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

[Max Reitz]

On 29.10.19 13:19, Vladimir Sementsov-Ogievskiy wrote:
> 29.10.2019 15:11, Max Reitz wrote:
>> On 29.10.19 13:05, Vladimir Sementsov-Ogievskiy wrote:
>>> 29.10.2019 14:55, Max Reitz wrote:
>>>> 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.
>>>
>>> Hmm, but when allocating new cluster in qcow2, it's not guaranteed to be 
>>> zero,
>>> as it may be reused previously allocated cluster..
>>
>> Hm, right.  We could special-case something for beyond the EOF, but I
>> don’t know whether that really makes it better.
>>
>> OTOH, maybe allocations at the EOF are the real bottleneck.  Reusing
>> existing clusters should be rare enough that maybe the existing code
>> which explicitly writes zeroes is sufficient.
> 
> But, as I understand pre-EOF fallocates are safe in xfs? So, we may just drop 
> calling
> fallocate past-EOF (it's zero anyway) and do fallocate pre-EOF (it's safe) ?

But probably slow still.  And there’s the question of how much
complexity we want to heap onto this.

Max

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