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From: Ivan Voras <ivoras@fer.hr>
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To: Pete French <petefrench@ticketswitch.com>
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Cc: freebsd-stable@freebsd.org
Subject: Re: Dissapointing performance of ciss RAID 0+1 ?
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Pete French wrote:
>> It would be interesting for you to track iostat (i.e. run "iostat 1") 
>> with and without modified vfs.read_max and see if there's a difference.
> 
> On the file: KB/t is about 127.5 with both sizes. Rate is 39 on with
> the read_max set to 8, but 115 with read_max set to 64.

Ok, this might mean the time has come to increase the default value for 
vfs.read_max.

> On the raw device: KB/t is always 128. rate is 41 with the size set to 8
> but rises to 57 with the size set to 64! How can the vfs parameters affect
> access to the raw device ?

Don't know. Maybe it's a statistical anomaly (burst)?

>> In a similar experiment, you could watch gstat (also before and after) 
>> and see if it reports the difference.
> 
> On the file: read_max=8 gives 75% busy, 42 meg/sec. 64 gives 99.7% and 120
> On the device: both sizes give the same results - 98% busy, 59 meg/second
> 
> I am not sure this is helping my understanding! :-)

That makes two of us :) I think I'll leave this thread to someone with 
more knowledge of VFS to explain.