Re: [PATCH RFC 0/2] btrfs: remove the metadata readahead mechanism

David Sterba
2021-12-07T15:41:06Z

On Tue, Dec 07, 2021 at 03:53:29PM +0100, David Sterba wrote:
> On Tue, Dec 07, 2021 at 08:01:04PM +0800, Qu Wenruo wrote:
> > On 2021/12/7 19:56, Filipe Manana wrote:
> > > On Tue, Dec 07, 2021 at 07:43:49PM +0800, Qu Wenruo wrote:
> > >> On 2021/12/7 19:02, Filipe Manana wrote:
> > >>> On Tue, Dec 07, 2021 at 03:43:58PM +0800, Qu Wenruo wrote:
> > >>>> This is originally just my preparation for scrub refactors, but when the
> > >>>> readahead is involved, it won't just be a small cleanup.
> > >>>>
> > >>>> The metadata readahead code is introduced in 2011 (surprisingly, the
> > >>>> commit message even contains changelog), but now only one user for it,
> > >>>> and even for the only one user, the readahead mechanism can't provide
> > >>>> much help in fact.
> > >>>>
> > >>>> Scrub needs readahead for commit root, but the existing one can only do
> > >>>> current root readahead.
> > >>>
> > >>> If support for the commit root is added, is there a noticeable speedup?
> > >>> Have you tested that?
> > >>
> > >> Will craft a benchmark for that.
> > >>
> > >> Although I don't have any HDD available for benchmark, thus would only
> > >> have result from SATA SSD.
>
> I'm doing some tests, in a VM on a dedicated HDD.

There's some measurable difference:

With readahead:

Duration: 0:00:20
Total to scrub: 7.02GiB
Rate: 236.92MiB/s

Duration: 0:00:48
Total to scrub: 12.02GiB
Rate: 198.02MiB/s

Without readahead:

Duration: 0:00:22
Total to scrub: 7.02GiB
Rate: 215.10MiB/s

Duration: 0:00:50
Total to scrub: 12.02GiB
Rate: 190.66MiB/s

The setup is: data/single, metadata/dup, no-holes, free-space-tree,
there are 8 backing devices but all reside on one HDD.

Data generated by fio like

fio --rw=randrw --randrepeat=1 --size=3000m \
--bsrange=512b-64k --bs_unaligned \
--ioengine=libaio --fsync=1024 \
--name=job0 --name=job1 \

and scrub starts right away this. VM has 4G or memory and 4 CPUs.

The difference is 2 seconds, roughly 4% but the sample is not large
enough to be conclusive.