RAID-10 and 4 HDDs - how many HDDs can break without loosing data?

RAID-10 and 4 HDDs - how many HDDs can break without loosing data?

[Tomasz Chmielewski]

I want to set up RAID-10 on 4 400 GB drives, on not-so powerful machine 
(600 MHz ARM, Thecus n4100) - to have a total of 800 GB storage, with 
some protection against a drive failure.

As the machine is not very powerful, I don't want to use RAID-5 nor RAID-6.


Previously, I would use RAID-0 on top of two RAID-1s:

     ----RAID0----
     |           |
   RAID1       RAID1
  |    |       |    |
HDD1 HDD2   HDD3 HDD4


With RAID-0 on top of RAID-1, with 4 drives, I would achieve:
- 100% chance of recovery if any single, one disk fails,
- 50% chance of recovery if any two disk failure.


Now with RAID-10 in the Linux kernel, what are my chances of recovering 
from a single and two disk failure?

        RAID10
  ------------------
  |    |      |    |
HDD1 HDD2  HDD3 HDD4

Is it also 100% if one disk fails, and 50% if two disks fail?

Or perhaps, with 4 drives, RAID-10 can survive any 2 disks failure?


-- 
Tomasz Chmielewski
http://wpkg.org

RE: RAID-10 and 4 HDDs - how many HDDs can break without loosing data?

[Andrew Rechenberg Lists]

I'm not sure if the kernel raid10 module is the same as building a RAID0
md device out of multiple RAID1 devices.

We used to use the later to create a RAID10 device by striping Linux SW
RAID1 devices.  We had great luck creating a RAID10 array out of a large
number of disk spindles.

So in your /etc/raidtab (or /etc/mdadm.conf) your RAID0 /dev/md2 would
consist of striping across /dev/md0 and /dev/md1.  In your diagram below
your HDD1 and HDD2 would be mirrored to create either md0 or md1 and
HDD3 and HDD4 would then be the other md device.

I know how that works and it worked reliably for me so that's what I
would recommend doing.

BTW, you could only survive a 2 disk failure if they were the "correct"
2 disks.  If both HDD1 and HDD2 would fail then you would be out of
luck.  Again, I've never used the kernel raid10 module, and I haven't
been able to find any good (visual) information on the module to be able
to determine how the module would survive disk failures.  I would assume
it's the same as the striping of RAID1 devices that I described above,
but I don't like to assume ;)

Good luck,
Andy.

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Tomasz
Chmielewski
Sent: Thursday, August 10, 2006 4:48 AM
To: linux-raid@vger.kernel.org
Subject: RAID-10 and 4 HDDs - how many HDDs can break without loosing
data?

I want to set up RAID-10 on 4 400 GB drives, on not-so powerful machine
(600 MHz ARM, Thecus n4100) - to have a total of 800 GB storage, with
some protection against a drive failure.

As the machine is not very powerful, I don't want to use RAID-5 nor
RAID-6.


Previously, I would use RAID-0 on top of two RAID-1s:

     ----RAID0----
     |           |
   RAID1       RAID1
  |    |       |    |
HDD1 HDD2   HDD3 HDD4


With RAID-0 on top of RAID-1, with 4 drives, I would achieve:
- 100% chance of recovery if any single, one disk fails,
- 50% chance of recovery if any two disk failure.


Now with RAID-10 in the Linux kernel, what are my chances of recovering
from a single and two disk failure?

        RAID10
  ------------------
  |    |      |    |
HDD1 HDD2  HDD3 HDD4

Is it also 100% if one disk fails, and 50% if two disks fail?

Or perhaps, with 4 drives, RAID-10 can survive any 2 disks failure?


--
Tomasz Chmielewski
http://wpkg.org
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RAID-10 and 4 HDDs - how many HDDs can break without loosing data?

[Tomasz Chmielewski]

I want to set up RAID-10 on 4 400 GB drives, on a not-so powerful 
machine (600 MHz ARM, Thecus n4100) - to have a total of 800 GB storage, 
with some protection against a drive failure.

As the machine is not very powerful, I don't want to use RAID-5 nor RAID-6.


Previously, I would use RAID-0 on top of two RAID-1s:

     ----RAID0----
     |           |
   RAID1       RAID1
  |    |       |    |
HDD1 HDD2   HDD3 HDD4


With RAID-0 on top of RAID-1, with 4 drives, I would achieve:
- 100% chance of recovery if any single, one disk fails,
- 50% chance of recovery if any two disks fail.


Now with RAID-10 in the Linux kernel, what are my chances of recovering 
from a single and two disk failure?

        RAID10
  ------------------
  |    |      |    |
HDD1 HDD2  HDD3 HDD4

Is it also 100% if one disk fails, and 50% if two disks fail?

Or perhaps, with 4 drives, RAID-10 can survive any 2 disks failure?


-- 
Tomasz Chmielewski
http://wpkg.org

Re: RAID-10 and 4 HDDs - how many HDDs can break without loosing data?

[Luca Berra]

On Thu, Aug 10, 2006 at 10:49:37AM +0200, Tomasz Chmielewski wrote:
>With RAID-0 on top of RAID-1, with 4 drives, I would achieve:
>- 100% chance of recovery if any single, one disk fails,
>- 50% chance of recovery if any two disks fail.
>
>Now with RAID-10 in the Linux kernel, what are my chances of recovering 
>from a single and two disk failure?

>Is it also 100% if one disk fails, and 50% if two disks fail?

if the number of copies is 2 yes

>Or perhaps, with 4 drives, RAID-10 can survive any 2 disks failure?
nope

note that with raid10 the number of copies for each stripe does not need
to be a divisor of the number of drives.
e.g you can have 4 drives and 3 stripe copies.
in this case you are able to survive any 2 disk failure. but the usable
space is DRIVE_SIZE * 1.33

L.


-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
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