On Saturday,  1 February 2003 at 23:22:13 +0100, Manuel Bouyer wrote:
> On Sat, Feb 01, 2003 at 07:54:27PM +0100, Pavel Cahyna wrote:
>>>
>>> Right, this is a deficiency in how we handle layered disk I/O.  Right
>>> now we are limited to MAXPHYS (64k) for each "disk".  If this "disk"
>>
>> Why this limit? Is this a heritage from PDP-11 with its (i think) 64kB address
>> space?
>
> Some popular disk standart (IDE comes to mind, there may be others) don't
> support transfers > 64k.

No, that's not correct:

  #  dd if=/dev/ad0s1c of=/dev/null bs=128k &
  # iostat ad0 1
        tty             ad0             cpu
   tin tout  KB/t tps  MB/s  us ni sy in id
   164  158  0.00   0  0.00   9  0  4  3 85
     1  340 128.00 190 23.71   3  0  8 12 78
     0   44 128.00 188 23.52   1  0  7  5 88
     0  155 128.00 187 23.37   2  0  5  5 88
     0  155 128.00 191 23.91   2  0  6  5 88
     0  154 128.00 191 23.89   6  0  6  5 82
     0  155 128.00 191 23.89   6  0  6  3 84
  ^C
  # uname -a
  FreeBSD echunga.lemis.com 4.7-STABLE FreeBSD 4.7-STABLE #0: Wed Dec 18 14:20:56 CST 2002     grog@echunga.lemis.com:/src/FreeBSD/4-STABLE-ECHUNGA/src/sys/compile/ECHUNGA  i386
  # dmesg | grep ad0
  ad0: 13042MB <WDC WD136BA> [26500/16/63] at ata0-master UDMA66

I don't know what the real limit is, though.

Greg
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