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Date: Tue, 17 Dec 2024 08:11:04 +0100
From: Christoph Hellwig <hch@lst.de>
To: John Garry <john.g.garry@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, brauner@kernel.org, djwong@kernel.org,
	cem@kernel.org, dchinner@redhat.com, ritesh.list@gmail.com,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, martin.petersen@oracle.com
Subject: Re: [PATCH v2 0/7] large atomic writes for xfs
Message-ID: <20241217071104.GB19358@lst.de>
References: <20241210125737.786928-1-john.g.garry@oracle.com> <20241213143841.GC16111@lst.de> <51f5b96e-0a7e-4a88-9ba2-2d67c7477dfb@oracle.com> <20241213172243.GA30046@lst.de> <9e119d74-868e-4f60-9ed7-ed782d5433da@oracle.com>
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On Fri, Dec 13, 2024 at 05:43:09PM +0000, John Garry wrote:
>> So if the redo log uses buffered I/O I can see how that would bloat writes.
>> But then again using buffered I/O for a REDO log seems pretty silly
>> to start with.
>>
>
> Yeah, at the low end, it may make sense to do the 512B write via DIO. But 
> OTOH sync'ing many redo log FS blocks at once at the high end can be more 
> efficient.
>
> From what I have heard, this was attempted before (using DIO) by some 
> vendor, but did not come to much.

I can't see how buffered I/O will be fast than an optimized direct I/O
implementation.  Then again compared to very dumb dio code that doesn't
replace the caching in the page I can easily see how dio would be
much worse.  But given that people care about optimizing this workload
enough to look into changes all over the kernel I/O stack I would
expected that touching the code to write the redo log should not be
out of the picture.