megaraid_sas (H710), fast SSD's, poor RAID0 performance

megaraid_sas (H710), fast SSD's, poor RAID0 performance

[Mark Lehrer]

Hello there, I have a server in our test lab (specs below) that
doesn't appear to be getting nearly the write speeds that I was
expecting (test details below).  In "top" terminology, there is one
CPU core at about 30% SI and each core shows anywhere from 1 to 15 %
SY and about the same for WA.

My main question: what tools do I need to get some visibility as to
what is going on within the megaraid_sas module and on the SAS
controller itself?  I have gone about as far as I can go with
utilities such as top and MegaCli.

For simplicity, I am only focusing on raw write speed straight to the
block device.  I have a few benchmarks I use, mainly Orion and some of
the Phoronix suite, but Currently the easiest test is just:

dd if=/dev/zero of=/dev/sdc ibs=1048576 obs=1048576

The main symptom is that with a single disk I can push > 300MB/sec,
and with a pair in RAID0 I can get over 600MB/sec, but with 5 of these
in RAID0 it never goes above around 700MB/sec.  My next test will be
to use each disk as a separate device and try software raid (or just
writing to each disk simultaneously).


Thanks,
Mark

p.s. system specs:

Dell 720xd, H710 ("mini") controller
5x Micron P300 SATA SLC SSD's