Re: Is it possible that certain physical disk doesn't implement flush correctly?

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/3/30 下午8:57, Supercilious Dude wrote:
> Hi,
> 
> This would give a false positive on any controller with a cache as all
> requests would take 0ms until the cache is full and the controller has
> to actually flush to disk.

I'm purposing to measure the execution time of flush/fsync, not write.

And if flush takes 0ms, it means it doesn't really write cached data
onto disk.

Thanks,
Qu

> I am using an HP P841 controller in my test
> system and it has a 4GB cache making every IO instant unless there are
> enough of them that they can't be flushed to the disks as quickly as
> they come in - the latency variation is huge depending on load.
> 
> Regards
> 
> On Sat, 30 Mar 2019 at 12:34, Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>>
>> Hi,
>>
>> I'm wondering if it's possible that certain physical device doesn't
>> handle flush correctly.
>>
>> E.g. some vendor does some complex logical in their hdd controller to
>> skip certain flush request (but not all, obviously) to improve performance?
>>
>> Do anyone see such reports?
>>
>> And if proves to happened before, how do we users detect such problem?
>>
>> Can we just check the flush time against the write before flush call?
>> E.g. write X random blocks into that device, call fsync() on it, check
>> the execution time. Repeat Y times, and compare the avg/std.
>> And change X to 2X/4X/..., repeat above check.
>>
>> Thanks,
>> Qu
>>
>>


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