[plug] dd speed estimates and /dev/hdx grepping

[Craig Ringer]

> Does anyone have *any* idea how long a dd might take in the following
> situation assuming everything is hunky-dory?
> 
> Celeron 300 - 128MB RAM, two identical 40GB 7200RPM drives, booted with
> Debian 3.0 CD, dropped to console and:
> 
> dd if=/dev/hdd of=/dev/hda 

They're on different controllers, which is good in this case. Did you 
enable DMA and such before you started? If not, you could be in for a 
wait. I always run something like

hdparm -d 1 -m 16 -u 1 -c 1 /dev/hdx

before doing anything like that, to make sure the disks are running at 
optimal performance.

Otherwise... you also haven't specified a block size, which means dd 
will be using 512B. Block sizes of up to about 4mb can help 
considerably, or so I've heard.

> It has been going for 5 hours so far with no errors - when I switch back to
> the install console the screen redraw is VERY slow - indicating the machine
> is doing some thinking.

I've seen that kind of time on machines at about that spec, though I was 
copying a 1.2GB disk over the network to another host. You might be in 
for quite a wait, especially with the settings you've (not) used.

> On the another note, if one wanted to grep a filesystem image for stuff, can
> one do that directly on a device or would it need to be dd'd into one or
> many standard files first?  'grep -b blah /dev/hda'

... will work fine.

Craig Ringer


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