change existing RAID10 to "far layout"

[Roberto Ragusa]

On 12/31/2011 02:11 AM, Reindl Harald wrote:
>>> for better performance you could use RAID10 in the far layout instead (raid10,f2),
>>> this is a direct replacement with the enhanced raid10 driver, which gives double
>>> the sequential read speed compared to raid1. Use "--level=10 -p f2" as additional
>>> parameters when creating arrays with mdadm.
> 
> hmm is there a way to change this in an existing RAID10?
> i think i would like this for / where read-performance is much
> more important and /dev/md2 is perfectly with near-layout
> for write-performance (hughe data, virtual machines)

It is apparently impossible to change the layout from near to far.
If you think about how different they are, it is difficult to
imagine how a conversion could be done.

The best option is to boot from a rescue image, copy the root partition
somewhere else, destroy and redo the RAID and copy the root partition back.

In that case, you may want to consider the offset layout too.
After some experiments when making a filesystem for VM files, the best
option proved to be offset=2 with chunksize=4MiB.
Very good linear read speed, and good write performance even on random writes.

# cat /proc/mdstat
Personalities : [raid10]
md10 : active raid10 sdd5[3] sdc5[2] sdb3[1] sda3[0]
      285196288 blocks super 1.2 4096K chunks 2 offset-copies [4/4] [UUUU]

unused devices: <none>
# hdparm -t /dev/md10

/dev/md10:
 Timing buffered disk reads:  922 MB in  3.02 seconds = 305.11 MB/sec

-- 
   Roberto Ragusa    mail at robertoragusa.it