Re: Linux 2.6.29

[Chris Mason]

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


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