For sequential reading, this is not true. For random reading and
writing I agree with you in theory, but benchmarks show that it is not
so, at least for Linux RAID, viz the above URL.

i agree with you, since linux algorith for raid1 is closest head, not
round robin or time based

there´s some patch on internet (google it: round robin raid1 linux)
for roundrobin, but none for time based =(
it´s a point of optimization of today raid1 algorithm

round robin (may be at this mail list)
http://www.spinics.net/lists/raid/msg30003.html

2011/2/1 Keld Jørn Simonsen [off-list ref]:

On Tue, Feb 01, 2011 at 05:02:46PM +0100, Keld Jørn Simonsen wrote:

quoted

On Tue, Feb 01, 2011 at 11:01:33AM +0100, David Brown wrote:

quoted

On 31/01/2011 23:52, Keld Jørn Simonsen wrote:

quoted

raid1+0 and Linux MD raid10 are similar, but significantly different
in a number of ways. Linux MD raid10 can run on only 2 drives.
Linux raid10,f2 has almost RAID0 striping performance in sequential read.
You can have an odd number of drives in raid10.
And you can have as many copies as you like in raid10,

You can make raid10,f2 functionality from raid1+0 by using partitions.
For example, to get a raid10,f2 equivalent on two drives, partition them
into equal halves.  Then make md0 a raid1 mirror of sda1 and sdb2, and
md1 a raid1 mirror of sdb1 and sda2.  Finally, make md2 a raid0 stripe
set of md0 and md1.

I don't think you get the striping performance of raid10,f2 with this
layout. And that is one of the main advantages of raid10,f2 layout.
Have you tried it out?

As far as I can see the layout of blocks are not alternating between the
disks. You have one raid1 of sda1 and sdb2, there a file is allocated on
blocks sequentially on sda1 and then mirrored on sdb2, where it is also
sequentially allocated. That gives no striping.

Well, maybe the RAID0 layer provides the adequate striping.
I am noy sure, but it looks like it could hold in theory.
One could try it out.

One advantage of this scheme could be improved probability
When 2 drives fail, eg. in the case of a 4 drive array.
The probability of survival of a running system could then
be enhaced form 33 % to 66 %.

One problem could be the choice of always the lowest block number, which
is secured in raid10,f2, but not in a raid0 over raid1 (or raid10,n2) scenario.

best regards
keld
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Roberto Spadim
Spadim Technology / SPAEmpresarial
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