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Date: Fri, 27 Sep 2002 12:07:55 -0700
From: Andrew Morton <akpm@digeo.com>
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To: "Justin T. Gibbs" <gibbs@scsiguy.com>
CC: James Bottomley <James.Bottomley@steeleye.com>, Jens Axboe <axboe@suse.de>,
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       Mathieu Chouquet-Stringer <mathieu@newview.com>,
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Subject: Re: Warning - running *really* short on DMA buffers while doingfile  
 transfers
References: <200209271721.g8RHLTn05231@localhost.localdomain> <2543856224.1033153019@aslan.btc.adaptec.com>
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"Justin T. Gibbs" wrote:
> 
> ...
> > The evidence is here:
> >
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=103302456113997&w=1
> 
> Which unfortunately characterizes only a single symptom without breaking
> it down on a transaction by transaction basis.  We need to understand
> how many writes were queued by the OS to the drive between each read to
> know if the drive is actually allowing writes to pass reads or not.
> 

Given that I measured a two-second read latency with four tags,
that would be about 60 megabytes of write traffic after the
read was submitted.  Say, 120 requests.  That's with a tag
depth of four.

Not sure how old the disk is.  It's a 36G Fujitsu SCA-2.  Manufactured
in 2000, perhaps??