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I am currently restoring a usb stick with a dd image, I have previously created. However is noticed very poor write speed around ~7 MB/s although the stick is usually much faster.
After looking around a little, I noticed that a lot of data is being read from the USB drive which explains the poor write performance. Nothing but dd is accessing the device at that time.
Edited atop
line:
sdd: busy 101% | read 18020 | write 613 | MBr/s 7.04 | MBw/s 7.1
I noticed a similar behavior when writing directly to my md array using dd. However when writing to a mounted filesystem with dd, this does not happen.
So my question is why is read from the usb stick when writing directly to the block device and whether this can be prevented to improve write performance?
What specific
dd
command switches are you running? And what variety of file systems are you running/writing? – ice13berg – 2015-06-01T10:46:16.850@ice13berg I used
dd if=/dev/sdX > lbzip2 image-dd.bz2
to create the image and am now usinglbzcat image-dd.bz2 | dd of=/deb/sdX
to restore it. – Antagonym – 2015-06-01T10:48:58.537Regarding the md array: RAID 5 or RAID 6? If so, are you using the stripe size as dd blocksize? – Hennes – 2015-06-01T10:50:26.610
@Hennes It's a RAID-6 and in the case of RAID i do expect some read activity when writing due to parity calculation but less than I have seen. I did not set any blocksize for the dd command on the RAID array. – Antagonym – 2015-06-01T10:54:57.747