On Jul 14, 2005, at 1:50 AM, Michael van Elst wrote:

> dan4l-nospam@verizon.net (Dan LaBell) writes:
>
>> but I decided to test it out on my pc first.   It seemed completely
>> zeroed (fresh) , but
>> to verify that ,and to do a read-test, I did  a: dd if=/dev/rwd1d
>> bs=128k | sum
>> Afterwards, there would be about a 3 second window, where I could 
>> type,
>> maybe switch vt's,
>> but, then a long hang, which seemed last around a minute, possibly
>> longer, before any kind of responsiveness was returned.  I'm wondering
>> if the affect could be tuned away?
>
> This must be unrelated, since you are reading from a raw device. No
> filecache activity is involved.
>
> I guess you fall over the end of rwd1d. The size of the raw device
> does not reflect the size of the disk, so it is possible to read
> beyond the end and generate I/O errors that are then naively retried.
>
Similiar to trying to read from an empty floppy drive?  But with no 
errors, to console?
And dd reporting no errors, and reading the correct size.  I find this 
hard to believe.
Also, if I wait 30 seconds or so, and then Ctrl-c I also get a hang, 
but not so long.
leading to the hypothesis of some kind of vm tuning issue.

I've never needed to specify size (count) , with using any device file, 
when reading it just works, and when writing, perhaps a simple "short 
write" and dd reports NNNN + 1 etc, unless of course I want to operate 
on the less than the full size.

In fact, I'm so used to this behavior on unix, especially when zeroing 
a device, that when I
zero a file, I sometimes forget to add count=