Re: dd - unequal speed from partition to partition, partition to file, file to partition

[Stan Bischof <stan@worldbadminton.com>]

In comp.os.linux.misc bolega <gnuist006@gmail.com> wrote:
> dd - unequal speed from partition to partition, partition to file,
> file to partition
> 
> /dev/sda1 to /dev/sda2            around 25Meg/s
> /dev/sda1 to a file_sda1          around 12Meg/s
> file_sda1 to /dev/sda1             around  5Meg/s
> 
> Why ???
> 
> I have played with various block sizes such 1M, 4M, 500M
> 
> In the all three cases, its from the same disk to itself   OR   from
> different disks, eg SATA to USB ... if I recall correctly.
> 
> Please give an explanation why read from a file to disk partition is
> so slow ???
> 

Well, the obvious answer ( maybe even the right one ! ) is that
different disks and interfaces have different speeds available,
different cache sizes and so on.

Transferrring from one SATA disk to another SATA on the same bus 
will definitely be a different speed than transferring from the
same disk to another disk mounted on USB. Transferring from one
disk to another will be different ( likely faster ) than
from one disk to elsewhere on the same disk.

In each case the data goes through a path- and the slowest
link in that path will determine the overall speed. Then
you get into other items like cache size- small files may be very
fast if they fit into cache while big files can be much
slower since they overflow the cache. Lots of variables.

There is absolutely no reason to expect the various paths
to be the same speed , not even taking into account the
software used.

Stan