From: Michal Rostecki <mrostecki@suse.de>
To: Anand Jain <anand.jain@oracle.com>
Cc: Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
David Sterba <dsterba@suse.com>,
"open list:BTRFS FILE SYSTEM" <linux-btrfs@vger.kernel.org>,
open list <linux-kernel@vger.kernel.org>,
Michal Rostecki <mrostecki@suse.com>
Subject: Re: [PATCH RFC 6/6] btrfs: Add roundrobin raid1 read policy
Date: Fri, 12 Feb 2021 17:12:58 +0000 [thread overview]
Message-ID: <20210212171246.GA20817@wotan.suse.de> (raw)
In-Reply-To: <20210211155533.GB1263@wotan.suse.de>
Hi Anand,
re: inflight calculation
On Thu, Feb 11, 2021 at 03:55:33PM +0000, Michal Rostecki wrote:
> > It is better to have random workloads in the above three categories
> > of configs.
> >
> > Apart from the above three configs, there is also
> > all-non-rotational with hetero
> > For example, ssd and nvme together both are non-rotational.
> > And,
> > all-rotational with hetero
> > For example, rotational disks with different speeds.
> >
> >
> > The inflight calculation is local to btrfs. If the device is busy due to
> > external factors, it would not switch to the better performing device.
> >
>
> Good point. Maybe I should try to use the part stats instead of storing
> inflight locally in btrfs.
I tried today to reuse the inflight calculation which is done for
iostat. And I came to conclusion that it's impossible without exporting
some methods from the block/ subsystem.
The thing is that there are two methods of calculating inflight. Which
one of them is used, depends on queue_is_mq():
https://github.com/kdave/btrfs-devel/blob/9d294a685fbcb256ce8c5f7fd88a7596d0f52a8a/block/genhd.c#L1163
And if that condition is true, I noticed that part_stats return 0, even
though there are processed requests (I checked with fio inside VM).
In general, those two methods of checking inflight are:
1) part_stats - which would be trivial to reuse, just a matter of one
simple for_each_possible_cpu() loop with part_stat_local_read_cpu()
https://github.com/kdave/btrfs-devel/blob/9d294a685fbcb256ce8c5f7fd88a7596d0f52a8a/block/genhd.c#L133-L146
2) blk_mq_in_flight() - which has a lot of code and unexported
structs inside the block/ directory, double function callback;
definitely not easy to reimplement easily in btrfs without copying
dozens of lines
https://github.com/kdave/btrfs-devel/blob/9d294a685fbcb256ce8c5f7fd88a7596d0f52a8a/block/blk-mq.c#L115-L123
Well, I tried copying the whole blk_mq_in_flight() function with all
dependencies anyway, hard to do without causing modpost errors.
So, to sum it up, I think that making 2) possible to reuse in btrfs
would require at lest exporting the blk_mq_in_flight() function,
therefore the change would have to go through linux-block tree. Which
maybe would be a good thing to do in long term, not sure if it should
block my patchset entirely.
The question is if we are fine with inflight stats inside btrfs.
Personally I think we sholdn't be concerned too much about it. The
inflight counter in my patches is a percpu counted, used in places which
already do some atomic operations.
Thanks,
Michal
next prev parent reply other threads:[~2021-02-12 17:14 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-09 20:30 [PATCH RFC 0/6] Add roundrobin raid1 read policy Michal Rostecki
2021-02-09 20:30 ` [PATCH RFC 1/6] btrfs: Add inflight BIO request counter Michal Rostecki
2021-02-09 20:30 ` [PATCH RFC 2/6] btrfs: Store the last device I/O offset Michal Rostecki
2021-02-09 20:30 ` [PATCH RFC 3/6] btrfs: Add stripe_physical function Michal Rostecki
2021-02-09 20:30 ` [PATCH RFC 4/6] btrfs: Check if the filesystem is has mixed type of devices Michal Rostecki
2021-02-10 4:08 ` Michał Mirosław
2021-02-10 12:50 ` Michal Rostecki
2021-02-12 18:26 ` Michal Rostecki
2021-02-12 23:36 ` Michał Mirosław
2021-02-15 14:40 ` Michal Rostecki
2021-02-10 10:09 ` Filipe Manana
2021-02-10 12:55 ` Michal Rostecki
2021-02-09 20:30 ` [PATCH RFC 5/6] btrfs: sysfs: Add directory for read policies Michal Rostecki
2021-02-13 10:19 ` Greg KH
2021-02-15 14:35 ` Michal Rostecki
2021-02-15 14:59 ` Greg KH
2021-02-09 20:30 ` [PATCH RFC 6/6] btrfs: Add roundrobin raid1 read policy Michal Rostecki
2021-02-10 4:24 ` Michał Mirosław
2021-02-10 12:29 ` Michal Rostecki
2021-02-10 12:58 ` Michał Mirosław
2021-02-10 19:23 ` Michal Rostecki
2021-02-11 2:27 ` Michał Mirosław
2021-02-11 12:35 ` Michal Rostecki
2021-02-10 8:20 ` Anand Jain
2021-02-11 15:55 ` Michal Rostecki
2021-02-12 17:12 ` Michal Rostecki [this message]
2021-02-10 6:52 ` [PATCH RFC 0/6] " Anand Jain
2021-02-10 12:18 ` Michal Rostecki
2021-02-10 14:00 ` Michal Rostecki
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