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From: Supercilious Dude <supercilious.dude@gmail.com>
To: Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	linux-block@vger.kernel.org
Subject: Re: Is it possible that certain physical disk doesn't implement flush correctly?
Date: Sat, 30 Mar 2019 12:57:29 +0000	[thread overview]
Message-ID: <CAGmvKk7bYkaNXGe4agwLWmM_YM7Sjfn1NjBheQ=FzcJmUkZcGg@mail.gmail.com> (raw)
In-Reply-To: <dc1735e6-e410-e593-7059-3728eb427886@gmx.com>

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@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
>
>

  reply	other threads:[~2019-03-30 12:57 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-30 12:31 Is it possible that certain physical disk doesn't implement flush correctly? Qu Wenruo
2019-03-30 12:57 ` Supercilious Dude [this message]
2019-03-30 13:00   ` Qu Wenruo
2019-03-30 13:04     ` Supercilious Dude
2019-03-30 13:09       ` Qu Wenruo
2019-03-30 13:14         ` Supercilious Dude
2019-03-30 13:24           ` Qu Wenruo
2019-03-31 22:45             ` J. Bruce Fields
2019-03-31 23:07               ` Alberto Bursi
2019-03-31 11:27 ` Alberto Bursi
2019-03-31 12:00   ` Qu Wenruo
2019-03-31 13:36     ` Hannes Reinecke
2019-03-31 14:17       ` Qu Wenruo
2019-03-31 14:37         ` Hannes Reinecke
2019-03-31 14:40           ` Qu Wenruo
2019-03-31 12:21   ` Andrei Borzenkov
2019-04-01 11:55   ` Austin S. Hemmelgarn
2019-04-01 12:04 ` Austin S. Hemmelgarn

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