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

To: Michael Tokarev <mjt@xxxxxxxxxx>
Subject: Re: sparsify - utility to punch out blocks of 0s in a file
From: Eric Sandeen <sandeen@xxxxxxxxxx>
Date: Sun, 05 Feb 2012 17:55:41 -0600
Cc: ext4 development <linux-ext4@xxxxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <4F2F146B.6050003@xxxxxxxxxxxxxxxx>
References: <4F2D8F30.3090802@xxxxxxxxxx> <4F2D90B6.4070008@xxxxxxxxxx> <4F2F146B.6050003@xxxxxxxxxxxxxxxx>
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On 2/5/12 5:44 PM, Michael Tokarev wrote:
> 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.

ok

> 
>>      /*
>>       * 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.

well, we can only punch on block boundaries.  But I suppose I should swap
round_up and round_down, so that we never punch anything that isn't *inside*
the specified range.

> []
>>      /*
>>       * 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?

yes, and yes (but unlikely i think)...


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

I probably have some boundary condition & error checking to do yet :)

Thanks for the review,
-Eric
 
> Thanks!
> 
> /mjt

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