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Date: Tue, 17 Feb 2009 15:25:27 -0600
From: Tom Judge <tom@tomjudge.com>
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To: Sebastiaan van Erk <sebster@sebster.com>
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Cc: freebsd-performance@freebsd.org
Subject: Re: Extremely slow write speeds to disk : FreeBSD 7.1 and Dell SAS
 6/iR Adapter
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Sebastiaan van Erk wrote:
> Dieter wrote:
>>>> But the issue is that writing to disk is extremely slow...i.e. 14Mb 
>>>> per
>>>> second....
>>>>
>>>> If I install any other linux distribution, like CentOS, write speek is
>>>> the way it should be..it averages around 190Mb+ per second
>>>>
>>>> I was hoping if it were at all possible for you to assist me in with a
>>>> later RAID Controller Driver for the SAS 6/iR Adapter
>>>>       
>>> Try:
>>> hw.mpt.enable_sata_wc=1
>>> in /boot/loader.conf and reboot.
>>>
>>> Assuming you are using SATA drives, the write cache is disabled by 
>>> default. This reenables it. The mpt manpage holds more information.
>>>     
>>
>> Just keep in mind that turning the disk's write cache on puts your
>> data at risk.  :-(
>>
>> The correct solution is NCQ.
> Just wondering, if you have a battery on your RAID array, is this a 
> problem? Wouldn't the cache get written anyway in case of a crash?
>
> Regards,
> Sebastiaan
Hi,

hw.mpt.enable_sata_wc tunable enables the on disk write cache not the 
controller write cache.

Tom