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Re: sparsify - utility to punch out blocks of 0s in a file

To: Eric Sandeen <sandeen@xxxxxxxxxx>
Subject: Re: sparsify - utility to punch out blocks of 0s in a file
From: Michael Tokarev <mjt@xxxxxxxxxx>
Date: Mon, 06 Feb 2012 03:44:43 +0400
Cc: ext4 development <linux-ext4@xxxxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <4F2D90B6.4070008@xxxxxxxxxx>
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References: <4F2D8F30.3090802@xxxxxxxxxx> <4F2D90B6.4070008@xxxxxxxxxx>
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On 05.02.2012 00:10, Eric Sandeen wrote:
[]

Just a very quick look:

>  * sparsify - utility to punch out blocks of 0s in a file
> int main(int argc, char **argv)
> {
[]
>       if (optind == argc) {
>               printf("Error: no filename specified\n");
>               usage();
>       }
> 
>       fname = argv[optind++];

There's no handling of the case when there are more than one file
specified on the command line.


>       /*
>        * Normalize to blocksize-aligned range:
>        * round start down, round end up - get all blocks including the range 
> specified
>        */
> 
>       punch_range_start = round_down(punch_range_start, blocksize);
>       punch_range_end = round_up(punch_range_end, blocksize);
>       min_hole = round_up(min_hole, blocksize);
>       if (!min_hole)
>               min_hole = blocksize;

I think this deserves some bold warning if punch_range_start
or punch_hole_end is not a multiple of blocksize.

[]
>       /*
>        * Read through the file, finding block-aligned regions of 0s.
>        * If the region is at least min_hole, punch it out.
>        * This should be starting at a block-aligned offset
>        */
> 
>       while ((ret = read(fd, readbuf, min_hole)) > 0) {
> 
>               if (!memcmp(readbuf, zerobuf, min_hole)) {

Now this is interesting.  Can ret be < min_hole?  Can a read
in a middle of a file be shorter than specified?

How it will work together with some other operation being done
at the same file -- ftruncate anyone?

Thanks!

/mjt

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