Michael Tokarev wrote:
> Raid10 IS RAID1+0 ;)
It's just that linux raid10 driver can utilize more.. interesting ways
to lay out the data.
This is misleading, and adds to the confusion existing even before linux
raid10. When you say raid10 in the hardware raid world, what do you mean?
Stripes of mirrors? Mirrors of stripes? Some proprietary extension?
What Neil did was generalize the concept of N drives - M copies, and called it
10 because it could exactly mimic the layout of conventional 1+0 [*]. However
thinking about md level 10 in the terms of RAID 1+0 is wrong. Two examples
(there are many more):
* mdadm -C -l 10 -n 3 -o f2 /dev/md10 /dev/sda1 /dev/sdb1 /dev/sdc1
Odd number of drives, no parity calculation overhead, yet the setup can still
suffer a loss of a single drive
* mdadm -C -l 10 -n 2 -o f2 /dev/md10 /dev/sda1 /dev/sdb1
This seems useless at first, as it effectively creates a RAID1 setup, without
preserving the FS format on disk. However md10 has read balancing code, so one
could get a single thread sustained read at a speed twice what he could
possibly get with md1 in the current implementation
I guess I will sit down tonight and craft some patches to the existing md* man
pages. Some things are indeed left unsaid.
Peter
[*] The layout is the same but the functionality is different. If you have 1+0
on 4 drives, you can survive a loss of 2 drives as long as they are part of
different mirrors. mdadm -C -l 10 -n 4 -o n2 <drives> however will _NOT_
survive a loss of 2 drives.
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