kern/38970: slow crashdump on amd64 machine

[Simon Burge <simonb%NetBSD.org@localhost>]

>Number:         38970
>Category:       kern
>Synopsis:       slow crashdump on amd64 machine
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jun 17 06:35:00 +0000 2008
>Originator:     Simon Burge
>Release:        NetBSD -current - problem has been around for a while
>Organization:
>Environment:
        Architecture: amd64
        Machine: amd64
>Description:
        I've got two amd64 machines here.

        A single CPU Opteron with 1GB of RAM and system disk is a
        Fujitsu MHT2080BH on a svwsata.  That machine can write a crash
        dump at ~12MB/sec.

        Another machine is an 8 CPU Intel machine with 16GB of RAM and
        the system disk is a Seagate ST3500630NS on an ahcisata.  This
        machine can only write a crash dump out at about 200kB/sec.
        Each new number ticking down takes around 5 seconds to appears.
        Relevent parts of dmesg for the disk are:

          ahcisata0 at pci0 dev 31 function 2: vendor 0x8086 product 0x2681
          ahcisata0: interrupting at ioapic0 pin 19
          ahcisata0: AHCI revision 1.1, 6 ports, 32 command slots, features 
0x86226000
          atabus1 at ahcisata0 channel 0
          ahcisata0 port 0: device present, speed: 1.5Gb/s
          wd0 at atabus1 drive 0: <ST3500630NS>
          wd0: quirks 2<FORCE_LBA48>
          wd0: 465 GB, 969021 cyl, 16 head, 63 sec, 512 bytes/sect x 976773168 
sectors

>How-To-Repeat:
        Boot to single user and "reboot -d".

>Fix:
        None given.