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Re: [PATCH 1/6] fs: add hole punching to fallocate

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate
From: Ric Wheeler <rwheeler@xxxxxxxxxx>
Date: Fri, 28 Jan 2011 13:13:16 -0500
Cc: Lawrence Greenfield <leg@xxxxxxxxxx>, "Ted Ts'o" <tytso@xxxxxxx>, Josef Bacik <josef@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, joel.becker@xxxxxxxxxx, cmm@xxxxxxxxxx, cluster-devel@xxxxxxxxxx
In-reply-to: <20110112124431.GP28803@dastard>
References: <1289248327-16308-1-git-send-email-josef@xxxxxxxxxx> <20101109011222.GD2715@dastard> <20101109033038.GF3099@xxxxxxxxx> <20101109044242.GH2715@dastard> <20101109214147.GK3099@xxxxxxxxx> <20101109234049.GQ2715@dastard> <AANLkTimwmJ_ZoE9oAuA1WGhCgK585jDznqnc6k0=9Ntb@xxxxxxxxxxxxxx> <20110112124431.GP28803@dastard>
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On 01/12/2011 07:44 AM, Dave Chinner wrote:
On Tue, Jan 11, 2011 at 04:13:42PM -0500, Lawrence Greenfield wrote:
On Tue, Nov 9, 2010 at 6:40 PM, Dave Chinner<david@xxxxxxxxxxxxx>  wrote:
The historical reason for such behaviour existing in XFS was that in
1997 the CPU and IO latency cost of unwritten extent conversion was
significant,
.....

(Take for example a trusted cluster filesystem backend that checks the
object checksum before returning any data to the user; and if the
check fails the cluster file system will try to use some other replica
stored on some other server.)
IOWs, all they want to do is avoid the unwritten extent conversion
overhead. Time has shown that a bad security/performance tradeoff
decision was made 13 years ago in XFS, so I see little reason to
repeat it for ext4 today....
I'd make use of FALLOC_FL_EXPOSE_OLD_DATA. It's not the CPU overhead
of extent conversion. It's that extent conversion causes more metadata
operations than what you'd have otherwise,
Yes, that's the "IO latency" part of the cost I mentioned above.

which means systems that
want to use O_DIRECT and make sure the data doesn't go away either
have to write O_DIRECT|O_DSYNC or need to call fdatasync().
Seriously, we tell application writers _all the time_ that they
*must* use fsync/fdatasync to guarantee their data is on stable
storage and that they cannot rely on side-effects of filesystem or
storage specific behaviours (like ext3 ordered mode) to do that job
for them.

You're suggesting that by introducing FALLOC_FL_EXPOSE_OLD_DATA,
applications can rely on filesystem/storage specific behaviour to
guarantee data is on stable storage without the use of
fdatasync/fsync. Wht you describe is definitely storage specific,
because volatile write caches still needs the fdatasync to issue a
cache flush.

Do you see the same conflict here that I do?


The very concept seems quite "non-enterprise". I also agree that the cost of maintaining extra mount options (and code) for something that no sane end user would ever do seems to be a loss.

Why wouldn't you want to convert the punched hole to an unwritten extent?

Thanks!

Ric

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