linux-ext4 - Re: [PATCH 0/1] iomap regression for aio dio 4k writes

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Date:   Thu, 22 Jun 2023 22:32:33 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jeremy Bongio <bongiojp@...il.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Allison Henderson <allison.henderson@...cle.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 0/1] iomap regression for aio dio 4k writes

On Thu, Jun 22, 2023 at 09:59:29AM +1000, Dave Chinner wrote:
> Ah, you are testing pure overwrites, which means for ext4 the only
> thing it needs to care about is cached mappings. What happens when
> you add O_DSYNC here?

I think you mean O_SYNC, right?  In a pure overwrite case, where all
of the extents are initialized and where the Oracle or DB2 server is
doing writes to preallocated, pre-initialized space in the tablespace
file followed by fdatasync(), there *are* no post-I/O data integrity
operations which are required.

If the file is opened O_SYNC or if the blocks were not preallocated
using fallocate(2) and not initialized ahead of time, then sure, we
can't use this optimization.

However, the cases where databases workloads *are* doing overwrites
and using fdatasync(2) most certainly do exist, and the benefit of
this optimization can be a 20% throughput.  Which is nothing to sneeze
at.

What we might to do is to let the file system tell the iomap layer via
a flag whether or not there are no post-I/O metadata operations
required, and then *if* that flag is set, and *if* the inode has no
pages in the page cache (so there are no invalidate operations
necessary), it should be safe to skip using queue_work().  That way,
the file system has to affirmatively state that it is safe to skip the
workqueue, so it shouldn't do any harm to other file systems using the
iomap DIO layer.

What am I missing?

Cheers,

						- Ted