Re: What's the typical RAID10 setup?

[Roberto Spadim <roberto@spadim.com.br>]

ok, read/write scheduling is another task..
the best algorithm is time based (optimize to minimal time)
it´s not the round robin neither the closer head algorithm
if you want performace use minimal time to execute (time based)

in this topic (many emails) there´s a important thing. resolve the
probability problem and make it 'official', include numbers and
context

the best algorithm for read and write isn´t the question about
probability of how many mirros can i lose, how many disks can i lose
(maybe can be if we change context to more source based, MAYBE)

2011/2/1 David Brown <david@westcontrol.com>:
> On 01/02/2011 14:50, Jon Nelson wrote:
>>
>> On Tue, Feb 1, 2011 at 4:01 AM, David Brown<david@westcontrol.com>  wrote:
>>>
>>> On 31/01/2011 23:52, Keld Jørn Simonsen wrote:
>>>>
>>>> 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.
>>>
>>> If you have three disks, you can do that too:
>>>
>>> md0 = raid1(sda1, sdb2)
>>> md1 = raid1(sdb1, sdc2)
>>> md2 = raid1(sdc1, sda2)
>>> md3 = raid0(md0, md1, md2)
>>>
>>> As far as I can figure out, the performance should be pretty much the
>>> same
>>> (although wrapping everything in a single raid10,f2 is more convenient).
>>
>> The performance will not be the same because. Whenever possible, md
>> reads from the outermost portion of the disk -- theoretically the
>> fastest portion of the disk (by 2 or 3 times as much as the inner
>> tracks) -- and in this way raid10,f2 can actually be faster than
>> raid0.
>>
>
> This would presumably apply to all raid1 arrangements, not just raid10 -
> when md has a choice to read from more than one place it will prefer the
> outermost place.  In the arrangement I described above, the raid pairs such
> as md0 each have one have on an inner partition, and one half on an outer
> partition.  /If/ md is smart enough, then it will do the same here and read
> from the outer partition by preference.
>
> The question is, does md determine the "outermost" copy by track number
> relative to the partition, or by absolute track number on the disk?  If it
> is the former, then I see your point - with my raid 1 + 0 arrangement the
> innermost and outermost partitions will be viewed the same.  If it is the
> later, then my arrangement will work equally well.
>
> On a related note, if you mix an SSD and a HD (partition) in a mirror, will
> md prefer to read from the SSD first?  I know it is possible to use the
> "write-mostly" flag to force all reads to come from the SSD (assuming it
> hasn't failed), but it would be nice to get parallel reads from the HD as
> well whenever the read is large enough or when there are multiple reads in
> parallel.
>
>
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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