Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of blocks

Ric Wheeler
2016-03-17T13:49:21Z

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@fromorbit.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