Re: Linux 2.6.29

Chris Mason
2009-03-30T18:58:00Z

On Mon, 2009-03-30 at 14:39 -0400, Mark Lord wrote:
> Chris Mason wrote:
> >
> > I had some fun trying things with this, and I've been able to reliably
> > trigger stalls in write cache of ~60 seconds on my seagate 500GB sata
> > drive. The worst I saw was 214 seconds.
> ..
>
> I'd be more interested in how you managed that (above),
> than the quite different test you describe below.
>
> Yes, different, I think. The test below just times how long a single
> chunk of data might stay in-drive cache under constant load,
> rather than how long it takes to flush the drive cache on command.
>
> Right?
>
> Still, useful for other stuff.
>

That's right, it is testing for starvation in a single sector, not for
how long the cache flush actually takes. But, your remark from higher
up in the thread was this:

>
> Anything in the drive's write cache very probably made
> it to the media within a second or two of arriving there.
>

Sorry if I misread things. But the goal is just to show that it really
does matter if we use a writeback cache with or without barriers. The
test has two datasets:

1) An area that is constantly overwritten sequentially
2) A single sector that stores a critical bit of data.

#1 is the filesystem log, #2 is the filesystem super. This isn't a
specialized workload ;)

-chris