Re: SATA RAID5 speed drop of 100 MB/s

[Michael Tokarev <mjt@tls.msk.ru>]

Justin Piszcz wrote:
> Don't forget about max_sectors_kb either (for all drives in the SW RAID5
> array)
> 
> max_sectors_kb = 8
> $ dd if=/dev/zero of=file.out6 bs=1M count=10240
> 10737418240 bytes (11 GB) copied, 55.4848 seconds, 194 MB/s
> 
> max_sectors_kb = 128
> 10737418240 bytes (11 GB) copied, 22.6298 seconds, 474 MB/s

Well.  You're comparing something different.  Yes, this
thread is about linux software raid5 in the first place,
but I were commenting about [NT]CQ within a single drive.

Overall, yes, the larger your reads/writes to the drive
becomes, the faster its linear performance is.  Yet you
have to consider real workload instead of very synthetic
dd test.  It may be good approcsimation of a streaming
video workload (when you feed a large video file over
network or something like that), but even with this,
you probably want to feed several files at once (different
files to different clients), so single-threaded test
here isn't very useful.  IMHO anyway, and good for a
personal computer test.

/mjt