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).

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