linux-kernel - Re: Zero length files - an alternative approach?

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Message-ID:  <yw1xab744lqi.fsf@thrashbarg.mansr.com>
Date:	Sun, 29 Mar 2009 12:22:29 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-ext4@...r.kernel.org
Subject:  Re: Zero length files - an alternative approach?

Graham Murray <graham@...rray.org.uk> writes:

> Just a thought on the ongoing discussion of dataloss with ext4 vs ext3.
>
> Taking the common scenario:
> Read oldfile
> create newfile file
> write newfile data
> close newfile
> rename newfile to oldfile
>
> When using this scenario, the application writer wants to ensure that
> either the old or new content are present. With delayed allocation, this
> can lead to zero length files. Most of the suggestions on how to address
> this have involved syncing the data either before the rename or making
> the rename sync the data.
>
> What about, instead of 'bringing forward' the allocation and flushing of
> the data, would it be possible to instead delay the rename until after
> the blocks for newfile have been allocated and the data buffers flushed?
> This would keep the performance benefits of delayed allocation etc and
> also satisfy the applications developers' apparent dislike of using
> fsync(). It would give better performance that syncing the data at
> rename time (either using fsync() or automatically) and satisfy the
> requirements that either the old or new content is present.

Consider this scenario:

1. Create/write/close newfile
2. Rename newfile to oldfile
3. Open/read oldfile.  This must return the new contents.
4. System crash and reboot before delayed allocation/flush complete
5. Open/read oldfile.  Old contents now returned.

This rollback isn't obviously, to me at least, without problems of its
own.

-- 
Måns Rullgård
mans@...sr.com

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