Richard Scobie wrote:
MRK wrote:
I spent some time trying to optimize it but that was the best I could
get. Anyway both my benchmark and Richard's one imply a very
significant bottleneck somehwere.
This bottleneck is the SAS controller, at least in my case. I did the
same math regarding streaming performance of one drive times number of
drive and wondered where the shortfall was, after tests showed I could
only streaming read at 850MB/s on the same array.
A query to an LSI engineer got the following response, which basically
boils down to "you get what you pay for" - SAS vs SATA drives.
"Yes, you're at the "practical" limit.
With that setup and SAS disks, you will exceed 1200 MB/s. Could go
higher than 1,400 MB/s given the right server chipset.
However with SATA disks, and the way they break up data transfers, 815
to 850 MB/s is the best you can do.
Under SATA, there are multiple connections per I/O request.
* Command Initiator -> HDD
* DMA Setup Initiator -> HDD
* DMA Activate HDD -> Initiator
* Data HDD -> Initiator
* Status HDD -> Initiator
And there is little ability with typical SATA disks to combine traffic
from different I/Os on the same connection. So you get lots of
individual connections being made, used, & broken.
Contrast that with SAS which has typically 2 connections per I/O, and
will combine traffic from more than 1 I/O per connection. It uses the
SAS links much more efficiently."
Firstly: Happy Easter! :-)
Secondly:
If this is true then one won't achieve higher speeds even on RAID-0. If
anybody can test this... I cannot right now
I am a bit surprised though. The SATA "link" is one per drive, so if 1
drive is able to do 90MB/sec, N drives on N cables should do Nx90MB/sec.
If this is not so, then the chipset of the controller must be the
bottleneck.
If this is so, the newer LSI controllers at 6.0gbit/sec could be able to
do better (they supposedly have a faster chip). Also maybe one could buy
more controller cards and divide drives among those. These two
workarounds would still be cheaper than SAS drives.
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