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Date:   Fri, 27 May 2022 21:59:20 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Eric Wheeler <bcache@lists.ewheeler.net>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Keith Busch <kbusch@kernel.org>, Coly Li <colyli@suse.de>,
        Adriano Silva <adriano_da_silva@yahoo.com.br>,
        Bcache Linux <linux-bcache@vger.kernel.org>,
        Matthias Ferdinand <bcache@mfedv.net>,
        linux-block@vger.kernel.org
Subject: Re: [RFC] Add sysctl option to drop disk flushes in bcache? (was:
 Bcache in writes direct with fsync)
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On Fri, May 27, 2022 at 06:52:22PM -0700, Eric Wheeler wrote:
> Adriano who started this thread (cc'ed) reported that setting 
> queue/write_cache to "write back" provides much higher latency on his NVMe 
> than "write through"; I tested a system here and found the same thing.
>
> [...]
>
> Is this expected?

Once you do that, the block layer ignores all flushes and FUA bits, so
yes it is going to be a lot faster.  But also completely unsafe because
it does not provide any data durability guarantees.