Re: Suboptimal raid6 linear read speed

[Charles Polisher <cpolish@surewest.net>]

On Tue, Jan 15, 2013 at 11:55:07PM +1100, Peter Rabbitson wrote:
> On Tue, Jan 15, 2013 at 07:49:10AM -0500, Phil Turmel wrote:
> > You are neglecting each drive's need to skip over parity blocks.  If the
> > array's chunk size is small, the drives won't have to seek, just wait
> > for the platter spin.  Larger chunks might need a seek.
> 
> > Either way, you
> > won't get better than (single drive rate) * (n-2) where "n" is the
> > number of drives in your array. (Large sequential reads.)
> 
> This can't be right. As far as I know the md layer is smarter than that, and
> includes various anticipatory codepaths specifically to leverage multiple
> drives in this fashion. Fwiw raid5 does give me the near-expected speed
> (n * single drive).

Happen to be working with comparative benchmarks looking for
relative throughput, varying the number of active drives in the
array and the RAID level. Clearly in this data RAID6 sequential
writes are bottlenecked by the 2 parity stripes. RAID6 setup
increases from 2 non-parity drives in the 4 drive configuration
to 6 non-parity drives in the 8 drive configuration, so one
might hope for 3x advantage. Yet the data show an advantage of
only 1.83 for reads. My guess is the need to read the parity
stripes is again a limiting factor. Next benchmark will vary
stripe and stride.

                                        Advantage     Advantage
                                        vs 4 drives   vs RAID0
Config  Drives  Seq write   Seq  read   Write  Read   Write Read
------  ------  ----------  ----------  ----- -----   ----  ----
RAID0   4        8.1MB/sec   9.3MB/sec   1.00  1.00   1.00  1.00
RAID0   8       16.8MB/sec  15.0MB/sec   2.07  1.61   1.00  1.00
 
RAID1   4        2.1MB/sec   3.6MB/sec   1.00  1.00   0.25  0.38
RAID1   8        1.6MB/sec   3.6MB/sec   0.76  1.00   0.09  0.24

RAID5   4       16.8MB/sec   9.1MB/sec   1.00  1.00   2.07  0.97
RAID5   8       17.2MB/sec  14.9MB/sec   1.02  1.63   2.12  1.60

RAID6   4       12.6MB/sec   7.9MB/sec   1.00  1.00   1.55  0.84
RAID6   8       14.4MB/sec  14.5MB/sec   1.63  1.83   1.77  1.55

RAID10  4        4.0MB/sec   7.3MB/sec   1.00  1.00   0.49  0.78
RAID10  8        6.3MB/sec  13.4MB/sec   1.57  1.83   0.37  0.89

Yes, these drives are *really* slow (Connor CP 30548). 
The math doesn't change.
-- 
Charles