Guido van Rooij wrote:
Anyway, I created a gm device and a partition. Now the read performance
is not what I'd expect.
I have the partition on two SATA devices on different controlers.
I get around 60MB/s for each disk. I can get that speed from both disks
simultaneously.
Now when I dd from the gm device, I don't get any speed higher than that.
I tried with -b split -s <various sizes>, -b round-robin, -b load.
(dd-ing as done with a bs of 1m; I see the transaction size is 128Kb,
unless the split method is used, in which case the transaction size
gies down. When round-robin is used, the transaction size is 128Kb/s,
but the number of transaction per second goes down.).
I cannot explain why I should not get a higher read speed. Anyone?
Hee Guido,
I've once ran several of these types of tests for some of the disks I
collected over time. Even wrote a page on that topic. More or less as a
consequence of a paper you mailed me a while ago on NFS performance :)
If you want:
The narative on this (don't dare calling it an article.)
http://www.tegenbosch28.nl/FreeBSD/Performance/Raw-disk/
And I've compared a WD800 SATA disk with a gmirror of 2 the same disks.
http://www.tegenbosch28.nl/FreeBSD/Performance/Raw-disk/wd800-sata/
http://www.tegenbosch28.nl/FreeBSD/Performance/Raw-disk/wd800-sata/
The fact that these are not mentioned in the article is that I never got
around into looking why the graphs look the way that they look.
Especially the write ones need some serious consideration.
If you want the scripts, for some DIY: Just give me a buzz...
Running this for a large disk takes al long time (>1 day)
I'm currently running it on a 250Gb disk.
Probably there'll be flack from people telling you not to use dd for
disk benchmarking. Given its simple approach it does let you understand
what you are doing.
--WjW
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