linux-kernel - Re: [PATCH v2] mm: implement write-behind policy for sequential file writes

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Date:   Fri, 20 Sep 2019 16:05:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2] mm: implement write-behind policy for sequential file writes

On Fri, Sep 20, 2019 at 12:35 AM Konstantin Khlebnikov
<khlebnikov@...dex-team.ru> wrote:
>
> This patch implements write-behind policy which tracks sequential writes
> and starts background writeback when file have enough dirty pages.

Apart from a spelling error ("contigious"), my only reaction is that
I've wanted this for the multi-file writes, not just for single big
files.

Yes, single big files may be a simpler and perhaps the "10% effort for
90% of the gain", and thus the right thing to do, but I do wonder if
you've looked at simply extending it to cover multiple files when
people copy a whole directory (or unpack a tar-file, or similar).

Now, I hear you say "those are so small these days that it doesn't
matter". And maybe you're right. But partiocularly for slow media,
triggering good streaming write behavior has been a problem in the
past.

So I'm wondering whether the "writebehind" state should perhaps be
considered be a process state, rather than "struct file" state, and
also start triggering for writing smaller files.

Maybe this was already discussed and people decided that the big-file
case was so much easier that it wasn't worth worrying about
writebehind for multiple files.

            Linus