Date:	Thu, 17 Mar 2016 09:49:15 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Chris Mason <clm@...com>, sandeen@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>, Ric Wheeler <rwheeler@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Gregory Farnum <greg@...gs42.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	shane.seymour@....com, Bruce Fields <bfields@...ldses.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Jeff Layton <jlayton@...chiereds.net>
Subject: Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of
 blocks

On 03/16/2016 06:23 PM, Chris Mason wrote:
> On Tue, Mar 15, 2016 at 05:51:17PM -0700, Chris Mason wrote:
>> On Tue, Mar 15, 2016 at 07:30:14PM -0500, Eric Sandeen wrote:
>>> On 3/15/16 7:06 PM, Linus Torvalds wrote:
>>>> On Tue, Mar 15, 2016 at 4:52 PM, Dave Chinner <david@...morbit.com> wrote:
>>>>>> It is pretty clear that the onus is on the patch submitter to
>>>>>> provide justification for inclusion, not for the reviewer/Maintainer
>>>>>> to have to prove that the solution is unworkable.
>>>> I agree, but quite frankly, performance is a good justification.
>>>>
>>>> So if Ted can give performance numbers, that's justification enough.
>>>> We've certainly taken changes with less.
>>> I've been away from ext4 for a while, so I'm really not on top of the
>>> mechanics of the underlying problem at the moment.
>>>
>>> But I would say that in addition to numbers showing that ext4 has trouble
>>> with unwritten extent conversion, we should have an explanation of
>>> why it can't be solved in a way that doesn't open up these concerns.
>>>
>>> XFS certainly has different mechanisms, but is the demonstrated workload
>>> problematic on XFS (or btrfs) as well?  If not, can ext4 adopt any of the
>>> solutions that make the workload perform better on other filesystems?
>> When I've benchmarked this in the past, doing small random buffered writes
>> into an preallocated extent was dramatically (3x or more) slower on xfs
>> than doing them into a fully written extent.  That was two years ago,
>> but I can redo it.
> So I re-ran some benchmarks, with 4K O_DIRECT random ios on nvme (4.5
> kernel).  This is O_DIRECT without O_SYNC.  I don't think xfs will do
> commits for each IO into the prealloc file?  O_SYNC makes it much
> slower, so hopefully I've got this right.
>
> The test runs for 60 seconds, and I used an iodepth of 4:
>
> prealloc file: 32,000 iops
> overwrite:    121,000 iops
>
> If I bump the iodepth up to 512:
>
> prealloc file: 33,000 iops
> overwrite:   279,000 iops
>
> For streaming writes, XFS converts prealloc to written much better when
> the IO isn't random.  You can start seeing the difference at 16K
> sequential O_DIRECT writes, but really its not a huge impact.  The worst
> case is 4K:
>
> prealloc file: 227MB/s
> overwrite:     340MB/s
>
> I can't think of sequential workloads where this will matter, since they
> will either end up with bigger IO or the performance impact won't get
> noticed.
>
> -chris

I think that these numbers are the interesting ones, see a 4x slow down is 
certainly significant.

If you do the same patch after hacking XFS preallocation as Dave suggested with 
xfs_db, do we get most of the performance back?

Ric

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