Beginner's question on RAID 1+0

What is RAID 1+0? A mirror of 2 stripe sets? Or a a stripe assembled
from lots of 2-disk mirrors?

I've heard that one of those ways is bad, and thus the widely used RAID 1+0
uses the second way. Why the first way is bad?

--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation

http://www.storagecraft.com

Maxim S. Shatskih wrote:
What is RAID 1+0? A mirror of 2 stripe sets? Or a a stripe assembled
from lots of 2-disk mirrors?

I've heard that one of those ways is bad, and thus the widely used RAID 1+0
uses the second way. Why the first way is bad?

If you take two RAID-0 stripe sets and make them mirror each other, the
loss of a single disk in each set results in loss of data (and if the
sets get large enough, the probability of that occurring is not all that
low). When you instead mirror disk pairs and string the pairs together
in a RAID-0 configuration, you only lose data if both disks in a single
pair fail (the probability of this being far lower than that of the
first case with any significant number of pairs - in fact, the ratio of
data-loss probabilities may be about equal to the number of pairs, 2
pairs giving you about half the probability of data loss, 3 pairs giving
you about 1/3, etc.).

- bill

On Wed, 26 Jan 2005 02:50:20 +0300, "Maxim S. Shatskih"
wrote:

What is RAID 1+0? A mirror of 2 stripe sets? Or a a stripe assembled
from lots of 2-disk mirrors?

I've heard that one of those ways is bad, and thus the widely used RAID 1+0
uses the second way. Why the first way is bad?

0+1 isn;t "bad" per se, it's just been ousted by a newer and, imo,
better option 1+0.

You are correct in your understanding of the layout. it's the second
one you presented. The reason 0+1 is now considered bad is because
the loss of a single drive causes the entire mirror'd plex to fall
off, leaving you with a raid 0 plex of X drives. At the time this was
the top of the heap for performance and protection. But enter
1+0.....

This scenario allows you to lose up to half the drives in the raid set
as long as 2 of them are not the same mirror'd plex. Illustration is
generally better.


stripe
1-1 mirror 1
2-2 mirror 2
3-3 mirror 3

The mirror's are the base plex's, then those plex's are striped. This
scenario allows you to lose the X drives and still keep running.
Basically half as long as they are the "right" half.


stripe
X-1 mirror 1
2-X mirror 2
X-3 mirror 3

Most array vendors and LVM software will do 1+0 now but that wasn't
always true. So 0+1 was your best bet, particularly if you were using
software.

Hope this helps.

~F

If you take two RAID-0 stripe sets and make them mirror each other, the

Thanks!

Is there any performance issues in this choice?

--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation

http://www.storagecraft.com

Maxim S. Shatskih wrote:
If you take two RAID-0 stripe sets and make them mirror each other, the


Thanks!

Is there any performance issues in this choice?

Yes, and Faeandar sort of alluded to them.

If you make two conventional RAID-0 stripes and then mirror them, then
if you lose a disk in one of them that entire stripe may become
unavailable (the software *could* be suave enough to use the remaining
disks in the sick RAID-0 set for read requests that didn't require data
on the failed member, but I wouldn't bet on it). This means that the
disks in the remaining RAID-0 set are now shouldering the entire
workload if just one disk in the other set fails.

With mirrored disk pairs, the only performance impact when a single disk
fails is that all read requests which would normally have been split
between it and its partner now fall on the partner alone. But since
that's only 1/Nth of the total request load (for N disk pairs), you've
more or less still got 2N - 1 disks sharing the total load rather than
just N (though the partner of the failed disk can in some cases still
become a hot spot and drag down overall throughput more than that -
e.g., if the data is striped so finely over the disk pairs that most
read requests hit most of the pairs rather than having each read
normally target only one or two pairs).

- bill

RAID 1+0 is a mirroring of 2 stripes sets without a parity disk - this
provides awesome performance with almost no redundancy. It is also expensive
as you require 50% of your disk space to be mirrored - yikes!
"Maxim S. Shatskih" wrote in message
...
What is RAID 1+0? A mirror of 2 stripe sets? Or a a stripe
assembled
from lots of 2-disk mirrors?

I've heard that one of those ways is bad, and thus the widely used
RAID 1+0
uses the second way. Why the first way is bad?

--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation

http://www.storagecraft.com

Monte Oates wrote:
RAID 1+0 is a mirroring of 2 stripes sets without a parity disk - this
provides awesome performance with almost no redundancy. It is also expensive
as you require 50% of your disk space to be mirrored - yikes!

That doesn't make _any_ sense at all.
RAID 1 (with or without +0) provides _full_ redundancy.

Google is your friend...

Scott

There's a huge difference in reliability between striping mirrored sets vs
mirroring striped sets.

You ALWAYS want to stripe mirrored sets. What this gives you in
reliability is the best of all worlds - way better than standard RAID-5 in
fact. The reason is that the probability of losing both drives in a
two-disk mirror set in the same time window is very small. However, if you
stripe mirror sets, losing a single drive loses and entire 1/2 of the whole
mirror set, and as the number of drives in the entire configuration grows,
the probability of losing two drives at the same time grows.

Does this short explanation help at all?

On 22 Feb 2005 08:53:09 GMT, Scott Howard wrote:

Monte Oates wrote:
RAID 1+0 is a mirroring of 2 stripes sets without a parity disk - this
provides awesome performance with almost no redundancy. It is also expensive
as you require 50% of your disk space to be mirrored - yikes!

That doesn't make _any_ sense at all.
RAID 1 (with or without +0) provides _full_ redundancy.

Google is your friend...

Scott

--- jls
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I do not speak in any authorized capacity for anyone,
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