On 30/08/2012 22:28, Kent Overstreet wrote:

On Thu, Aug 30, 2012 at 01:18:54PM +0100, Jonathan Tripathy wrote:

quoted

On 30.08.2012 08:21, Jonathan Tripathy wrote:

quoted

On 30/08/2012 08:15, Jonathan Tripathy wrote:

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Hi There,

On my WIndows DomU (Xen VM) which is running on a LV which is
using bcache (against two SSD in MDRAID1 and a MD-RAID10 spindle
array), I ran an IOMeter test for about 2 hours (with 30 workers
and a io depth of 256). This was a very heavy workload (Got an
average iops of about 6.5k). After I stopped the test, I then
went back to fio on my Linux Xen Host (Dom0). The random write
performance isn't as good as it was before I started the IOMeter
test. It used to be about 25k and now showed about 7k iops. I
assumed that maybe this was due to the fact that bcache was
writing out dirty data to the spindles so the SSD was busy.

However, this morning, after the spindles have calmed down,
performance of fio is still not great (still about 7k).

Is there something wrong here? What is expected behavior?

Thanks

BTW, I can confirm that this isn't an SSD issue, as I have a
partition on the SSD that I kept seperate from bcache and I'm getting
excellent (about 28k) iops performance there.

It's as if after the heavy workload I did with IOMeter, bcache has
somehow throttled the writeback cache?

Any help is appreciated.

I'd like to add that a reboot pretty much solves the issue. This
leads me to believe that there is a bug in the bcache code that
causes performance to drop the more it gets used.

Any ideas?

Weird!

Yeah, that definitely sounds like a bug. I'm going to have to try and
reproduce it and go hunting. Can you think of anything that might help
with reproducing it?
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Hi Kent,

I'm going to try and reproduce it myself as well. I just used IOMeter in 
a Windows DomU with 30 workers, each having an io depth of 256. A *very* 
heavy workload indeed, but my point was to see if I could break 
something. Unless the issue is specific to windows causing problems 
(NTFS or whatever), I'm guessing running fio with 30 jobs and an iodepth 
of 256 would probably produce a similar load.

BTW, do you have access to a Xen node for testing?

Thanks