Re: [PATCH 0/9] xfs file non-exclusive online defragment

[Wengang Wang <wen.gang.wang@oracle.com>]

> On Dec 14, 2023, at 7:15 PM, Dave Chinner <david@fromorbit.com> wrote:
> 
> On Thu, Dec 14, 2023 at 01:35:02PM -0800, Darrick J. Wong wrote:
>> On Thu, Dec 14, 2023 at 09:05:21AM -0800, Wengang Wang wrote:
>>> Background:
>>> We have the existing xfs_fsr tool which do defragment for files. It has the
>>> following features:
>>> 1. Defragment is implemented by file copying.
>>> 2. The copy (to a temporary file) is exclusive. The source file is locked
>>>   during the copy (to a temporary file) and all IO requests are blocked
>>>   before the copy is done.
>>> 3. The copy could take long time for huge files with IO blocked.
>>> 4. The copy requires as many free blocks as the source file has.
>>>   If the source is huge, say it’s 1TiB,  it’s hard to require the file
>>>   system to have another 1TiB free.
>>> 
>>> The use case in concern is that the XFS files are used as images files for
>>> Virtual Machines.
>>> 1. The image files are huge, they can reach hundreds of GiB and even to TiB.
>>> 2. Backups are made via reflink copies, and CoW makes the files badly fragmented.
>>> 3. fragmentation make reflink copies super slow.
>>> 4. during the reflink copy, all IO requests to the file are blocked for super
>>>   long time. That makes timeout in VM and the timeout lead to disaster.
>>> 
>>> This feature aims to:
>>> 1. reduce the file fragmentation making future reflink (much) faster and
>>> 2. at the same time,  defragmentation works in non-exclusive manner, it doesn’t
>>>   block file IOs long.
>>> 
>>> Non-exclusive defragment
>>> Here we are introducing the non-exclusive manner to defragment a file,
>>> especially for huge files, without blocking IO to it long. Non-exclusive
>>> defragmentation divides the whole file into small pieces. For each piece,
>>> we lock the file, defragment the piece and unlock the file. Defragmenting
>>> the small piece doesn’t take long. File IO requests can get served between
>>> pieces before blocked long.  Also we put (user adjustable) idle time between
>>> defragmenting two consecutive pieces to balance the defragmentation and file IOs.
>>> So though the defragmentation could take longer than xfs_fsr,  it balances
>>> defragmentation and file IOs.
>> 
>> I'm kinda surprised you don't just turn on alwayscow mode, use an
>> iomap_funshare-like function to read in and dirty pagecache (which will
>> hopefully create a new large cow fork mapping) and then flush it all
>> back out with writeback.  Then you don't need all this state tracking,
>> kthreads management, and copying file data through the buffer cache.
>> Wouldn't that be a lot simpler?
> 
> Hmmm. I don't think it needs any kernel code to be written at all.
> I think we can do atomic section-by-section, crash-safe active file
> defrag from userspace like this:
> 
> scratch_fd = open(O_TMPFILE);
> defrag_fd = open("file-to-be-dfragged");
> 
> while (offset < target_size) {
> 
> /*
>  * share a range of the file to be defragged into
>  * the scratch file.
>  */
> args.src_fd = defrag_fd;
> args.src_offset = offset;
> args.src_len = length;
> args.dst_offset = offset;
> ioctl(scratch_fd, FICLONERANGE, args);
> 
> /*
>  * For the shared range to be unshared via a
>  * copy-on-write operation in the file to be
>  * defragged. This causes the file needing to be
>  * defragged to have new extents allocated and the
>  * data to be copied over and written out.
>  */
> fallocate(defrag_fd, FALLOC_FL_UNSHARE_RANGE, offset, length);
> fdatasync(defrag_fd);
> 
> /*
>  * Punch out the original extents we shared to the
>  * scratch file so they are returned to free space.
>  */
> fallocate(scratch_fd, FALLOC_FL_PUNCH, offset, length);
> 
> /* move onto next region */
> offset += length;
> };
> 
> As long as the length is large enough for the unshare to create a
> large contiguous delalloc region for the COW, I think this would
> likely acheive the desired "non-exclusive" defrag requirement.
> 
> If we were to implement this as, say, and xfs_spaceman operation
> then all the user controlled policy bits (like inter chunk delays,
> chunk sizes, etc) then just becomes command line parameters for the
> defrag command...


Ha, the idea from user space is very interesting!
So far I have the following thoughts:
1). If the FICLONERANGE/FALLOC_FL_UNSHARE_RANGE/FALLOC_FL_PUNCH works on a FS without reflink
     enabled.
2). What if there is a big hole in the file to be defragmented? Will it cause block allocation and writing blocks with
    zeroes.
3). In case a big range of the file is good (not much fragmented), the ‘defrag’ on that range is not necessary.
4). The use space defrag can’t use a try-lock mode to make IO requests have priorities. I am not sure if this is very important.

Maybe we can work with xfs_bmap to get extents info and skip good extents and holes to help case 2) and 3).

I will figure above out.
Again, the idea is so amazing, I didn’t reallize it.

Thanks,
Wengang