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Date:      Mon, 14 Jul 2003 12:34:41 +0930
From:      Greg 'groggy' Lehey <grog@FreeBSD.org>
To:        Marc Wiz <marc@wiz.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: vinum and hot-swapping
Message-ID:  <20030714030441.GU94666@wantadilla.lemis.com>
In-Reply-To: <20030714030137.GG50589@freshaire.wiz.com>
References:  <200307130245.h6D2j8HB000556@soth.ventu> <20030713191453.GF23909@dan.emsphone.com> <20030714003258.GP94666@wantadilla.lemis.com> <20030714015746.GE50589@freshaire.wiz.com> <20030714021914.GR94666@wantadilla.lemis.com> <20030714030137.GG50589@freshaire.wiz.com>

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On Sunday, 13 July 2003 at 22:01:37 -0500, Marc Wiz wrote:
> On Mon, Jul 14, 2003 at 11:49:14AM +0930, Greg 'groggy' Lehey wrote:
>> On Sunday, 13 July 2003 at 20:57:46 -0500, Marc Wiz wrote:
>>> On Mon, Jul 14, 2003 at 10:02:58AM +0930, Greg 'groggy' Lehey wrote:
>>>>> I don't believe vinum can optimize full-stripe writes, though, since
>>>>> FreeBSD can only do I/O in 64k max chunks,
>>>>
>>>> 128 kB.
>>>
>>> I thought physio was limited to 64 KB?  The man page on dump states
>>> this.  Please correct me if I am wrong.
>>
>> Physio is limited to MAXPHYS, which you'll find defined in
>> /usr/include/sys/param.h:
>>
>>   #ifndef MAXPHYS
>>   #define MAXPHYS		(128 * 1024)	/* max raw I/O transfer size */
>>   #endif
>
> That's nice to know.  What prevents it from being increased beyond
> 128K?

There are a couple of things, I think, but I can't recall what.
Individual controllers and host adaptors sometimes have quirks.

>> I've taken a look at the dump code (yuk!), and it no longer places any
>> limitation on the block size.  I'll fix the man page.
>
> Thanks.  I have tried increasing the block size but going past 64K
> does not seem to help performance.  I will have to check the dump
> code to see if it is limiting itself to a max of 64K.

Check the dump with iostat 1.  I think that the CAM layer currently
restricts all I/O to 60 kB.   If that's the case, you'll probably find
that 60 kB blocks will be faster than 64 (which will give you an
average block size of 30).

Greg
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