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

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Message-ID: <AANLkTimbkstru_nUxnd7R8Zg=ioB3skTntedq_dLxpZm@mail.gmail.com>
Date:	Sun, 19 Dec 2010 17:39:01 +0100
From:	Olaf van der Spek <olafvdspek@...il.com>
To:	Calvin Walton <calvin.walton@...il.com>
Cc:	Ric Wheeler <ricwheeler@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: Atomic non-durable file write API

On Sat, Dec 18, 2010 at 11:15 PM, Calvin Walton <calvin.walton@...il.com> wrote:
> Hmm. I’m doing a little interpretation of what Olaf said here; but I
> think you may have misunderstood the question?
>
> He doesn’t care about whether or not the file is securely written to
> disk (durable); however he doesn’t want to see any partially written
> files. In other words, something like
>
>     1. Write to temp file
>     2. Rename temp file over original file

Meta data, including file owner, should be preserved.
Ideally, no temp files should be visible either.

> Where the rename is only committed to disk once the entire contents of
> the file have been written securely – whenever that may eventually
> happen.
>
> He doesn’t want to synchronously wait for the file to be written,
> because the new data isn’t particularly important. The only important
> thing is that the file either contains the old or new data after a
> filesystem crash; not incomplete data. So, it’s more of an ordering
> problem, I think? (Analogous to putting some sort of barrier between the
> file write/close and the file rename to maintain ordering.)
>
> Hopefully I’ve interpreted the original question correctly, because this
> is something I would find interesting as well.

Yes, you did.

Olaf
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