Re: Question about non asynchronous aio calls.

[Avi Kivity <avi@scylladb.com>]

On 10/08/2015 02:46 PM, Dave Chinner wrote:
> On Thu, Oct 08, 2015 at 11:23:07AM +0300, Gleb Natapov wrote:
>> On Thu, Oct 08, 2015 at 08:21:58AM +0300, Avi Kivity wrote:
>>>>>> I fixed something similar in ext4 at the time, FWIW.
>>>>> Makes sense.
>>>>>
>>>>> Is there a way to relax this for reads?
>>>> The above mostly only applies to writes. Reads don't modify data so
>>>> racing unaligned reads against other reads won't given unexpected
>>>> results and so aren't serialised.
>>>>
>>>> i.e. serialisation will only occur when:
>>>> 	- unaligned write IO will serialise until sub-block zeroing
>>>> 	  is complete.
>>>> 	- write IO extending EOF will serialis until post-EOF
>>>> 	  zeroing is complete
>>>
>>> By "complete" here, do you mean that a call to truncate() returned, or that
>>> its results reached the disk an unknown time later?
>>>
> No, I'm talking purely about DIO here. If you do write that
> starts beyond the existing EOF, there is a region between the
> current EOF and the offset the write starts at. i.e.
>
>     0             EOF            offset     new EOF
>     +dddddddddddddd+..............+nnnnnnnnnnn+
>
> It is the region between EOF and offset that we must ensure is made
> up of either holes, unwritten extents or fully zeroed blocks before
> allowing the write to proceed. If we have to zero allocated blocks,
> then we have to ensure that completes before the write can start.
> This means that when we update the EOF on completion of the write,
> we don't expose stale data in blocks that were between EOF and
> offset...

Thanks.  We found, experimentally, that io_submit(write_at_eof) followed 
by (without waiting) io_submit(write_at_what_would_be_the_new_eof) 
occasionally blocks.

So I guess we have to employ a train algorithm here and keep at most one 
aio in flight for append loads (which are very common for us).

>
>> I think Brian already answered that one with:
>>
>>    There are no such pitfalls as far as I'm aware. The entire AIO
>>    submission synchronization sequence triggers off an in-memory i_size
>>    check in xfs_file_aio_write_checks(). The in-memory i_size is updated in
>>    the truncate path (xfs_setattr_size()) via truncate_setsize(), so at
>>    that point the new size should be visible to subsequent AIO writers.
> Different situation as truncate serialises all IO. Extending the file
> via truncate also runs the same "EOF zeroing" that the DIO code runs
> above, for the same reasons.

Does that mean that truncate() will wait for inflight aios, or that new 
aios will wait for the truncate() to complete, or both?

>
>>>> 	- truncate/extent manipulation syscall is run
>>> Actually, we do call fallocate() ahead of io_submit() (in a worker thread,
>>> in non-overlapping ranges) to optimize file layout and also in the belief
>>> that it would reduce the amount of blocking io_submit() does.
> fallocate serialises all IO submission - including reads. Unlike
> truncate, however, it doesn't drain the queue of IO for
> preallocation so the impact on AIO is somewhat limited.
>
> Ideally you want to limit fallocate calls to large chunks at a time.
> If you have a 1:1 mapping of fallocate calls to write calls, then
> you're likely making things worse for the AIO submission path
> because you'll block reads as well as writes. Doing the allocation
> in the write submission path will not block reads, and only writes
> that are attempting to do concurrent allocations to the same file
> will serialise...

We have a 1:8 ratio (128K:1M), but that's just random numbers we guessed.

Again, not only for reduced xfs metadata, but also to reduce the amount 
of write amplification done by the FTL. We have a concurrent append 
workload on many files, and files are reclaimed out of order, so larger 
extends means less fragmentation for the FTL later on.

>
> If you want to limit fragmentation without adding and overhead on
> XFS for non-sparse files (which it sounds like your case), then the
> best thing to use in XFS is the per-inode extent size hints. You set
> it on the file when first creating it (or the parent directory so
> all children inherit it at create), and then the allocator will
> round out allocations to the size hint alignment and size, including
> beyond EOF so appending writes can take advantage of it....

We'll try that out.  That's fsxattr::fsx_extsize?

What about small files that are eventually closed, do I need to do 
anything to reclaim the preallocated space?

>
>>> A final point is discoverability.  There is no way to discover safe
>>> alignment for reads and writes, and which operations block io_submit(),
>>> except by asking here, which cannot be done at runtime.  Interfaces that
>>> provide a way to query these attributes are very important to us.
>> As Brian pointed statfs() can be use to get f_bsize which is defined as
>> "optimal transfer block size".
> Well, that's what posix calls it. It's not really the optimal IO
> size, though, it's just the IO size that avoids page cache RMW
> cycles. For direct IO, larger tends to be better, and IO aligned to
> the underlying geometry of the storage is even better. See, for
> example, the "largeio" mount option, which will make XFS report the
> stripe width in f_bsize rather than the PAGE_SIZE of the machine....
>

Well, random reads will still be faster with 512 byte alignment, yes? 
and for random writes, you can't just make those I/Os larger, you'll 
overwrite something.

So I read "optimal" here to mean "smallest I/O size that doesn't incur a 
penalty; but if you really need more data, making it larger will help".

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