[OpenIndiana-discuss] Recommendations for fast storage

[Richard Elling]

For the context of ZPL, easy answer below :-) ...

On Apr 16, 2013, at 4:12 PM, Timothy Coalson <tsc5yc at mst.edu> wrote:

> On Tue, Apr 16, 2013 at 6:01 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.
>> 
> 
> There is another part to his question, specifically whether a single random
> read that falls within one block of the file hits more than one top level
> vdev -

No.

> to put it another way, whether a single block of a file is striped
> across top level vdevs.  I believe every block is allocated from one and
> only one vdev (blocks with ditto copies allocate multiple blocks, ideally
> from different vdevs, but this is not the same thing), such that every read
> that hits only one file block goes to only one top level vdev unless
> something goes wrong badly enough to need a ditto copy.

Correct.
 -- richard

--

Richard.Elling at RichardElling.com
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