Re: [RFC 0/3] block/file-posix: Work around XFS bug

[Max Reitz <mreitz@redhat.com>]

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On 29.10.19 12:48, Vladimir Sementsov-Ogievskiy wrote:
> 29.10.2019 11:50, Max Reitz wrote:
>> On 28.10.19 12:25, Vladimir Sementsov-Ogievskiy wrote:
>>> 28.10.2019 14:04, Kevin Wolf wrote:
>>>> Am 27.10.2019 um 13:35 hat Stefan Hajnoczi geschrieben:
>>>>> On Fri, Oct 25, 2019 at 11:58:46AM +0200, Max Reitz wrote:
>>
>> [...]
>>
>>>>>> (3) Drop handle_alloc_space(), i.e. revert c8bb23cbdbe32f.
>>>>>>       To my knowledge I’m the only one who has provided any benchmarks for
>>>>>>       this commit, and even then I was a bit skeptical because it performs
>>>>>>       well in some cases and bad in others.  I concluded that it’s
>>>>>>       probably worth it because the “some cases” are more likely to occur.
>>>>>>
>>>>>>       Now we have this problem of corruption here (granted due to a bug in
>>>>>>       the XFS driver), and another report of massively degraded
>>>>>>       performance on ppc64
>>>>>>       (https://bugzilla.redhat.com/show_bug.cgi?id=1745823 – sorry, a
>>>>>>       private BZ; I hate that :-/  The report is about 40 % worse
>>>>>>       performance for an in-guest fio write benchmark.)
>>>>>>
>>>>>>       So I have to ask the question about what the justification for
>>>>>>       keeping c8bb23cbdbe32f is.  How much does performance increase with
>>>>>>       it actually?  (On non-(ppc64+XFS) machines, obviously)
>>>>>>
>>>>>>       Advantages:
>>>>>>       + Trivial
>>>>>>       + No layering violations
>>>>>>       + We wouldn’t need to keep track of whether the kernel bug has been
>>>>>>         fixed or not
>>>>>>       + Fixes the ppc64+XFS performance problem
>>>>>>
>>>>>>       Disadvantages:
>>>>>>       - Reverts cluster allocation performance to pre-c8bb23cbdbe32f
>>>>>>         levels, whatever that means
>>>>>
>>>>> My favorite because it is clean and simple, but Vladimir has a valid
>>>>> use-case for requiring this performance optimization so reverting isn't
>>>>> an option.
>>>>
>>>> Vladimir also said that qcow2 subclusters would probably also solve his
>>>> problem, so maybe reverting and applying the subcluster patches instead
>>>> is a possible solution, too?
>>>
>>> I'm not sure about ssd case, it may need write-zero optimization anyway.
>>
>> What exactly do you need?  Do you actually need to write zeroes (e.g.
>> because you’re storing images on block devices) or would it be
>> sufficient to just drop the COW areas when bdrv_has_zero_init() and
>> bdrv_has_zero_init_truncate() are true?
> 
> Hmm, what do you mean? We need to zero COW areas. So, original way is to
> write real zeroes, optimized way is fallocate.. What do you mean by drop?
> Mark sublusters as zeroes by metadata?

Why do you need to zero COW areas?  For normal files, any data will read
as zero if you didn’t write anything there.

> But still we'll have COW areas in subcluster, and we'll need to directly zero
> them.. And fallocate will most probably be faster on ssd ext4 case..
> 
>>
>> I’m asking because Dave Chinner said
>> (https://bugzilla.redhat.com/show_bug.cgi?id=1765547#c7) that
>> fallocate() is always slow at least with aio=native because it needs to
>> wait for all concurrent AIO writes to finish, and so it causes the AIO
>> pipeline to stall.
>>
>> (He suggested using XFS extent size hints to get the same effect as
>> write-zeroes for free, basically, but I don’t know whether that’s really
>> useful to us; as unallocated areas on XFS read back as zero anyway.)
>>
>>>> We already have some cases where the existing handle_alloc_space()
>>>> causes performance to actually become worse, and serialising requests as
>>>> a workaround isn't going to make performance any better. So even on
>>>> these grounds, keeping commit c8bb23cbdbe32f is questionable.
>>>>
>>>
>>> Can keeping handle_alloc_space under some config option be an option?
>>
>> Hm.  A config option is weird if you’re the only one who’s going to
>> enable it.  But other than that I don’t have anything against it.
>>
> 
> It's just a bit easier for us to maintain config option, than out-of-tree patch.
> On the other hand, it's not a real problem to maintain this one patch in separate.
> It may return again to the tree, when XFS bug fixed.

We’ll still have the problem that fallocate() must stall aio=native
requests.

Max


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