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

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Date:	Fri, 07 Dec 2012 17:02:32 -0500
From:	Ric Wheeler <rwheeler@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>,
	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 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

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