On 7/5/2013 8:33 PM, Christoph Anton Mitterer wrote:

One more on that, which I've just thought about:

On Thu, 2013-07-04 at 18:07 -0400, Phil Turmel wrote:

quoted

I believe dmcrypt is single-threaded, too.

Even if that is still true... and it with dmcrypt below MD it would run
with _one_ thread _per_ physical device... while the other way round
(dmcrypt on top of MD) it would run with _only one_ thread over the
_whole_ MD device (and thus also all disks)...

...would the former really give a performance benefit (or wouldn't it be
actually much worse).

Yes.

Since even though you'd have one dmcrypt thread per disk now,... each of
them would have to en/decrypt the "same" actual data for different
devices... so while you have 4x threads... you need to do 4x
en/decryption works.

This would only be true if using n-way mirroring.

Which wouldn't be the case when having dmcrypt on top of MD... sure
you'd only have one thread...

You misunderstand the way this works.  With striped md RAID each chunk
goes to a different disk.  All chunks contain dissimilar data.  If you
use dmcrypt at the lowest level of the stack you have one dmcrypt thread
per chunk, each processing 1/n of the load.  There is no duplication of
work.

-- 
Stan