Re: [PATCH RFC 0/5] fs, ext4: Physical blocks placement hint for fallocate(0): fallocate2(). TP defrag.

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 11.03.2020 22:26, Andreas Dilger wrote:
> On Mar 3, 2020, at 2:57 AM, Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>> On 02.03.2020 19:56, Theodore Y. Ts'o wrote:
>>> Kirill,
>>>
>>> In a couple of your comments on this patch series, you mentioned
>>> "defragmentation".  Is that because you're trying to use this as part
>>> of e4defrag, or at least, using EXT4_IOC_MOVE_EXT?
>>>
>>> If that's the case, you should note that input parameter for that
>>> ioctl is:
>>>
>>> struct move_extent {
>>> 	__u32 reserved;		/* should be zero */
>>> 	__u32 donor_fd;		/* donor file descriptor */
>>> 	__u64 orig_start;	/* logical start offset in block for orig */
>>> 	__u64 donor_start;	/* logical start offset in block for donor */
>>> 	__u64 len;		/* block length to be moved */
>>> 	__u64 moved_len;	/* moved block length */
>>> };
>>>
>>> Note that the donor_start is separate from the start of the file that
>>> is being defragged.  So you could have the userspace application
>>> fallocate a large chunk of space for that donor file, and then use
>>> that donor file to defrag multiple files if you want to close pack
>>> them.
>>
>> The practice shows it's not so. Your suggestion was the first thing we tried,
>> but it works bad and just doubles/triples IO.
>>
>> Let we have two files of 512Kb, and they are placed in separate 1Mb clusters:
>>
>> [[512Kb file][512Kb free]][[512Kb file][512Kb free]]
>>
>> We want to pack both of files in the same 1Mb cluster. Packed together on block
>> device, they will be in the same server of underlining distributed storage file
>> system. This gives a big performance improvement, and this is the price I aimed.
>>
>> In case of I fallocate a large hunk for both of them, I have to move them
>> both to this new hunk. So, instead of moving 512Kb of data, we will have to move
>> 1Mb of data, i.e. double size, which is counterproductive.
>>
>> Imaging another situation, when we have
>> [[1020Kb file]][4Kb free]][[4Kb file][1020Kb free]]
>>
>> Here we may just move [4Kb file] into [4Kb free]. But your suggestion again
>> forces us to move 1Mb instead of 4Kb, which makes IO 256 times worse! This is
>> terrible! And this is the thing I try prevent with finding a new interface.
> 
> One idea I had, which may work for your use case, is to run fallocate() on
> the *1MB-4KB file* to allocate the last 4KB in that hunk, then use that block
> as the donor file for the 1MB+4KB file.  The ext4 allocation algorithms should
> always give you that 4KB chunk if it is free, and that avoids the need to try
> and force the allocator to select that block through some other method.

Do you mean the following:

1)fallocate() 4K at the end of *1MB-4KB* the first file (==> this increases the file length).
2)EXT4_IOC_MOVE_EXT *4KB* the second file in that new hunk.
3)truncate 4KB at the end of the first file.

?

If so, this can't be an online defrag, since some process may want to increase *1MB-4KB*
file in between. This will just bring to data corruption.
Another problem is that power lose between 1 and 3 will result in that file length remain
*1MB* instead of *1MB-4KB*.

So, we still need some kernel support to implement this.