Date:	Sat, 05 Sep 2009 00:56:16 +0800
From:	Peng Tao <bergwolf@...il.com>
To:	Greg Freemyer <greg.freemyer@...il.com>
CC:	linux-ext4@...r.kernel.org, Theodore Tso <tytso@....edu>,
	Akira Fujita <a-fujita@...jp.nec.com>
Subject: Re: [PATCH] e4defrag: fallocate donor file only once

Greg Freemyer wrote:
> On Thu, Sep 3, 2009 at 11:08 PM, Peng Tao<bergwolf@...il.com> wrote:
>> On Thu, Sep 3, 2009 at 5:30 PM, Greg Freemyer<greg.freemyer@...il.com> wrote:
>>> On Thu, Sep 3, 2009 at 4:00 AM, Peng Tao<bergwolf@...il.com> wrote:
>>>> On Thu, Sep 3, 2009 at 6:09 AM, Greg Freemyer<greg.freemyer@...il.com> wrote:
>>>>> On Wed, Sep 2, 2009 at 11:35 AM, Peng Tao<bergwolf@...il.com> wrote:
>>>>>> If we allocate the donor file once for all, it will have a better chance
>>>>>> to be continuous.
>>>>>>
>>>>>> Signed-off-by: "Peng Tao" <bergwolf@...il.com>
>>>>> Seems like an improvement, but I'm not seeing any special handling for
>>>>> sparse files.  (Not before or after this patch.)
>>>>>
>>>>> Seems like there should be an outer loop that identifies contiguous
>>>>> data block sets in a sparse file and defrags them individually as
>>>>> opposed to trying to defrag the entire file at once.
>>>>>
>>>>> My impression is that with a large sparse file, e4defrag currently
>>>>> (with or without this patch) would fallocate a full non-sparse donor
>>>>> set of blocks the full size of the original file, then swap in just
>>>>> the truly allocated blocks?
>>>> Thanks for the reminder. The original code takes good care of sparse
>>>> files in join_extents(). Please ignore my patch.
>>>>
>>>> Sorry for the noise.
>>> RFC from a more ext4 knowledgeable person than me:
>>>
>>> The code in e4defrag still looks way to complex.  I don't see why it
>>> needs to know so much about extents and groups.
>>>
>>> I just looked at util/copy_sparse.c
>>>
>>> It simply loops through all the blocks in the source file and calls
>>> ioctl(fd, FIBMAP, &b) to see if they are allocated vs. sparse,
>>>
>>> If allocated it copies the block from src to dest.  Pretty straight
>>> forward and has none of the complexity of e4defrag.
>>>
>>> Seems to me e4defrag should have the actual defrag_file() rewritten to
>>> be something like:
>>>
>>> defrag_file()  {
>>>    loop through the blocks looking for the contiguous set of data blocks.
>>>          defrag_contiguous_data(start_block, num_blocks)
>>> }
>>>
>>> defrag_contiguous_data(start_block, num_blocks) {
>>>    // allocate one full extent at a time and donate the blocks to orig file
>>>    for(start=start_block; start < start_block, num_blocks; start+=chunk) {
>>>          fallocate(chunk);
>>>          move_ext(orig, donor, start, 0, chunk);
>>>      }
>>> }
>>>
>>> And then set chunk to be the max size of one extent.  Maybe the
>>> "chunk" should be bigger than one extent?
>>>
>>> Also, I did not put any logic in above to show testing to see if the
>>> new file is less fragmented than the original.  That will add to the
>>> complexity, but hopefully the actual defrag logic can be as relatively
>>> simple as the above instead of what it is now.
>>>
>>> Anyway, t would be a major change to e4defrag, but it seems that it
>>> would give ext4 a much better chance to reorganize itself by calling
>>> fallocate on full extent size chunks at minimum, instead of what the
>>> code currently does.
>> Hi, Greg,
>>
>> The current e4defrag is doing most of work exactly same as your RFC,
>> and in a nicer manner. If you look into the code path, you'll see that
>> its logic is very much like the RFC except that it first fallocates a
>> donor file to see if a defragmentation is really necessary so it won't
>> have to fall back during defragmentation, which IMO is a good design
>> point.
>>
>> Please correct me if I understand anything wrong.
> 
> I've looked a lot more at the current code.  I'm pretty sure this is right:
> 
> First, assume defrag of a non-sparse 1TB file.
> 
> The current code will walk the extent tree and create a single extent
> group that covers the full 1TB, then call fallocate to try to get 1TB
> of donor blocks.  Then compare the number of extents in the original
> and the donor.  If the donor has less it will swap in the donor
> blocks.
> 
> It seems much smarter work on extent size chunks (or whatever best
> fits the kernels block structure.
> 
> ie.
> 
> for (start_block=0; start_block < max_blocks; start_block+=
> max_blocks_in_extent)
> 
>       current_extents = num_extents_in_block_range(start_block,
> start+max_blocks_in_extent);
> 
>       if (current_extents == 1) continue;
> 
>       // allocate a sparse file with perfectly aligned donor blocks as
> currently required by kernel
>       fallocate(start_block * block_size, max_blocks_in_extent * block_size);
> 
>       donor_extents = num_extents_in_block_range(start_block,
> start+max_blocks_in_extent);
> 
>      if (donor_extents < current_extents)
>             donate_donor_blocks_to_orig(start_block,
> start+max_blocks_in_extent);
> 
> )
> 
> And in the case of a sparse file, it seems much easier to understand
> if the above is called on each logically contiguous set or data
> blocks.  Seriously, why bother the kernel by making it able to accept
> a block range that has holes in it.
Agreed. If the kernel doesn't have to deal with holes, the EXT4_IOC_MOVE_EXT
ioctl can be much simplified.
> 
> It seems reasonable for the kernel to check the block range being
> passed in and if the orig files has a hole in the middle of it, then
> return an error.
> 
> Back to e4defrag, even if the code is not greatly simplified, the
> above seems like it would use far less resources than the current
> code.   Think about a large file that has the first 90% of the blocks
> defrag'ed.  The above would cause just the tail to be defrag'ed, not
> the entire file.
Yes, it makes sense. Are you planning some patch for above changes?
> 
> Greg
> 


-- 
Best Regards,
Peng Tao
State Key Laboratory of Networking and Switching Technology
Beijing Univ. of Posts and Telecoms.
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