RAID level for personal archival?...

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Derrick Coetzee...

Posted: Fri Sep 18, 2009 6:41 am

Guest

Hi all,

I'm building a RAID solution for personal archiving (general large
things like raw photos, full disk backups, DVD rips, that sort of
thing). I picked up a 7-disk NAS tower that I'm filling with 2 TB
desktop-class SATA disks. Much of this is irreplacable data - RAID
failure would be very bad.

The question I have is: is RAID 5 sufficient, or is RAID 6 needed?
Because this is for personal archival, the system would only be turned
on occasionally (much longer per-disk mean time to failure), and in
the event of a disk failure, could be turned off until the disk is
replaced (it would never run in degraded mode). On the other hand,
rebuilding a 2 TB disk could take a while, and this interval could by
itself provide ample opportunity for a second disk failure
(particularly if my disks happened to come from a single batch).
Should I play it safe and go for RAID 6 here?

Thanks for your advice!

--
Derrick Coetzee

David Brown...

Posted: Fri Sep 18, 2009 12:19 pm

Guest

Derrick Coetzee <dcnews at (no spam) moonflare.com> wrote:

Quote:

A rebuild runs in degraded mode, and is the hardest task for the disks
since everything on all disks needs to be read again. It is therefore a
likely time for other disks to fail. This is particularly true if all
the disks are from the same batch (and they are obviously subject to
roughly the same environment).

What about running raid 10 instead? It's not as space efficient, but
it's arguably safer than raid 6 (raid 6 can survive two disk fails -
/if/ it can survive the rebuild afterwards. raid 10 can survive one
disk fail, followed by more failures as long as you don't hit both
drives in a pair. And rebuild is much faster and easier).

Cydrome Leader...

Posted: Fri Sep 18, 2009 3:43 pm

Guest

"Derrick Coetzee <dcnews at (no spam) moonflare.com>" <dcoetzee at (no spam) gmail.com> wrote:

Quote:

It would be bad to lose your raid, but you still need to back it up. Trust
me, a "NAS tower + desktop class sata disks" will fail at some point, and
you will lose all your stuff.

Quote:

The question I have is: is RAID 5 sufficient, or is RAID 6 needed?

5 would be sketchy for such large drives. 6 is better.

Quote:

Because this is for personal archival, the system would only be turned
on occasionally (much longer per-disk mean time to failure), and in
the event of a disk failure, could be turned off until the disk is
replaced (it would never run in degraded mode). On the other hand,

you don't know that it won't run in degraded mode unless you have some
magic way to stop the array upon any failure, which really don't do you
any good anyways.

Quote:

rebuilding a 2 TB disk could take a while, and this interval could by
itself provide ample opportunity for a second disk failure
(particularly if my disks happened to come from a single batch).
Should I play it safe and go for RAID 6 here?

yes, raid6 may be better. I'd suggest using a decent RAID controller if
you can too. Any controller built into a motherboard is garbage by the
way.

Again, you still need backups no matter what you do. you said it yourself-
the data is irreplacable. A RAID array can help reduce the chance of
HARDWARE data loss, but that's just one of many ways you can lose data in
the first place. It helps, but is never a complete answer by itself.

Bill Todd...

Posted: Sat Sep 19, 2009 5:15 am

Guest

Derrick Coetzee <dcnews at (no spam) moonflare.com> wrote:

Quote:

Data sitting in one location can never be considered really safe - even
if it's replicated in that one location. So if you're at all serious
about protecting against its loss, your first priority would be to
maintain copies in at least two locations.

Given that, given that you'll apparently never need to update any
archived data in the archive, and given that you don't appear to expect
to need to access the data often, why use RAID at all rather than just
dump the data to replicated (and then geographically separated)
archive-quality DVD or similar optical media, or even tape? The fact
that a single RAID array might be more convenient to use is irrelevant
if your real priority is protection against data loss.

If you choose to go the RAID route anyway you'll definitely need a RAID
controller (whether hardware or software) that can ride through
detection of a bad block when reconstructing data after a disk failure.
RAID-6 should normally be able to do this, but since the likelihood of
a second whole-disk failure during reconstruction is several orders of
magnitude lower than that of hitting a bad block on one of the surviving
disks a reasonably-designed RAID-5 controller should be able to ride
through bad blocks as well, though of course the resulting block will be
corrupt after reconstruction (especially with large files the likelihood
that this will affect more than one file is vanishingly small, so think
of it as single-file corruption rather than array failure).

Or you could choose to maintain an unreplicated off-site archive (on
disk, optical media, or tape) plus all the same data (or at least all
that you really care about) on a running system at your primary
location. This would keep all the data readily accessible while
maintaining a remote copy just in case - plus have the advantage that
neither site used a RAID implementation that might have difficulty
dealing with bad blocks during reconstruction.

Just food for thought,

- bill

Bill Todd...

Posted: Mon Sep 21, 2009 1:09 am

Guest

mrvelous1 wrote:

....

Quote:

By the time go through with all of these recommendations you will have
effectively built yourself a Data Center.

Not really - perhaps you got confused by the length of the explanations
to the point that you lost track of their substance.

And at what cost? Maybe

Quote:

you should consider augmenting a simpler cheaper solution with some of
these online services that offer a cloud model, where you backup your
data to their servers and they are contractually obligated to insure
your data safety. This gets you the ability to restore in case you rm
the hell out of it (brain cramp), restore in case of theft or damage
(beer spill), and pretty much insures that if you upgrade your
hardware, or are unable to get a replacement drive or RAID card, that
you have an offsite/offline solution to get the data back.

The cloud model is attractive in some regards (e.g., ease of use and,
when your backup needs are limited, cost), less so in others. You'll
probably want to back up to at least two different cloud vendors to
guard against the possibility that one will suddenly cease to exist
(especially when dealing with a low-cost - or even free, for small
volumes - cloud vendor, since this end of the market hasn't even begun
to shake itself out yet). And good luck finding a vendor (especially a
low-cost one) who will come anywhere near guaranteeing your data's
safety (another good reason to use two of them) - which is not to say
that some don't take very reasonable precautions to protect your data,
just that they're not stupid when it comes to 'contractual obligations'.

By the

Quote:

time you are finished with all the fault tolerances these folks have
kindly suggested, you may have very well paid for years of an offsite
backup already...

Unlikely, since the least expensive suggestion that satisfies all the
criteria mentioned above is simply to archive to at least one off-site,
off-line set of media (which you periodically verify are still good)
while retaining an on-site copy as well if you don't choose to replicate
the archived data to a second off-site location. This also gets around
the need to use RAID in any form in your archive, thus avoiding Ralph's
concerns about low-end RAID implementation quality.

- bill

Maxim S. Shatskih...

Posted: Mon Sep 21, 2009 9:47 am

Guest

Quote:

I'm building a RAID solution for personal archiving (general large
things like raw photos, full disk backups, DVD rips, that sort of
thing). I picked up a 7-disk NAS tower that I'm filling with 2 TB
desktop-class SATA disks. Much of this is irreplacable data - RAID
failure would be very bad.

Maybe tape drive is better?

--
Maxim S. Shatskih
Windows DDK MVP
maxim at (no spam) storagecraft.com
http://www.storagecraft.com

...

Posted: Mon Sep 21, 2009 10:17 am

Guest

On Thu, 17 Sep 2009 23:41:31 -0700 (PDT), "Derrick Coetzee
<dcnews at (no spam) moonflare.com>" <dcoetzee at (no spam) gmail.com> wrote:

Quote:

I wouldn't use desktop-class drives for anything I REALLY wanted to be
kept. For something like that I would use high-end server-class or
NAS-class drives. With automatic backup to a really good tape drive
system. And mirroring to a remote location would be good, too.

And, in any case you need a really good UPS/power conditioner.

And throw in some lightening protection.

<snip>

--
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