Re: [RFC][PATCH 0/3] add FALLOC_FL_NO_HIDE_STALE flag in fallocate

[Eric Sandeen <sandeen@redhat.com>]

On 4/17/12 11:59 PM, Andreas Dilger wrote:

...

> Secondly, your test program is not doing random writes to disk, but
> rather doing writes at 64kB intervals. There is logic in the
> uninitialized extent handling that will write zeros to an entire
> extent, rather than create many fragmented uninitialized extents.  It
> may be possible that you are zeroing out the entire file, and writing
> 16x as much data as you expect.
> 
> Cheers, Andreas

I don't think the testcase as written is triggering that behavior, though
other similar testcases might.  In this case the left-over uninit extents
are large enough that they don't get zeroed:

File size of /mnt/scratch/test is 268435456 (65536 blocks, blocksize 4096)
 ext logical physical expected length flags
   0       0    34816               1 
   1       1    34817              15 unwritten
   2      16    34832               1 
   3      17    34833              15 unwritten
   4      32    34848               1 
   5      33    34849              15 unwritten
...

Good guess though :)

-Eric