* striping and mirroring with only two disks
@ 2004-01-31 19:53 Keld Jørn Simonsen
2004-01-31 21:29 ` Guy
2004-01-31 21:36 ` Nathan Lewis
0 siblings, 2 replies; 6+ messages in thread
From: Keld Jørn Simonsen @ 2004-01-31 19:53 UTC (permalink / raw)
To: linux-raid
Hi!
Is it possible to both do striping and mirroring if you only have 2 disks?
I was thinking of having the chunks organized on each disk so
that striping reads and writes could be done. Eg. by having
disk1: chunk0 chunk2 chunk4 ...... chunk1 chunk3 chunk5
disk2: chunk1 chunk3 chunk5 ...... chunk0 chunk2 chunk4
Would such a layout of chunks allow for striped reading and writing?
Best regards
keld
^ permalink raw reply [flat|nested] 6+ messages in thread
* RE: striping and mirroring with only two disks
2004-01-31 19:53 striping and mirroring with only two disks Keld Jørn Simonsen
@ 2004-01-31 21:29 ` Guy
2004-01-31 21:36 ` Nathan Lewis
1 sibling, 0 replies; 6+ messages in thread
From: Guy @ 2004-01-31 21:29 UTC (permalink / raw)
To: 'Keld Jørn Simonsen', linux-raid
The reason people stripe is performance. The performance gain is by having
more disk heads working. If you stripe a single disk (or 2 if mirrored), it
will thrash! This will hurt/kill performance. Just mirror.
Oh, you could do it if you want. It will work. Create a mirror (RAID1)
with each pair of chunks, 6 mirrors total (with your example). Then stripe
the mirrors (RAID0).
-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Keld Jørn Simonsen
Sent: Saturday, January 31, 2004 2:53 PM
To: linux-raid@vger.kernel.org
Subject: striping and mirroring with only two disks
Hi!
Is it possible to both do striping and mirroring if you only have 2 disks?
I was thinking of having the chunks organized on each disk so
that striping reads and writes could be done. Eg. by having
disk1: chunk0 chunk2 chunk4 ...... chunk1 chunk3 chunk5
disk2: chunk1 chunk3 chunk5 ...... chunk0 chunk2 chunk4
Would such a layout of chunks allow for striped reading and writing?
Best regards
keld
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: striping and mirroring with only two disks
2004-01-31 19:53 striping and mirroring with only two disks Keld Jørn Simonsen
2004-01-31 21:29 ` Guy
@ 2004-01-31 21:36 ` Nathan Lewis
2004-01-31 23:55 ` Keld Jørn Simonsen
1 sibling, 1 reply; 6+ messages in thread
From: Nathan Lewis @ 2004-01-31 21:36 UTC (permalink / raw)
To: Keld Jørn Simonsen, linux-raid
Even if it were possible, your write performance would be TERRIBLE (imaging
writing the chunks in order - 0, 1, 2, 3.... Your disk heads would be
thrashing constantly). The RAID1 code already does read-balancing. Not
the same thing as striping, but certainly increases performance on reads,
especially when more than one read is being done at a time.
If you wanted to try it, I think you might be able to accomplish it with
partitions. Partition your two disks in half, so you have:
disk1 a & b
disk2 a & b
Then mirror disk1a with disk2b (say as /dev/md0), and likewise mirror
disk1b with disk2a (say as /dev/md1). Then create a RAID0 array (/dev/md2)
from those two mirrors (md0 and md1).
You could also do it the other way:
create two raid0 arrays (disk1a+disk2a=/dev/md0) (disk1b+disk2b=/dev/md1)
then mirror them as /dev/md2.
At 01:53 PM 1/31/2004, Keld Jørn Simonsen wrote:
>Hi!
>
>Is it possible to both do striping and mirroring if you only have 2 disks?
>
>I was thinking of having the chunks organized on each disk so
>that striping reads and writes could be done. Eg. by having
>
>disk1: chunk0 chunk2 chunk4 ...... chunk1 chunk3 chunk5
>disk2: chunk1 chunk3 chunk5 ...... chunk0 chunk2 chunk4
>
>Would such a layout of chunks allow for striped reading and writing?
>
>Best regards
>keld
>-
>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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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: striping and mirroring with only two disks
2004-01-31 21:36 ` Nathan Lewis
@ 2004-01-31 23:55 ` Keld Jørn Simonsen
2004-02-02 4:58 ` Neil Brown
0 siblings, 1 reply; 6+ messages in thread
From: Keld Jørn Simonsen @ 2004-01-31 23:55 UTC (permalink / raw)
To: Nathan Lewis; +Cc: Keld Jørn Simonsen, linux-raid
On Sat, Jan 31, 2004 at 03:36:21PM -0600, Nathan Lewis wrote:
> Even if it were possible, your write performance would be TERRIBLE (imaging
> writing the chunks in order - 0, 1, 2, 3.... Your disk heads would be
> thrashing constantly).
Well, I am not sure that will be so. Most modern disks today have
buffers of 2 MB - 8 MB on the unit, and writes would thus be buffered
there, meaning that the drive would be striping.
> The RAID1 code already does read-balancing. Not
> the same thing as striping, but certainly increases performance on reads,
> especially when more than one read is being done at a time.
Yes, striping is best when there is only one read or write in operation.
I think tho, that lonely sequential reads or writes are very common, even on servers.
>
> If you wanted to try it, I think you might be able to accomplish it with
> partitions. Partition your two disks in half, so you have:
> disk1 a & b
> disk2 a & b
>
> Then mirror disk1a with disk2b (say as /dev/md0), and likewise mirror
> disk1b with disk2a (say as /dev/md1). Then create a RAID0 array (/dev/md2)
> from those two mirrors (md0 and md1).
>
> You could also do it the other way:
> create two raid0 arrays (disk1a+disk2a=/dev/md0) (disk1b+disk2b=/dev/md1)
> then mirror them as /dev/md2.
I think that is a nice way to get what I was thinking of.
Nice in the way that it is already possible with existing code.
But will the mirroring work? Or, if one of the disks fails, can I be
sure that the other will have all the data, and that I can recreate
the combined md from the two parts?
Why has nobody thought of it before? All descriptions I have seen of
RAID0+1 says that it needs 4 disks.
Best regards
keld
>
>
> At 01:53 PM 1/31/2004, Keld Jørn Simonsen wrote:
> >Hi!
> >
> >Is it possible to both do striping and mirroring if you only have 2 disks?
> >
> >I was thinking of having the chunks organized on each disk so
> >that striping reads and writes could be done. Eg. by having
> >
> >disk1: chunk0 chunk2 chunk4 ...... chunk1 chunk3 chunk5
> >disk2: chunk1 chunk3 chunk5 ...... chunk0 chunk2 chunk4
> >
> >Would such a layout of chunks allow for striped reading and writing?
> >
> >Best regards
> >keld
> >-
> >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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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: striping and mirroring with only two disks
2004-01-31 23:55 ` Keld Jørn Simonsen
@ 2004-02-02 4:58 ` Neil Brown
2004-02-02 8:46 ` Hendrik Visage
0 siblings, 1 reply; 6+ messages in thread
From: Neil Brown @ 2004-02-02 4:58 UTC (permalink / raw)
To: Keld Jørn Simonsen; +Cc: Nathan Lewis, linux-raid
On Sunday February 1, keld@dkuug.dk wrote:
>
> Why has nobody thought of it before? All descriptions I have seen of
> RAID0+1 says that it needs 4 disks.
Just because it hasn't been implemented doesn't mean it hasn't been
thought of....
I started work on a "raid10" personality a while back, which could do
what you describe, or could stripe/mirror across 3 drives and other
combination.
It feel by the wayside as some things often do.
NeilBrown
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: striping and mirroring with only two disks
2004-02-02 4:58 ` Neil Brown
@ 2004-02-02 8:46 ` Hendrik Visage
0 siblings, 0 replies; 6+ messages in thread
From: Hendrik Visage @ 2004-02-02 8:46 UTC (permalink / raw)
To: Neil Brown; +Cc: Keld Jørn Simonsen, Nathan Lewis, linux-raid
On Mon, Feb 02, 2004 at 03:58:27PM +1100, Neil Brown wrote:
> On Sunday February 1, keld@dkuug.dk wrote:
>
> I started work on a "raid10" personality a while back, which could do
> what you describe, or could stripe/mirror across 3 drives and other
> combination.
Sounds like the RAID1E and RAID5E of some of the IBM ServeRAID controllers...
1E: Works across minimum of 3 drives, with the stripes written as:
Drive: 1 2 3
a b c
b c a
d e f
e f d
Ie. you still lose half the disk space, but will work over unequal
number of wrives with still a single drive failure resilience and a full
speed read.
5E: minimum 4drives, similar story just a tad different (No, I've mislaid
the documentation :()
^ permalink raw reply [flat|nested] 6+ messages in thread
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2004-01-31 19:53 striping and mirroring with only two disks Keld Jørn Simonsen
2004-01-31 21:29 ` Guy
2004-01-31 21:36 ` Nathan Lewis
2004-01-31 23:55 ` Keld Jørn Simonsen
2004-02-02 4:58 ` Neil Brown
2004-02-02 8:46 ` Hendrik Visage
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