I see.

Well, whether MD is smart enough to rearrange a queue when the arrays
are sharing disks or not, it still means that you can't run your
processes in parallel.

I did see MD put the arrays in queue during resyncs when I had a
similar setup. I didn't benchmark the arrays at that time, so I don't
have solid numbers of how much of a performance penalty that setup
caused.

On Wed, Oct 14, 2009 at 9:23 AM, Antonio Perez [off-list ref] wrote:

Majed B. wrote:

quoted

I think I already answered your question:

quoted

For desktop usage it's OK to use that setup since you won't be writing
to / and the other segments a lot at the same time.

quoted

If you're running an application which writes a lot of data to / and
you require to read/write a lot of data of the rest of the disk, it
will conflict and slow things down a lot.

Basically, you're partitioning each disk and making each partition
belong to an array.

If you misunderstood part, or I did, let me know :)

Thanks Majeb. I believe I understand you.

This is what I get from your comment:

If the disks are setup with several partitions, and the corresponding
partitions belong to a md array as this:
       sd[a..d]1 --> md1
       sd[a..d]2 --> md2
       sd[a..d]3 --> md3

If md1 is used as / and md2 is used as /home (or /data) there will be no
concurrent reads to both places.

While this may be true, depending on the specific application, it is not
what I am asking.
________________________________________________________________________

The point is:

If md1 is used as /data1 and md2 is used as /data2 will it work?

Will the md system be aware that those are on the same disk (spindle) and
use the correct queuing on the reads for the best reading speed possible?

Or will md get confused on the correct read sequence causing additional head
seeks which will degrade overall performance.

Sorry if I confuse you more, this is not an "simple" question.

Please read Robin Hill answer. :-)

--
Antonio Perez

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