linux-kernel - Re: [ANNOUNCE] Ramback: faster than a speeding bullet

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Date:	Fri, 14 Mar 2008 08:12:37 -0400
From:	Ric Wheeler <ric@....com>
To:	Benny Amorsen <benny+usenet@...rsen.dk>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

Benny Amorsen wrote:
> Ric Wheeler <ric@....com> writes:
> 
>> The only really safe default is to disable the write cache by default
>> or possibly dynamically disable the write cache when barriers are not
>> supported by a drive. Both have a severe performance impact and I am
>> not sure that for most casual users it is a good trade.
> 
> So people ARE running their disks in a mode similar to Ramback.
> 
> 
> /Benny
> 

We have been looking at write performance with RAM disk, battery backed 
Clariion array & slow laptop drives in another thread on fs-devel, but 
the rough numbers should be interesting.

If you are not doing an fsync() at the end of writing a file, you are 
writing to the page cache (as long as it fits in DRAM) so you are 
basically getting thousands of small files/sec.

We did a test which showed the following for synchronous (fsync()) 
writers of small files with a SLES10/SP1 kernel but the results still 
hold for upstream kernels (at least for the order of magnitude).

Ramdisk test backed testing showed over 4600 small 4k files/sec with 1 
thread.

Midrange array (looks like a ramdisk behind a fibre channel port) hit 
around 778 files/sec with 1 thread.

With a local disk, write cached enabled and barriers on, you are getting 
around 47 4k files/sec.

The tests were run on ext3, different file systems perform differently 
but all fall in the same order of magnitude of performance with the same 
class of storage behind it ;-)

ric
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