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Mike Marley
2004-12-07, 17:43
Oh, and one more thing. If for some reason the hardware raid device
fails, you are screwed, right? It's my understanding that once a raid
breaks all bets are off. Right?


Mike M.

JJ
2004-12-07, 18:21
Not necessarily. Some RAID controllers and software are better than
others in that the configuration can be transferred to a new controller if
the controller itself fails. Some (expensive) systems can use redundant
controllers. Some have the configuration embedded somehow on the disks
themselves to protect against controller failure.

Keep in mind, though, that RAID is NOT a backup solution. It protects
only against the failure of disk drives. Period. While that assurance is
nice for critical systems must run 24x7, it's probably a secondary concern
for home music servers where 99.99%+ of the files seldom if ever change.
Still, RAID 5 has advantages for a large music server. You can create
very large disk volumes easily. Those volumes aren't susceptible to the
(all to common) loss of a hard drive. As a backup strategy, though, RAID
doesn't cover you in many other real-world scenarios. Do an 'oops' and
accidentally delete a file (or an entire directory tree) and you're
screwed.

One solution for backup is to hard disk. For instance, you could
periodically copy the files to a seperatet disk based backup system. Or
use a more sophisticated grandfather-father-son incremental backup. This
works fine when the amount of data is relatively small. An example might
be a small business with maybe 25 GBytes of critical data. Backing up to
a 250 GBytes hard disk gives the ability to store at least 10 generations
of backups, more if you do incremental backups. But as the amount of data
increases, hard disk backup may become prohibitively expensive. At a
minimum you need an equal amount of hard disk space as your data. And
that gives you just one generation of backup. If you've spent thousands
of dollars for a multi-terrabyte array to store your music, then you can
expect to spend almost as much or more for a hard disk backup system.

Tape backup is economical in terms of media, but for the amounts of
storage we're talking about, the cost of drives and robotic tape changing
systems are really only feasible for commercial installations.

My view is that DVD-R is currently the best approach for backup of large
music collections. The thing that really makes this a no-brainer is that
our music files very seldom change. So you back up your perfect rips to
DVD-R _once_ and then store them somewhere safe. If you're not a file
trader, and you have rips from disks that you own, then you have the
originals on hand anyway, so what you're really backing up is the time and
the _work_ that you put into ripping, tagging and organizing your
collection.


----- Original Message -----
From: "Mike Marley" <mmarley (AT) accupoll (DOT) com>
To: <discuss (AT) lists (DOT) slimdevices.com>
Sent: Tuesday, December 07, 2004 5:43 PM
Subject: [slim] LaCie Ethernet disks - how noisy are they


> Oh, and one more thing. If for some reason the hardware raid device
> fails, you are screwed, right? It's my understanding that once a raid
> breaks all bets are off. Right?

Jack Coates
2004-12-07, 18:24
Mike Marley wrote:
> Oh, and one more thing. If for some reason the hardware raid device
> fails, you are screwed, right? It's my understanding that once a raid
> breaks all bets are off. Right?
>
>
> Mike M.

If the RAID card dies, you're SOL unless you can purchase another of
exactly the same type. This doesn't just mean the same brand, it means
the same brand, chipset, and in some cases firmware revision.

Far better to do software RAID 5 and let the OS rebuild if anything goes
wrong, IMHO.

Here's a quick refresher (N is the number of physical disks):

RAID 0 is striping, data goes across all disks with no parity. Speed
yes, safety no. Usable capacity is N.

RAID 1 is mirroring, the same data goes onto both disks. Speed is a wash
(reads are faster, writes are slower), but it's fairly safe. Limited to
two disks. Usable capacity is N/2.

RAID 2,3,4 are unused today, as they stank.

RAID 5 is striped-data-with-striped-parity, data and parity go across
all disks. Speed and safety are improved, and you can use as many disks
as you want (minimum of 3). Usable capacity is N-(1/N). You can afford
to lose one disk, and you better replace it and rebuild right smart.
Lose two disks and you're SOL.

With Linux software RAID or some really high-end hardware, you can do
multi-level RAID (point being increased performance or
mine-is-bigger-than-yours):

RAID 10 (AKA RAID 0+1) is striping across sets of mirrored pairs.
Requires at least 4 disks.

RAID 50 (AKA RAID 0+5) is striped-data-with-striped-parity across sets
of mirrored pairs. Requires at least six disks.

If you lose a disk controller, they're all hosed. If you get hacked or
accidentally delete everything, you're hosed. RAID is not a substitute
for backup to offsite or non-volatile media.

--
Jack at Monkeynoodle dot Org: It's a Scientific Venture...
Riding the Emergency Third Rail Power Trip since 1996!

Jack Coates
2004-12-07, 19:36
JJ wrote:
> Not necessarily. Some RAID controllers and software are better than
> others in that the configuration can be transferred to a new controller
> if the controller itself fails. Some (expensive) systems can use
> redundant controllers. Some have the configuration embedded somehow on
> the disks themselves to protect against controller failure.
>
> Keep in mind, though, that RAID is NOT a backup solution. It protects
> only against the failure of disk drives. Period. While that assurance
> is nice for critical systems must run 24x7, it's probably a secondary
> concern for home music servers where 99.99%+ of the files seldom if ever
> change. Still, RAID 5 has advantages for a large music server. You can
> create very large disk volumes easily. Those volumes aren't susceptible
> to the (all to common) loss of a hard drive. As a backup strategy,
> though, RAID doesn't cover you in many other real-world scenarios. Do
> an 'oops' and accidentally delete a file (or an entire directory tree)
> and you're screwed.
>

This is what I've been using to guard against the accidental backup
scenario... works well: http://www.monkeynoodle.org/comp/tools/backups.
Rsync for Windows can be found here:
http://www.itefix.no/phpws/index.php?module=pagemaster&PAGE_user_op=view_page&PAGE_id=6&MMN_position=23:23

--
Jack at Monkeynoodle dot Org: It's a Scientific Venture...
Riding the Emergency Third Rail Power Trip since 1996!

Daryle A. Tilroe
2004-12-08, 13:41
Jack Coates wrote:

> RAID 10 (AKA RAID 0+1) is striping across sets of mirrored pairs.
> Requires at least 4 disks.
>
> RAID 50 (AKA RAID 0+5) is striped-data-with-striped-parity across sets
> of mirrored pairs. Requires at least six disks.

10 and 01 are not the same. It makes a significant difference whether
you mirror first then stripe or stripe first then mirror. 10 is preferred
to 01. With 01 the loss of one drive essentially takes out at least one
more depending on how many drives are in the stripe group. However with
10 the loss of any disk keeps the stripe intact and theoretically you could
still loose one more drive (provided it is not the other half of the
degraded mirror) and have a functional system. Similar differences
apply to 50/05.

Here is a good link: http://www.pcguide.com/ref/hdd/perf/raid/levels/mult.htm

--
Daryle A. Tilroe

Jack Coates
2004-12-08, 22:14
> Jack Coates wrote:
>
>> RAID 10 (AKA RAID 0+1) is striping across sets of mirrored pairs.
>> Requires at least 4 disks.
>>
>> RAID 50 (AKA RAID 0+5) is striped-data-with-striped-parity across sets
>> of mirrored pairs. Requires at least six disks.
>
> 10 and 01 are not the same. It makes a significant difference whether
> you mirror first then stripe or stripe first then mirror. 10 is preferred
> to 01. With 01 the loss of one drive essentially takes out at least one
> more depending on how many drives are in the stripe group. However with
> 10 the loss of any disk keeps the stripe intact and theoretically you
> could
> still loose one more drive (provided it is not the other half of the
> degraded mirror) and have a functional system. Similar differences
> apply to 50/05.
>
> Here is a good link:
> http://www.pcguide.com/ref/hdd/perf/raid/levels/mult.htm
>

Huh... I've always mentally classed those configs under "too stupid to
seriously consider", along with RAID 4, and assumed that it was a just a
different typographical convention.

--
Jack At Monkeynoodle.Org: It's A Scientific Venture...
"Believe what you're told; there'd be chaos if everyone thought for
themselves." -- Top Dog hotdog stand, Berkeley, CA