Gmirror performanc (was Re: Gmirror question)

[Willem Jan Withagen]

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