Richard Scobie wrote:
MRK wrote:
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
Yes, both of these would work.
Someone posted previously on this list and was writing at 1.7GB/s
using 10 x 15K SAS drives md RAID0. He did mention the troughput was
higher with the LSI SAS2 cards, even with SAS1 port expanders connected.
Not so fast... actually I see a problem with previous deduction of what
is the bottleneck.
The answer from the LSI engineer leads to think that the bottleneck with
SATA is the number of IOPS, it's because there are 5 connections
established and then broken for each I/O. And this is independent from
the size transferred by each I/O operation via DMA (the overhead of the
data transfer is the same in SAS and SATA case, it's always the same DMA
chip doing the transfer).
However, if really the total number of IOPS is the bottleneck in SATA
with the 3.0gbit/sec LSI cards, why they don't slow down a single SSD
doing 4k random I/O?
Look at this
http://www.anandtech.com/show/2954/5
OCZ vertex LE doing 162 MB/sec at 4K aligned random writes, that means
41472 IOPS, independent and unmergeable requests. And that is SATA not SAS.
This is on Windows. Unfortunately we don't know the controller which was
used for this benchmark.
During MD-RAID sequential dd write I have seen linux (via iostat -x 1)
merging requests by a factor at least 400 (sometimes much higher), so I
suppose requests issued to the controller would be at least 1.6 MB long
(original requests are certainly not shorter than 4K, and 4K x 400=1.6MB).
If the system tops out at about 600MB/sec and writes issued are 1.6MB
long or more, it means that the controller tops out at 375 IOPS or less.
So how come the controller of the anandtech test above is capable of
doing 41472 IOPS?
This is also interesting:
Richard Scobie wrote:
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.
I think if you use dd to read from the 16 underlying devices
simultaneously, independently, and not using MD, (output to /dev/null)
you should obtain the full disk speed of 1.4 GB/sec or so (aggregated).
I think I did this test in the past and I noticed this. Can you try? I
don't have our big disk array in my hands any more :-(
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