linux-ext4 - Re: [PATCH 1/2] e2image: truncate raw image file to correct size

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Message-ID: <4F3D8D01.5040604@ubuntu.com>
Date:	Thu, 16 Feb 2012 18:10:57 -0500
From:	Phillip Susi <psusi@...ntu.com>
To:	Ted Ts'o <tytso@....edu>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2] e2image: truncate raw image file to correct size

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On 02/16/2012 05:58 PM, Ted Ts'o wrote:
> I don't see the bug here.  If there are no leftover sparse bytes,
> there's no need to write the last zero byte.  The whole point was to
> make sure i_size was set correctly, and if sparse==0, then i_size is
> correct.

- From what I can see, when sparse == 0, the last write does a seek to move
the file pointer, but doesn't write anything beyond the last hole, so i_size
is not updated.  This resulted in an image file I took of a 20gb fs being 124
MiB too small.  I can only assume that this is to be expected, and is the
reason for passing one byte of zero_buff to write_block instead of not giving
it any bytes to write, and just asking it to do the seek the way the loop does.

> ftruncate() happens to work today for Linux, but it's not guaranteed
> to do anything on all operating systems or even all file systems.  Per
> the standards spec:
> 
> 	If the file previously was smaller than this size, ftruncate()
> 	shall either increase the size of the file or fail.
> 
> Speaking of which, you're not checking the return value from ftruncate
> in your patch.  So I'd be happy if you checked ftruncate, and if it
> failed, falling back to the
> 
> 	if (sparse)
> 		write_block(fd, zero_buf, sparse-1, 1, -1);
> 
> code path.  That way, if ftruncate() happens to fail on NFS, or ceph,
> or some random file system that chose to meet the standards spec, but
> by failing if someone tries to increase the size of the file using
> ftruncate.  (Or some other OS; there are other operating systems,
> including GNU Hurd and BSD, and I don't know for sure how ftruncate
> behaves on all of those other OS's and file systems.)

Good idea.

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