dd(1) performance when copiing a disk to another

[Arne Wörner]

Hi!

--- Patrick Proniewski <patpro at patpro.net> wrote:
> > Can u find out, if DMA transfers are enabled for those discs?
> > What does dmesg say?
> 
> see end of mail for full dmesg output,
>
Looks good... :-)) But I never saw FBSD's kernel messages about
SATA drives... ;-)

> > Maybe atacontrol(8) says something useful about SATA discs,
> > too (e. g. atacontrol mode 0)?
> 
> # atacontrol mode 0
> Master = BIOSPIO
> Slave  = BIOSPIO
>
Hmm... 0 seems to be the wrong ata... Thats why the output does
not fit to SATA drives, I think...

> # dd if=/dev/ad4 of=/dev/null bs=1m count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes transferred in 17.647464 secs (59417943
> bytes/sec)
>
That seems to be 2 or about 2 times faster than disc->disc
transfer... But still slower, than I would have expected...
SATA150 sounds like the drive can do 150MB/sec...

As far as I know, SATA busses are independant from each other (no
master/slave; every drive gets its own cable)... Maybe "dd" cannot
issue a read request, while the write isn't completed? DMA
shouldn't be the problem, since the memory interface is quite fast
in your case...

So there remain the questions:
1. Why does the read speed drop in ur setting (maybe writing to
ad6 takes more time than reading from ad4? u could try to run two
dd processes one with if=ad4 and the other with if=ad6)?
2. Why can't we reach 150MB/sec?

> > (Maybe you could find a way to copy /dev/zero to /dev/ad6
> > without destroying the previous work... :-))
>
> well, not very easy both disk are the same size ;)
>
I thought of the first 1000 1MB blocks... :-)
The write speed might be interesting...
 
-Arne


		
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