Re: best base / worst case RAID 5,6 write speeds

[Phil Turmel <philip@turmel.org>]

Hi Mark,

On 12/16/2015 07:24 PM, Mark Knecht wrote:
> On Wed, Dec 16, 2015 at 7:31 AM, Dallas Clement
> <dallas.a.clement@gmail.com <mailto:dallas.a.clement@gmail.com>> wrote:
> 
>     Phil, the 16k chunk size has really given a boost to my RAID 5
>     sequential write performance measured with fio, bs=1408k.
> 
>     This is what I was getting with a 128k chunk size:
> 
>     iodepth=4 => 605 MB/s
>     iodepth=8 => 589 MB/s
>     iodepth=16 => 634 MB/s
>     iodepth=32 => 635 MB/s
> 
>     But this is what I'm getting with a 16k chunk size:
> 
>     iodepth=4 => 825 MB/s
>     iodepth=8 => 810 MB/s
>     iodepth=16 => 851 MB/s
>     iodepth=32 => 866 MB/s

Very interesting.  Good to see hypotheses supported by results.

> Dallas,
>    Hi. Just for kicks I tried Phil's idea (I think it was Phil) and

:-)

> sampled  stripe_cache_active
> by putting this command in a 1 second loop and running it today while I
> worked.
> 
> cat  /sys/block/md3/md/stripe_cache_active >> testCacheResults
> 
> My workload is _very_ different from what you're working on. This is a
> high-end desktop
> machine (Intel 980i Extreme processor, 24GB DRAM, RAID6) running 2
> Windows 7 VMs
> while I watch the stock market and program in MatLab. None the less I
> was somewhat 
> surprise at the spread in the number of active lines. The test ran for
> about 10 hours with 
> about 94% of the results being 0, but numbers ranging from 1 line to
> 2098 lines active
> at a single time. Also interesting to me was when that 2098 value hit it
> was apparently 
> all clear in less than 1 second as the values immediately following
> where back to 0.

Yeah, latencies are pretty low.  One-second samples will be fairly
random snapshots under most conditions.  Consider sampling much faster,
but building one-minute histograms and recording those.

Phil