Re: [dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics

[Wu Fengguang <fengguang wu gmail com>]

On Thu, Jan 26, 2012 at 10:25:33PM -0700, Andreas Dilger wrote:
[snip]
> Doesn't the kernel derive at least some idea of the speed of a device
> due to the writeback changes that you made?  It would be very useful
> if we could derive at least some rough metric for the device performance
> in the kernel and use that as input to the readahead window size as well.

Yeah we now have bdi->write_bandwidth (exported as "BdiWriteBandwidth"
in /debug/bdi/8:0/stats) for estimating the bdi write bandwidth.

However the value is not reflecting the sequential throughput in some
cases:

1) when doing random writes
2) when doing mixed reads+writes
3) when not enough IO have been issued
4) in the rare case, when writing to a small area repeatedly so that
   it's effectively writing to the internal disk buffer at high speed

So there are still some challenges in getting a reliably usable
runtime estimation.

Thanks,
Fengguang