Date:	Fri, 4 Sep 2009 15:10:45 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Peng Tao <bergwolf@...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

On Fri, Sep 4, 2009 at 12:56 PM, Peng Tao<bergwolf@...il.com> wrote:
> 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?

I'm "planning", but I doubt that I get to it for a few weeks.  If you
or someone else has time, that would be great.

Greg
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