[Bug 182158] dd slow when writing to partition of reading

[Michael Trunner]

Public bug reported:

Binary package hint: linux-image

I tried to copy a partition images over the network with nc and dd.
On my partition sda9 it was very fast about 55 Mb/s. On the sda2 I had only 8 Mb/s.

With dstat I found the reason: When writing data to the sda2, the kernel reads the same amount of data that it is writing.
On partition sda9 there was nearly no read disk access.

The problem exist even in init 1, so I think it is a kernel problem.
Inotify was off.

I used the followoing commands:

client:

nc -l -p 19000 | dd of=/dev/sda2
or
nc -l -p 19000 > /dev/sda2 (makes no difference)

server:

dd if=/dev/sda2 | nc targethost 19000

It tried the same with different computers on my laptop some partitions have the same problem.
Any Ideas?

** Affects: linux-meta (Ubuntu)
     Importance: Undecided
         Status: New

** Summary changed:

- dd slow when writing to partition of reading
+ slow raw writing to partition because of read access

-- 
slow raw writing to partition because of read access
https://bugs.launchpad.net/bugs/182158
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