e: ahd(4) poor performance

[Mark Terribile]

On Fri, 23 Jun 2006, Tom Samplonius wrote:

>On Fri, 23 Jun 2006, Matthew D. Fuller wrote:
>
>> On Sat, Jun 24, 2006 at 12:25:06AM +0300 I heard the voice of
>> Volodymyr Kostyrko, and lo! it spake thus:
>>>
>>>   Script:
>>>
>>> dd if=/dev/zero of=/dev/da0s1b count=20480&
>>> dd if=/dev/zero of=/dev/da0s1b count=20480&
>>     [...]
>>
>> This seems like it would be a record-shattering worst possible case
>> for the disk.  ...
>   I agree with that, but it happens to be an almost best case scenario
>for a write-back cache. ...
>   But in write-through (no write cache), it is definitely a nice way to
>thrash the disk.  You would get one write per block, per dd process.
>
>> ---
>> Matthew Fuller     (MF4839)   |  fullermd at over-yonder.net
>> Systems/Network Administrator |  http://www.over-yonder.net/~fullermd/
>>           On the Internet, nobody can hear you scream.
>
>Tom

Some years ago I ran some scripts like this to see what we could expect from
our disks.  When I piled operations like this up, I found that the sequences of
operations tended to synchronize.  It appeared that whenever two operations in
sequence found themselves on the same cylinder, they would be serviced very
quickly together (rotation, no seek).  Once synchronized, they stayed
synchronized and so unless the dd's were started with a very large time lag,
they would all end at about the same time.

Mark Terribile

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