Re: XFS/btrfs performance after IO-less dirty throttling

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Dec 19, 2011 at 09:57:16AM +0800, Dave Chinner wrote:
> On Fri, Dec 16, 2011 at 01:16:09PM +0800, Wu Fengguang wrote:
> > On Fri, Dec 16, 2011 at 12:25:08PM +0800, Dave Chinner wrote:
> > > On Fri, Dec 16, 2011 at 09:53:11AM +0800, Wu Fengguang wrote:
> > > > I'm indeed happy that you don't care that much on that regression
> > > > introduced by me ;-)
> > > 
> > > Heh.
> > > 
> > > BTW, do these tests run to ENOSPC?
> > 
> > Nope. Shall ENOSPC (performance) be tested?
> > 
> > > >                 10829.00        +4.3%     11296.00  TOTAL xfs:xfs_delalloc_enospc
> > > 
> > > This implies that it does.
> > 
> > Not really. The USB key partition size is 7.1GB.
> > Even in the fastest 1dd case, only 4GB data is written:
> 
> There's a couple of ways this can be tripping ENOSPC during delayed
> allocation. Speculative preallocation is the most likely cause given
> that for a 4GB file being written sequentially it will try to
> preallocate a 4GB chunk for the next delalloc extent....

Yeah I suspected some heuristic allocation, too.

> And by triggering this path, it will force data writeback to occur
> through the xfssyncd workqueue. i.e. the writeback behaviour that is
> occurring is not what you are expecting it to be - XFS is detecting
> a potential ENOSPC problem, and taking steps to flush delalloc data
> much faster than writeback is doing.

OK.

> > wfg@bee /export/writeback% cat fat/UKEY-thresh=100M/xfs-1dd-1-3.2.0-rc3/ls-files 
> > 131 -rw-rw-r-- 1 root root 4060078080 Dec  8 15:57 /fs/sdb3/zero-1
> 
> What's the dd command line you are using?

It's a loop of

        dd bs=$bs if=/dev/zero of=$mnt/zero-$i &

where bs=4k by default.

Thanks,
Fengguang