[OpenIndiana-discuss] Recommendations for fast storage

[Jay Heyl]

On Tue, Apr 16, 2013 at 4:01 PM, Jim Klimov <jimklimov at cos.ru> wrote:

> On 2013-04-16 23:56, Jay Heyl wrote:
>
>> result in more devices being hit for both read and write. Or am I wrong
>> about reads being issued in parallel to all the mirrors in the array?
>>
>
> Yes, in normal case (not scrubbing which makes a point of reading
> everything) this assumption is wrong. Writes do hit all devices
> (mirror halves or raid disks), but reads should be in parallel.
> For mechanical HDDs this allows to double average read speeds
> (or triple for 3-way mirrors, etc.) because different spindles
> begin using their heads in shorter strokes around different areas,
> if there are enough concurrent randomly placed reads.


Not to get into bickering about semantics, but I asked, "Or am I wrong
about reads being issued in parallel to all the mirrors in the array?", to
which you replied, "Yes, in normal case... this assumption is wrong... but
reads should be in parallel." (Ellipses intended for clarity, not argument
munging.) If reads are in parallel, then it seems as though my assumption
is correct. I realize the system will discard data from all but the first
reads and that using only the first response can improve performance, but
in terms of number of IOPs, which is where I intended to go with this, it
seems to me the mirrored system will have at least as many if not more than
the raid-zn system.

Or have I completely misunderstood what you intended to say?