Zero-filled blocks in HAMMER

[Matthew Dillon]

Urk.,  Sorry.  I got H2 and H1 mixed up.  Hammer2 does the zero testing.
Hammer1... actually might not.  But Hammer1 can do DEDUP and will DEDUP all
the zero'd blocks into never-never land.

Originally the intention was to support it in H1 via live_dedup, but live
dedup was disabled long ago (because it caused corruption and I gave up
trying to find out why).  Batch dedup will still catch the zerod blocks
though so eventually they won't take up any real space.

-Matt

On Mon, Aug 29, 2016 at 11:39 AM, Matthew Dillon <dillon at backplane.com>
wrote:

> For any copy-on-write filesystem there is no point pre-zeroing a file to
> 'allocate' its blocks, because any overwrite will reallocate the block.
> Thus, the expectation is that most copy-on-write filesystems will detect
> blocks which contain all-zeros and not bother to actually allocate disk
> space for such blocks, instead just leaving a 'hole' in the file (which
> reads as zeros when the file is read).  (If writing all zeros into a block
> which was previously non-zero, Hammer simply removes the block from the
> B+Tree to create a hole).
>
> In the original UFS, writing zeros would actually write blocks full of
> zeros into the file, but this was desireable because UFS is not a
> copy-on-write filesystem.  So the concept of preallocating space works.
> UFS also always supported file holes via lseek()/write() to skip the 'hole'
> you want to leave, as do numerous earlier filesystems.  So 'Sparse Files'
> is actually a very old concept.
>
> -Matt
>
>
> On Sun, Aug 28, 2016 at 9:25 AM, Tomohiro Kusumi <
> kusumi.tomohiro at gmail.com> wrote:
>
>> This might be off topic as I brought this from tux3 ml archive,
>> but what does "FS converted zerod blocks to hole like hammerfs"
>> supposed to mean here ?
>>
>> Is this talking about sparse file ?
>> (and do BSDs including DragonFly even support sparse file ?)
>>
>> http://phunq.net/pipermail/tux3/2015-August/002327.html
>> > Also, if FS converted zerod blocks to hole like hammerfs, simply ENOSPC
>> happens.
>>
>>
>> This is the result of
>> # dd if=/dev/zero of=xxx bs=16384 count=10
>> against a newly created fs, and there are 10 zero-filled data records,
>> which is what I was expecting to see in fs level from the way hammer
>> is implemented.
>>
>> --
>> HAMMER zone statistics
>>         zone #  blocks       items              used[B]
>>  used[%]
>>         zone 0  0            0                  0                   0
>>         zone 1  0            0                  0                   0
>>         zone 2  0            0                  0                   0
>>         zone 3  0            0                  0                   0
>>         zone 4  0            0                  0                   0
>>         zone 5  0            0                  0                   0
>>         zone 6  0            0                  0                   0
>>         zone 7  0            0                  0                   0
>>         zone 8  1            1                  4096
>> 0.0488281
>>         zone 9  1            4                  572
>>  0.00681877
>>         zone 10 1            10                 163840
>> 1.95312
>>         zone 11 0            0                  0                   0
>>         zone 12 0            0                  0                   0
>>         zone 13 0            0                  0                   0
>>         zone 14 0            0                  0                   0
>>         zone 15 0            0                  0                   0
>>         ------------------------------------------------------------
>> ----------
>>         total   3            15                 168508
>> 0.669591
>>
>
>
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