Hi Phil
So the first layout is the one you wanted. Each drive is ~4GB ? Or is this just a test setup?
It's not a test setup. Historical reasons. I started whith Linux around 1995 and
use software raid a long time. So I have this 4GB partition a long time and when
I decide to upgrade storage or a hd says goodby, I use a new 4GB partition...
Later I put more raid-arrays under a lvm, so I have no trouble with space on a
single partition.
Are there any experiences in which percentage the performance penalty is to expect?
I don't have percentages to share, no. They would vary a lot based on number of disks
and type of CPU. As an estimate though, you can expect raid6 to be about as fast as
raid5 when reading from a non-degraded array. Certain read workloads could even be faster,
as the data is spread over more spindles. It will be slower to write in all cases.
The extra "Q" parity for raid6 is quite complex to calculate. In a single disk failure situation,
both raid5 and raid6 will use the "P" parity to reconstruct the missing information, so
their single-degraded read performance will be comparable. With two disk failures,
raid6 performance plummets, as every read requires a complete inverse "Q" solution.
Of course, two disk failures in raid5 stops your system. So running at a crawl, with data intact, is better than no data.
That's the reason to think about a spare disc
You should also consider the odds of failure during rebuild, which is a serious concern for large raid5 arrays.
This was discussed recently on this list:
http://marc.info/?l=linux-raid&m=130754284831666&w=2
If your CPU has free cycles, I suggest you run raid6 instead of raid5+spare.
Phil
I think there are free cycles, so I should try it.
Thanks
Karsten
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