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Date: Fri, 07 Dec 2012 17:02:32 -0500
From: Ric Wheeler <rwheeler@redhat.com>
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To: "Theodore Ts'o" <tytso@mit.edu>, Chris Mason <chris.mason@fusionio.com>,
        Chris Mason <clmason@fusionio.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Christoph Hellwig <hch@infradead.org>,
        Martin Steigerwald <Martin@lichtvoll.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dave Chinner <david@fromorbit.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate
 UAPI
References: <201212051148.28039.Martin@lichtvoll.de> <CA+55aFzgP29WfB1yvnj+Q9-TsbX+b-TCCjTS_vPJ16DxMy6aDQ@mail.gmail.com> <20121206120532.GA14100@infradead.org> <20121207011628.GB16373@gmail.com> <50C22923.90102@redhat.com> <alpine.LFD.2.02.1212070956310.23641@air.linux-foundation.org> <20121207190306.GB14972@shiny> <20121207204325.GC29435@thunk.org> <20121207210932.GA25713@shiny> <50C262AE.5060701@redhat.com> <20121207215727.GF29435@thunk.org>
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On 12/07/2012 04:57 PM, Theodore Ts'o wrote:
> 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

Writing 4KB at a time to a disk cost XX units of time.

Writing to the same sector (especially for a HDD), cost XX units + a small amount.

I suggest that we try it out.

For SSD's, much better to use specific HW offload commands if possible like 
WRITE_SAME (zeroed) or UNMAP/TRIM to get that performance boost since no actual 
data is moved...

ric