linux-ext4 - Re: Atomic non-durable file write API

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Message-ID: <AANLkTimNK0ku5tBrA0DbSrNZe1jO6LcOobVVMOW2Kmxa@mail.gmail.com>
Date:	Sun, 26 Dec 2010 16:23:08 +0100
From:	Olaf van der Spek <olafvdspek@...il.com>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Dave Chinner <david@...morbit.com>, "Ted Ts'o" <tytso@....edu>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: Atomic non-durable file write API

On Sun, Dec 26, 2010 at 10:59 AM, Amir Goldstein <amir73il@...il.com> wrote:
> Whether or not the new data is durable is entirely up to the
> programmer to decide.

Right

> Perhaps all that is missing is an fdatawait(fd) API or fdatawait_async(fd) API,
> which should be very simple to implement (right?).
>
> So an editor that doesn't want to be too pushy, will just save a temp file,
> wait for it to sync on the system's free time and then swap the data with
> the original file in an atomic manner, which preserves metadata.
>
> If the system takes too long to sync, the editor can always issue
> fdatasync(fd) when it is tired of waiting.
>
> Will that make you happy, Olaf?
> If you are not happy from the new copy not being available to all
> system users until
> fdatasync is done, then we will call it "atomic, isolated non-durable
> file write API", OK?

No. Take the compiler case. Ideally you'd like file data updates to be
atomic, but waiting until an update hits disk before it's visible to
other processes is unacceptable.

IMO the use case for atomic non-durable file writes is very broad, so
you don't want to have these kinds of exceptions.

Olaf
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