Neil Brown wrote:
On Wednesday May 31, davidsen@xxxxxxx wrote:
Where I was working most recently some systems were using RAID5E (RAID5
with both the parity and hot spare distributed). This seems to be highly
desirable for small arrays, where spreading head motion over one more
drive will improve performance, and in all cases where a rebuild to the
hot spare will avoid a bottleneck on a single drive.
Is there any plan to add this capability?
I thought about it briefly....
As I understand it, the layout of raid5e when non-degraded is very
similar to raid6 - however the 'Q' block is simply not used.
This would be trivial to implement.
The interesting bit comes when a device fails and you want to rebuild
that distributed spare.
There are two possible ways that you could do this:
1/ Leave the spare where it is and write the correct data into each
spare. This would be fairly easy but would leave an array with an
very ... interesting layout of data.
When you add a replacement you just move everything back.
2/ reshape the array to be a regular raid5 layout. This would be hard
to do well without NVRAM as you are moving live data, but would result
in a neat and tidy array. Ofcourse adding a drive back in would be
interesting again...
I had previously only thought of option '2', and so discarded the idea
as not worth the effort. The more I think about it, the more possible
option 1 sounds.
I've put it back on my todo list, but I don't expect to get to it this
year. Ofcourse if someone else wants to give it a try, I'm happy to
make suggestions and review code.
I do appreciate being too busy, I'm just glad I have been able to
clarify the tradeoffs of RAID5e, and get it on your list at all. I did
look at the code a bit, and it would seem that if the "rebuild to hot
spare" code is modified to handle a distributed spare, then it looks as
if RAID6e might pretty much fall out. Feel free to tell me I'm dreaming.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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