linux-kernel - Re: Linux 2.6.29

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Message-ID: <49D57A82.1030006@garzik.org>
Date:	Thu, 02 Apr 2009 22:54:58 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	"Trenton D. Adams" <trenton.d.adams@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Trenton D. Adams wrote:
> On Thu, Apr 2, 2009 at 8:01 PM, Jeff Garzik <jeff@...zik.org> wrote:
>> Linus Torvalds wrote:
>> The most interesting thing I found:  the SSD does 80 MB/s for the first ~1
>> GB or so, then slows down dramatically.  After ~2GB, it is down to 32 MB/s.
>>  After ~4GB, it reaches a steady speed around 23 MB/s.
> 
> Isn't that the kernel IO queue, and the dd averaging of transfer
> speed?  For example, once you hit the dirty ratio limit, that is when
> it starts writing to disk.  So, the first bit you'll see really fast
> speeds, as it goes to memory, but it averages out over time to a
> slower speed.  As an example...

overwrite.c is a special program that does this, in a loop:

	write(buffer-N) data to pagecache
	start buffer-N write-out to storage
	wait for buffer-(N-1) write-out to complete

It uses the sync_file_range() system call, which is like fsync() on 
steroids, wearing cool sunglasses.

Regards,

	Jeff




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