Re: poor write performance

[Mark Nelson <mark.nelson@inktank.com>]

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. :)

>
> Thanks
>
> James
>