linux-kernel - Re: [PATCH v2 2/3] xfs: Prevent multiple wakeups of the same log space waiter

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Date:   Mon, 27 Aug 2018 00:39:06 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Waiman Long <longman@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] xfs: Prevent multiple wakeups of the same log
 space waiter

On Mon, Aug 27, 2018 at 10:21:34AM +1000, Dave Chinner wrote:
> tl; dr: Once you pass a certain point, ramdisks can be *much* slower
> than SSDs on journal intensive workloads like AIM7. Hence it would be
> useful to see if you have the same problems on, say, high
> performance nvme SSDs.

Note that all these ramdisk issues you mentioned below will also apply
to using the pmem driver on nvdimms, which might be a more realistic
version.  Even worse at least for cases where the nvdimms aren't
actually powerfail dram of some sort with write through caching and
ADR the latency is going to be much higher than the ramdisk as well.