Re: 2.6.36 io bring the system to its knees

[Chris Mason]

On Tue, Nov 02, 2010 at 07:47:15AM -0400, Sanjoy Mahajan wrote:
> Chris Mason <chris.mason@xxxxxxxxxx> wrote:
> 
> > > This has the appearance of some really bad IO or VM latency
> > > problem. Unfixed and present in stable kernel versions going from
> > > years ago all the way to v2.6.36.
> > 
> > Hmmm, the workload you're describing here has two special parts.
> > First it dramatically overloads the disk, and then it has guis doing
> > things waiting for the disk.
> 
> I think I see this same issue every few days when I back up my hard
> drive to a USB hard drive using rsync.  While the backup is running, the
> interactive response is bad.  A reproducible measurement of the badness
> is starting an rxvt with F8 (bound to "rxvt &" in my .twmrc).  Often it
> takes 8 seconds for the window to appear (as it just did about 2 minutes
> ago)!  (Starting a subsequent rxvt is quick.)

So this sounds like the backup is just thrashing your cache.  Latencies
starting an app are less surprising than latencies where a running app
doesn't respond at all.

Does rsync have the option to do an fadvise DONTNEED?

-chris
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