I would argue that file system stability is paramount for backups and performance is really a distant second.  who cares how fast the system is if it looses your data.  Also, Raid5 has been extensively tested for many years and it is VERY well know that there is a large performance penalty for the parity calculation.  *SOME* hardware raid cards can minimize this but in software raid on linux it is a big deal.  raid5 will be substantially slower than raid10 or raid1 that have no parity calculation.

That being said, if raid5(or6) is fast enough for you it is a mature and stable option and a good choice, but certainly comes with a performance penalty.

 

 

I second that!  though backuppc doesnt make heavy use of the root filesystem.  This is really a best practices piece of advice.  always put your data on a different partition or storage device than your system if you can(specifically in a server environment)
 

Really, if performance is a really big concern you need to do the math on what solution will give you the most active spindles vs latency.  You can take any latency penalties and apply them directly to the spindle count so a 3 spindle set vs a 4 spindle set that has a 20% latency penalty due to a parity check is more like a 3:3.2 ratio instead of 3:4.  consider that raid5 will have a 1/(n-1) performance penalty(roughly) due to the parity write.

examples:
raid10 with 6 drives in a raid0(r1-1+2, r1-3+4, r1-5+6), is 3 active spindles because the other three are mirrors but has a worst case safety of just 1 drive
raid10 in raid0(raid1-1+2+3, raid1-4+5+6) is just 2 spindles but is more resilient because you can loose 2 or more drives and keep the array up.

raid5 is 6-1 for parity and then 1/5 of that penalty for the parity write because 1/5 more data is written is equal to 4 active spindles and can tolerate 1 failure.
that math shows that the raid5 will have the highest thoroughput but this doesnt account for added latency which will be the same penalty of 1/(n-1) or 20%.

These are round numbers, kind of a rule of thumb.  6 volumes is about where raid5 actually catches up.  with a 4 drive set the raid5 penalty brings is to 2 active spindles and has a large 33% latency penalty because the array has to wait for all writes to complete while a raid10 is 2 active spindles without a latency hit.

raid6 shines with 10 or 12 spindles.  I say raid 6 because I wouldnt risk a large array to a single drive fault and a hotspare has a rebuild window that makes me nervous.  raid5 likes odd numbers of drives active(not including hotspare) and raid6 like even numbers.  I cant give a scientific explanation and can only explain it is a phenomenon.  if you use raid5/6, be sure to have 6 or more members active and also have a hotspare.