Software RAID 0+1 with mdadm.

Software RAID 0+1 with mdadm.

[Brad Dameron]

Been trying for days to get a software RAID 0+1 setup. This is on SuSe
9.2 with kernel 2.6.8-24.11-smp x86_64.

I am trying to setup a RAID 0+1 with 4 250gb SATA drives. I do the
following:

mdadm --create /dev/md1 --level=0 --chunk=4 --raid-devices=2 /dev/sdb1
/dev/sdc1
mdadm --create /dev/md2 --level=0 --chunk=4 --raid-devices=2 /dev/sdd1
/dev/sde1
mdadm --create /dev/md0 --level=1 --chunk=4 --raid-devices=2 /dev/md1
/dev/md2

This all works fine and I can mkreiserfs /dev/md0 and mount it. If I am
then to reboot /dev/md1 and /dev/md2 will show up in the /proc/mdstat
but not /dev/md0. So I create a /etc/mdadm.conf like so to see if this
will work:

DEVICE partitions
DEVICE /dev/md*
ARRAY /dev/md2 level=raid0 num-devices=2
UUID=5e6efe7d:6f5de80b:82ef7843:148cd518
   devices=/dev/sdd1,/dev/sde1
ARRAY /dev/md1 level=raid0 num-devices=2
UUID=e81e74f9:1cf84f87:7747c1c9:b3f08a81
   devices=/dev/sdb1,/dev/sdc1
ARRAY /dev/md0 level=raid1 num-devices=2  devices=/dev/md2,/dev/md1


Everything seems ok after boot. But again no /dev/md0 in /proc/mdstat.
But then if I do a mdadm --assemble --scan it will then load /dev/md0. 

Also do I need to create partitions? Or can I setup the whole drives as
the array?

I have since upgraded to mdadm 1.8 and setup a RAID10. However I need
something that is production worthy. Is a RAID10 something I could rely
on as well? Also under a RAID10 how do you tell it which drives you want
mirrored?

Any help appreciated. 

Thank you,
Brad Dameron
SeaTab Software
www.seatab.com

RE: Software RAID 0+1 with mdadm.

[Guy]

For a more stable array, build a RAID0 out of 2 RAID1 arrays.

Like this:

mdadm --create /dev/md1 --level=1 --chunk=4 --raid-devices=2 /dev/sdb1
/dev/sdc1
mdadm --create /dev/md2 --level=1 --chunk=4 --raid-devices=2 /dev/sdd1
/dev/sde1
mdadm --create /dev/md0 --level=0 --chunk=4 --raid-devices=2 /dev/md1
/dev/md2

You can put a file system directly on /dev/md0

Are all of the disks on the same cable?

Not sure about your booting issue.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Brad Dameron
Sent: Tuesday, January 25, 2005 5:28 PM
To: linux-raid@vger.kernel.org
Subject: Software RAID 0+1 with mdadm.

Been trying for days to get a software RAID 0+1 setup. This is on SuSe
9.2 with kernel 2.6.8-24.11-smp x86_64.

I am trying to setup a RAID 0+1 with 4 250gb SATA drives. I do the
following:

mdadm --create /dev/md1 --level=0 --chunk=4 --raid-devices=2 /dev/sdb1
/dev/sdc1
mdadm --create /dev/md2 --level=0 --chunk=4 --raid-devices=2 /dev/sdd1
/dev/sde1
mdadm --create /dev/md0 --level=1 --chunk=4 --raid-devices=2 /dev/md1
/dev/md2

This all works fine and I can mkreiserfs /dev/md0 and mount it. If I am
then to reboot /dev/md1 and /dev/md2 will show up in the /proc/mdstat
but not /dev/md0. So I create a /etc/mdadm.conf like so to see if this
will work:

DEVICE partitions
DEVICE /dev/md*
ARRAY /dev/md2 level=raid0 num-devices=2
UUID=5e6efe7d:6f5de80b:82ef7843:148cd518
   devices=/dev/sdd1,/dev/sde1
ARRAY /dev/md1 level=raid0 num-devices=2
UUID=e81e74f9:1cf84f87:7747c1c9:b3f08a81
   devices=/dev/sdb1,/dev/sdc1
ARRAY /dev/md0 level=raid1 num-devices=2  devices=/dev/md2,/dev/md1


Everything seems ok after boot. But again no /dev/md0 in /proc/mdstat.
But then if I do a mdadm --assemble --scan it will then load /dev/md0. 

Also do I need to create partitions? Or can I setup the whole drives as
the array?

I have since upgraded to mdadm 1.8 and setup a RAID10. However I need
something that is production worthy. Is a RAID10 something I could rely
on as well? Also under a RAID10 how do you tell it which drives you want
mirrored?

Any help appreciated. 

Thank you,
Brad Dameron
SeaTab Software
www.seatab.com

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Re: Software RAID 0+1 with mdadm.

[Luca Berra]

On Tue, Jan 25, 2005 at 02:28:21PM -0800, Brad Dameron wrote:
>Everything seems ok after boot. But again no /dev/md0 in /proc/mdstat.
>But then if I do a mdadm --assemble --scan it will then load /dev/md0. 
there is a bug in mdadm, see my mail "patches for mdadm 1.8.0" or wait
for 1.9.0

L.

-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
 /"\
 \ /     ASCII RIBBON CAMPAIGN
  X        AGAINST HTML MAIL
 / \

RE: Software RAID 0+1 with mdadm.

[Brad Dameron]

On Tue, 2005-01-25 at 15:04, Guy wrote:
> For a more stable array, build a RAID0 out of 2 RAID1 arrays.
> 
> Like this:
> 
> mdadm --create /dev/md1 --level=1 --chunk=4 --raid-devices=2 /dev/sdb1
> /dev/sdc1
> mdadm --create /dev/md2 --level=1 --chunk=4 --raid-devices=2 /dev/sdd1
> /dev/sde1
> mdadm --create /dev/md0 --level=0 --chunk=4 --raid-devices=2 /dev/md1
> /dev/md2
> 
> You can put a file system directly on /dev/md0
> 
> Are all of the disks on the same cable?
> 
> Not sure about your booting issue.
> 
> Guy
> 


Ya I did this setup as well. Still the same booting issue. Once it's
booted I can run mdadm --assemble --scan and it will find just the
stripe and then add it. I saw several people having this issue on a
google search. But never any solutions.

Brad Dameron
SeaTab Software
www.seatab.com

RE: Software RAID 0+1 with mdadm.

[J. Ryan Earl]

This bug that's fixed in 1.9.0, is in a bug when you create the array?  ie
do we need to use 1.9.0 to create the array.  I'm looking to do the same but
my bootdisk currently only has 1.7.soemthing on it.  Do I need to make a
custom bootcd with 1.9.0 on it?

Thanks,
-ryan

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org]On Behalf Of Luca Berra
Sent: Wednesday, January 26, 2005 9:17 AM
To: linux-raid@vger.kernel.org
Subject: Re: Software RAID 0+1 with mdadm.


On Tue, Jan 25, 2005 at 02:28:21PM -0800, Brad Dameron wrote:
>Everything seems ok after boot. But again no /dev/md0 in /proc/mdstat.
>But then if I do a mdadm --assemble --scan it will then load /dev/md0.
there is a bug in mdadm, see my mail "patches for mdadm 1.8.0" or wait
for 1.9.0

L.

--
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
 /"\
 \ /     ASCII RIBBON CAMPAIGN
  X        AGAINST HTML MAIL
 / \
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RE: Software RAID 0+1 with mdadm.

[Guy]

Sorry, I did not intend this to be the solution to your problem.  Just a
much more stable method for creating the 1+0 array.  With this method,
losing 1 disk only requires re-syncing 1 disk.  With the array as a 0+1, if
you lose 1 disk, you lose the whole RAID0 array, which then requires
re-syncing 2 disks of data.

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Brad Dameron
Sent: Wednesday, January 26, 2005 3:33 PM
To: linux-raid@vger.kernel.org
Subject: RE: Software RAID 0+1 with mdadm.

On Tue, 2005-01-25 at 15:04, Guy wrote:
> For a more stable array, build a RAID0 out of 2 RAID1 arrays.
> 
> Like this:
> 
> mdadm --create /dev/md1 --level=1 --chunk=4 --raid-devices=2 /dev/sdb1
> /dev/sdc1
> mdadm --create /dev/md2 --level=1 --chunk=4 --raid-devices=2 /dev/sdd1
> /dev/sde1
> mdadm --create /dev/md0 --level=0 --chunk=4 --raid-devices=2 /dev/md1
> /dev/md2
> 
> You can put a file system directly on /dev/md0
> 
> Are all of the disks on the same cable?
> 
> Not sure about your booting issue.
> 
> Guy
> 


Ya I did this setup as well. Still the same booting issue. Once it's
booted I can run mdadm --assemble --scan and it will find just the
stripe and then add it. I saw several people having this issue on a
google search. But never any solutions.

Brad Dameron
SeaTab Software
www.seatab.com



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RE: Software RAID 0+1 with mdadm.

[Neil Brown]

On Wednesday January 26, ryan@dynaconnections.com wrote:
> This bug that's fixed in 1.9.0, is in a bug when you create the array?  ie
> do we need to use 1.9.0 to create the array.  I'm looking to do the same but
> my bootdisk currently only has 1.7.soemthing on it.  Do I need to make a
> custom bootcd with 1.9.0 on it?

This issue that will be fixed in 1.9.0 has nothing to do with creating
the array.

It is only relevant for stacked arrays (e.g. a raid0 made out of 2 or
more raid1 arrays), and only if you are using
   mdadm --assemble --scan
(or similar) to assemble your arrays, and you specify the devices to
scan in mdadm.conf as
   DEVICES partitions
(i.e. don't list actual devices, just say to get them from the list of
known partitions).

So, no: no need for a custom bootcd.

NeilBrown

Re: Software RAID 0+1 with mdadm.

[Neil Brown]

On Tuesday January 25, brad@seatab.com wrote:
> Been trying for days to get a software RAID 0+1 setup. This is on SuSe
> 9.2 with kernel 2.6.8-24.11-smp x86_64.
> 
> I am trying to setup a RAID 0+1 with 4 250gb SATA drives. I do the
> following:
> 
> mdadm --create /dev/md1 --level=0 --chunk=4 --raid-devices=2 /dev/sdb1
> /dev/sdc1
> mdadm --create /dev/md2 --level=0 --chunk=4 --raid-devices=2 /dev/sdd1
> /dev/sde1
> mdadm --create /dev/md0 --level=1 --chunk=4 --raid-devices=2 /dev/md1
> /dev/md2
> 
> This all works fine and I can mkreiserfs /dev/md0 and mount it. If I am
> then to reboot /dev/md1 and /dev/md2 will show up in the /proc/mdstat
> but not /dev/md0. So I create a /etc/mdadm.conf like so to see if this
> will work:
> 
> DEVICE partitions
> DEVICE /dev/md*
> ARRAY /dev/md2 level=raid0 num-devices=2
> UUID=5e6efe7d:6f5de80b:82ef7843:148cd518
>    devices=/dev/sdd1,/dev/sde1
> ARRAY /dev/md1 level=raid0 num-devices=2
> UUID=e81e74f9:1cf84f87:7747c1c9:b3f08a81
>    devices=/dev/sdb1,/dev/sdc1
> ARRAY /dev/md0 level=raid1 num-devices=2  devices=/dev/md2,/dev/md1
> 
> 
> Everything seems ok after boot. But again no /dev/md0 in /proc/mdstat.
> But then if I do a mdadm --assemble --scan it will then load
> /dev/md0. 

My guess is that you are (or SuSE is) relying on "autodetect" to
assemble the arrays.  Autodetect cannot assemble an array made of
other arrays.  Just an array made of partitions.

If you disable the autodetect stuff and make sure 
  mdadm --assemble --scan
is in a boot-script somewhere, it should just work.

Also, you don't really want the "device=/dev/sdd1..." entries in
mdadm.conf.
They tell mdadm to require the devices to have those names.  If you
add or remove scsi drives at all, the names can change.  Just rely on
the UUID.

> 
> Also do I need to create partitions? Or can I setup the whole drives as
> the array?

You don't need partitions.

> 
> I have since upgraded to mdadm 1.8 and setup a RAID10. However I need
> something that is production worthy. Is a RAID10 something I could rely
> on as well? Also under a RAID10 how do you tell it which drives you want
> mirrored?

raid10 is 2.6 only, but should be quite stable.
You cannot tell it which drives to mirror because you shouldn't care.
You just give it a bunch of identical drives and let it put the data
where it wants.

If you really want to care (and I cannot imagine why you would - all
drives in a raid10 are likely to get similar load) then you have to
build it "by hand" - a raid0 of multiple raid1s.

NeilBrown

Re: Software RAID 0+1 with mdadm.

[Holger Kiehl]

>>
>> I have since upgraded to mdadm 1.8 and setup a RAID10. However I need
>> something that is production worthy. Is a RAID10 something I could rely
>> on as well? Also under a RAID10 how do you tell it which drives you want
>> mirrored?
>
> raid10 is 2.6 only, but should be quite stable.
> You cannot tell it which drives to mirror because you shouldn't care.
> You just give it a bunch of identical drives and let it put the data
> where it wants.
>
> If you really want to care (and I cannot imagine why you would - all
> drives in a raid10 are likely to get similar load) then you have to
> build it "by hand" - a raid0 of multiple raid1s.
>
But what about redundancy? The only reason why on some systems I choose
raid 1+0 and not raid10, is that I want the raid 1 always on two different
controllers or channels. So if there is some problem with the SCSI bus
you will only loose one half of your array. I think with raid10 it
can be that you loose your complete array, because a whole raid1 can
sit on a single controller/channel. Is this assumption correct, or
does raid10 have some magic to solve this?

Thanks,
Holger

RE: Software RAID 0+1 with mdadm.

[Guy]

RAID10 will work with an odd number of disks!  If really is cool!

But, since you need control of what is where, you don't want RAID10.

You should create RAID1 arrays.  Each of your RAID1 arrays should have 2
disks, each on a different controller/channel.  Then create a single RAID0
array out of the RAID1 arrays.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Holger Kiehl
Sent: Thursday, January 27, 2005 3:13 AM
To: Neil Brown
Cc: linux-raid
Subject: Re: Software RAID 0+1 with mdadm.

>>
>> I have since upgraded to mdadm 1.8 and setup a RAID10. However I need
>> something that is production worthy. Is a RAID10 something I could rely
>> on as well? Also under a RAID10 how do you tell it which drives you want
>> mirrored?
>
> raid10 is 2.6 only, but should be quite stable.
> You cannot tell it which drives to mirror because you shouldn't care.
> You just give it a bunch of identical drives and let it put the data
> where it wants.
>
> If you really want to care (and I cannot imagine why you would - all
> drives in a raid10 are likely to get similar load) then you have to
> build it "by hand" - a raid0 of multiple raid1s.
>
But what about redundancy? The only reason why on some systems I choose
raid 1+0 and not raid10, is that I want the raid 1 always on two different
controllers or channels. So if there is some problem with the SCSI bus
you will only loose one half of your array. I think with raid10 it
can be that you loose your complete array, because a whole raid1 can
sit on a single controller/channel. Is this assumption correct, or
does raid10 have some magic to solve this?

Thanks,
Holger
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Re: Software RAID 0+1 with mdadm.

[Luca Berra]

On Thu, Jan 27, 2005 at 04:24:34PM +1100, Neil Brown wrote:
>On Wednesday January 26, ryan@dynaconnections.com wrote:
>> This bug that's fixed in 1.9.0, is in a bug when you create the array?  ie
>> do we need to use 1.9.0 to create the array.  I'm looking to do the same but
>> my bootdisk currently only has 1.7.soemthing on it.  Do I need to make a
>> custom bootcd with 1.9.0 on it?
>
>This issue that will be fixed in 1.9.0 has nothing to do with creating
>the array.
>
>It is only relevant for stacked arrays (e.g. a raid0 made out of 2 or
>more raid1 arrays), and only if you are using
>   mdadm --assemble --scan
>(or similar) to assemble your arrays, and you specify the devices to
>scan in mdadm.conf as
>   DEVICES partitions
>(i.e. don't list actual devices, just say to get them from the list of
>known partitions).
actually the last statement is not true, a missing close(mdfd); causes
mdadm --assemble --scan to fail the first round even if you do specify
DEV /dev/mdX in mdadm.conf.

L.

-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
 /"\
 \ /     ASCII RIBBON CAMPAIGN
  X        AGAINST HTML MAIL
 / \

RAID-10 with odd number of disks (was Re: Software RAID 0+1 with mdadm.)

[Andy Smith]

[-- Attachment #1: Type: text/plain, Size: 415 bytes --]

On Thu, Jan 27, 2005 at 10:50:43AM -0500, Guy wrote:
> RAID10 will work with an odd number of disks!  If really is cool!

It will?  How?  Does it just make the last mirror "pair" have 3
disks or what?

If so then wouldn't it be better just to not have that disk under
md and use it for someting else?

-- 
> > > The optimum programming team size is 1.
Has Jurassic Park taught us nothing?
 -- pfilandr

[-- Attachment #2: Type: application/pgp-signature, Size: 187 bytes --]

RE: Software RAID 0+1 with mdadm.

[Brad Dameron]

By the way the patches did not fix my boot issue with the RAID 1+0. I am
using SuSe 9.2 if this helps. I've had to add in a few extra lines into
my boot.local to get it show up at boot.

Brad Dameron
SeaTab Software
www.seatab.com

On Wed, 2005-01-26 at 14:14, J. Ryan Earl wrote:
> This bug that's fixed in 1.9.0, is in a bug when you create the array?  ie
> do we need to use 1.9.0 to create the array.  I'm looking to do the same but
> my bootdisk currently only has 1.7.soemthing on it.  Do I need to make a
> custom bootcd with 1.9.0 on it?
> 
> Thanks,
> -ryan
> 
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org]On Behalf Of Luca Berra
> Sent: Wednesday, January 26, 2005 9:17 AM
> To: linux-raid@vger.kernel.org
> Subject: Re: Software RAID 0+1 with mdadm.
> 
> 
> On Tue, Jan 25, 2005 at 02:28:21PM -0800, Brad Dameron wrote:
> >Everything seems ok after boot. But again no /dev/md0 in /proc/mdstat.
> >But then if I do a mdadm --assemble --scan it will then load /dev/md0.
> there is a bug in mdadm, see my mail "patches for mdadm 1.8.0" or wait
> for 1.9.0
> 
> L.
> 
> --
> Luca Berra -- bluca@comedia.it
>         Communication Media & Services S.r.l.
>  /"\
>  \ /     ASCII RIBBON CAMPAIGN
>   X        AGAINST HTML MAIL
>  / \
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> -
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> the body of a message to majordomo@vger.kernel.org
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RE: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with mdadm.)

[Guy]

It rotates the pairs!
Assume 3 disks, A, B and C.
Each stripe would be on these disks:
A+B
C+A
B+C
A+B
C+A
B+C
...

Maybe not exactly as above, but if not, something similar.

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Andy Smith
Sent: Thursday, January 27, 2005 11:19 AM
To: 'linux-raid'
Subject: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with
mdadm.)

On Thu, Jan 27, 2005 at 10:50:43AM -0500, Guy wrote:
> RAID10 will work with an odd number of disks!  If really is cool!

It will?  How?  Does it just make the last mirror "pair" have 3
disks or what?

If so then wouldn't it be better just to not have that disk under
md and use it for someting else?

-- 
> > > The optimum programming team size is 1.
Has Jurassic Park taught us nothing?
 -- pfilandr

Re: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with mdadm.)

[Andy Smith]

[-- Attachment #1: Type: text/plain, Size: 352 bytes --]

On Thu, Jan 27, 2005 at 12:16:31PM -0500, Guy wrote:
> It rotates the pairs!
> Assume 3 disks, A, B and C.
> Each stripe would be on these disks:
> A+B
> C+A
> B+C
> A+B
> C+A
> B+C
> ...

Hmm, difficult to visualise and comprehend if there are any
differences as opposed to "normal" RAID-10.

Is this anything like how RAID-1E works on IBM ServeRAID?

[-- Attachment #2: Type: application/pgp-signature, Size: 187 bytes --]

Re: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with mdadm.)

[Andy Smith]

[-- Attachment #1: Type: text/plain, Size: 625 bytes --]

On Thu, Jan 27, 2005 at 05:27:00PM +0000, Andy Smith wrote:
> On Thu, Jan 27, 2005 at 12:16:31PM -0500, Guy wrote:
> > It rotates the pairs!
> > Assume 3 disks, A, B and C.
> > Each stripe would be on these disks:
> > A+B
> > C+A
> > B+C
> > A+B
> > C+A
> > B+C
> > ...
> 
> Hmm, difficult to visualise and comprehend if there are any
> differences as opposed to "normal" RAID-10.
> 
> Is this anything like how RAID-1E works on IBM ServeRAID?

This seems relevant:

http://www.adaptec.com/worldwide/product/markeditorial.html?sess=no&language=English+US&prodkey=raid_10_alternatives&type=White%20Papers

[-- Attachment #2: Type: application/pgp-signature, Size: 187 bytes --]

RE: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with mdadm.)

[Guy]

A normal RAID 10 requires an even number of disks.

Neil gave a detailed description.  Maybe 6 months ago.  Do a search for
RAID10 or raid10.

I just looked at the link you gave.  RAID-1E does look like the RAID10 that
md supports.  Until today, I had never seen RAID-1E.

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Andy Smith
Sent: Thursday, January 27, 2005 12:42 PM
To: 'linux-raid'
Subject: Re: RAID-10 with odd number of disks (was Re: Software RAID 0+1
with mdadm.)

On Thu, Jan 27, 2005 at 05:27:00PM +0000, Andy Smith wrote:
> On Thu, Jan 27, 2005 at 12:16:31PM -0500, Guy wrote:
> > It rotates the pairs!
> > Assume 3 disks, A, B and C.
> > Each stripe would be on these disks:
> > A+B
> > C+A
> > B+C
> > A+B
> > C+A
> > B+C
> > ...
> 
> Hmm, difficult to visualise and comprehend if there are any
> differences as opposed to "normal" RAID-10.
> 
> Is this anything like how RAID-1E works on IBM ServeRAID?

This seems relevant:

http://www.adaptec.com/worldwide/product/markeditorial.html?sess=no&language
=English+US&prodkey=raid_10_alternatives&type=White%20Papers

Re: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with mdadm.)

[berk walker]

In the past, I have found the quite often, too often, the disk errors  
happened in the 1st sectors of the disk (and I still have to reboot  
often).  It does not look good when losing a whole disk, eh?
b-


On Thu, 27 Jan 2005 12:16:31 -0500, Guy <bugzilla@watkins-home.com> wrote:

> It rotates the pairs!
> Assume 3 disks, A, B and C.
> Each stripe would be on these disks:
> A+B
> C+A
> B+C
> A+B
> C+A
> B+C
> ...
>
> Maybe not exactly as above, but if not, something similar.
>
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Andy Smith
> Sent: Thursday, January 27, 2005 11:19 AM
> To: 'linux-raid'
> Subject: RAID-10 with odd number of disks (was Re: Software RAID 0+1 with
> mdadm.)
>
> On Thu, Jan 27, 2005 at 10:50:43AM -0500, Guy wrote:
>> RAID10 will work with an odd number of disks!  If really is cool!
>
> It will?  How?  Does it just make the last mirror "pair" have 3
> disks or what?
>
> If so then wouldn't it be better just to not have that disk under
> md and use it for someting else?
>



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Re: Software RAID 0+1 with mdadm.

[linux]

The only issue is that the md code only auto-scans non-MD partitions when
assembling a RAID array.  You can use mdadm at boot time to put them
together as other people have shown, or you can add an option to the kernel
command line (LILO append="..." option; other boot loaders have something
similar) to help it out.

Since I have my root partition on RAID1+0, I have "md=4,/dev/md2,/dev/md3"
on the kernel command line.  This gets /dev/md4 (which is a RAID-0
made of two RAID-1s) going.

Oh, regarding data placement: I also have the RAID-1 split across
controllers.  Also, I have half the mirror in removable drive trays
for emergency insta-backup (fire or other natural disaster) uses.

There are uses for such things.