On 04/22/2013 07:01 AM, Mark Nelson wrote:
On 04/22/2013 06:48 AM, James Harper wrote:
My read speed is consistently around 40MB/second, and my write speed is
consistently around 22MB/second. I had expected better of read...
You may want to try increasing your read_ahead_kb on the OSD data disks
and see if that helps read speeds.
Default appears to be 128 and I was getting 40MB/second
Increasing to 256 takes me up to 48MB/second
Increasing to 512 takes me up to 53Mb/second
Any further increases don't do anything that I can measure
Is increasing read_ahead_kb good for general performance, or just for
impressing people with benchmarks? If the kernel spent time reading
ahead woult it hurt random read/write performance?
Potentially yes, but it depends on a lot of of factors. I suspect that
increasing it may be acceptable on modern drives, but you'll need to do
some testing to see how it goes in practice.
If anyone on the list knows how many sectors per track is typical for
modern 1-3TB drives I'm dying to know. That would help us guess at how
much data can be writen/read on average without imposing any head
movement. :)
Aha, sorry to reply to my own mail. I found some specifications for
Hitachi drives at least:
http://www.hgst.com/tech/techlib.nsf/products/Ultrastar_7K4000
look at section 4.2 of the "Ultrastar 7K4000 OEM Specification" document.
It specifies 310ktpi, or 310,000 tracks/inch.
Via google I found that this drive is using 5 800GB platters, meaning
there are 10 heads in this drive. Using hitachi's specifications:
(7,814,037,168 sectors / (310,000 tracks / inch * 3.5 inches)) / 10
heads * 512 bytes / sector = ~360KB/track head
So assuming my math is right, it looks like we can read up to around
360KB of data before hitting a head switch. Now unfortunately (or maybe
fortunately!) this is just the average case. Outer tracks will store
more data than inner tracks, so depending on what portion of the disk
you are doing the read from, you might introduce head switches more or
less often. It looks like even with a 256k or 512k read_ahead you
probably won't introduce a next-cylinder seek that often, though from
what I can find it's not going to be all that much more expensive vs a
head switch (2-3ms vs 1-2ms).
Mark
Thanks
James
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