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Date:   Wed, 25 May 2022 11:44:01 -0700 (PDT)
From:   Eric Wheeler <bcache@lists.ewheeler.net>
To:     Christoph Hellwig <hch@infradead.org>
cc:     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 Tue, 24 May 2022, Christoph Hellwig wrote:
> On Tue, May 24, 2022 at 02:34:23PM -0700, Eric Wheeler wrote:
> > Is this flag influced at all when /sys/block/sdX/queue/scheduler is set 
> > to "none", or does the write_cache flag operate independently of the 
> > selected scheduler?
> 
> This is up to the stacking driver.  dm and tend to pass through flushes
> where needed.
> 
> > This confirms what I have suspected all along: We have an LSI MegaRAID 
> > SAS-3516 where the write policy is "write back" in the LUN, but the cache 
> > is flagged in Linux as write-through:
> > 
> > 	]# cat /sys/block/sdb/queue/write_cache 
> > 	write through
> > 
> > I guess this is the correct place to adjust that behavior!
> 
> MegaRAID has had all kinds of unsafe policies in the past unfortunately.
> I'm not even sure all of them could pass through flushes properly if we
> asked them to :(

Thanks for the feedback, great info!

In your experience, which SAS/SATA RAID controllers are best behaved in 
terms of policies and reporting things like io_opt and 
writeback/writethrough to the kernel?