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Date: Fri, 09 Jan 2009 08:38:02 +0200
From: Danny Braniss <danny@cs.huji.ac.il>
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Cc: freebsd-scsi@freebsd.org
Subject: Re: ISCSI slow to read from DELL MD3000i 
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> Danny Braniss wrote:
> >> Danny Braniss wrote:
> >>     
> >>> wups, didn't read all your mail, sorry,
> >>> try increasing tht tag opening, in the config file add
> >>> 	tag = 64
> >>> for example, or via camcontrol
> >>>
> >>> danny
> >>>   
> >>>       
> >> I modify my iscsi.conf by adding tags = 64 (and no tag). Now works fine, 
> >> about 43MB/s. Thank You very match.
> >>
RTFM :-)

> >> But what means this parameter?
> >>     
> >
> > is the max number of commands that can be sent without waiting for ack. I set 
> > it
> > too concervative :-(
> >   
> I made many tests with different tags values and checked the writing 
> speed with dd.
> 0 (default) -> ~850kB/s
> 1 -> ~850kB/s
> 2 -> ~6.5MB/s
> 3 -> ~40MB/s
> 4 -> ~49MB/s
> 8 -> ~49MB/s
> ...
> 
> I think that NAS server sends ACK after 4 commands. Is it correct?
in a simplistic way yes, it depends on how much memory the target has, the
speed of its disks, and of course, the software :-)

>                                                                    Is it 
> possible that the NAS server modifies this behavior? Can I set tags at 4 
> or is it smarter to keep it higher?
> 
among many other things, it depends on how much memory you have, but I would set it 
somewhat higher, 8 or 16.

try running several dd's simultaneously.

danny