linux-kernel - Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate UAPI

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Date:	Fri, 7 Dec 2012 16:57:27 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Chris Mason <chris.mason@...ionio.com>,
	Chris Mason <clmason@...ionio.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Martin Steigerwald <Martin@...htvoll.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate
 UAPI

On Fri, Dec 07, 2012 at 04:42:06PM -0500, Ric Wheeler wrote:
> The other things that I think we should try would be to convert over
> larger chunks as we discussed on the list back in the summer (just
> because the user writes 4KB does not mean that we cannot flip over
> 1MB and zero that).

Writing a megabyte is not free.  If you assume that your HDD has a
sustained write throughput of 100-125 MB/s, writing a megabyte will
take 8-10ms.  It might be a win if you amortize it over a large number
of writes, but it doesn't help your 99.9 percentile latency numbers.
(99.9 percentile latency numbers matters because eventually you'll
have a user request which hits multiple serial long latency
operations, and then the delay looks **really** user visible.)

	    	     	       	     		- Ted
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