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

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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 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@xxxxxxx> 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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