On Thu, 17 Jun 2010 10:44:46 -0700
Gilad Arnold [off-list ref] wrote:

On Thu, Jun 17, 2010 at 07:26:31AM +1000, Neil Brown wrote:

quoted

A RAID5 on 2 devices would be a little slower than RAID1 on two
devices as there is more copying of data around in memory, and there
is no read balancing, but it shouldn't be much slower.

However with recent mdadm and kernel you can trivially convert a 2 drive
RAID1 to a 2 drive RAID5 while the array is online, so it should be easy to
experiment and change you mind about how you want it configured.

Thanks for the interesting comments, Neil.

Preparing for the upgrade I've been cleaning up data and ended up
realizing that I won't need to add another drive in the near future ;-)
I would like to change my RAID level, though, to improve performance,
and the idea of online layout conversion is very appealing.

I understand that a 2-drive RAID5 can be converted to a RAID1 already,
but it is also my understanding that RAID1 currently does not benefit
from striped reading performance, either entirely (due to
implementation) or partly (compared to RAID10 in f2 mode).  This leads
me to think that even with 2 drives, RAID10/f2 is a better choice than
RAID1. Is this a fair assessment?  And, if it is the case, can RAID5 be
converted to RAID10/f2 on-the-fly, or would I have to take the longer
path?  (i.e.  degrade RAID5, start RAID10 in degraded mode, copy data,
kill RAID5 and rebuild RAID10)

2-drive RAID10 f2  would be expected to provide better read throughput
(possibly twice as fast) at some cost to write throughput.  For many people
this is a worthwhile trade-off.  So it might be better for you.
Read throughput would degraded down to write throughput (i.e. slower than
RAID1) if the RAID10 were degraded.

There is currently no support for converting RAID10 arrays in any way.  You
have to copy the data.
With 2 drives, you could make a degraded RAID1 and a degraded RAID10 and copy
the data across.  Then add the RAID1 device into the RAID10 array.

NeilBrown