dan wrote:
> 
> I would argue that file system stability is paramount for backups and 
> performance is really a distant second.  who cares how fast the system 

If you can't perform your backups in a 24-hour period, performance 
becomes more important.

> 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.

He is using a hardware RAID card.

> 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.

You make it sounds like RAID-5 is incapable of saturating drive 
bandwidth.  I haven't seen this on any modern (2-yr-old or newer) 
machine with more than one CPU.  And he's using hardware RAID so the 
point is moot anyway.

> 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.

I think you have it backwards.  A stripe of three mirrors (your first 
example) means that you could lose up to three drives and still have 
data (as long as you're not losing both drives in a mirror).

> 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.

I think you're overestimating the performance penalty of latency.  This 
is worked around in most systems with caching (including a large amount 
of RAM on hardware controller cards), and only for writes.

> 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.

It is explainable if you know how many columns are in your RAID setup. 
Number of drives isn't important; an integral multiple of drives 
compared to the number of columns is.
-- 
Jim Leonard (trixter AT oldskool DOT org)            http://www.oldskool.org/
Help our electronic games project:           http://www.mobygames.com/
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A child borne of the home computer wars: http://trixter.wordpress.com/

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