Likelihood of read error, recover device failure raid10

[Wolfgang Mader <Wolfgang_Mader@brain-frog.de>]

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Hi,

I have two questions

1) Layout of raid10 in btrfs
btrfs pools all devices and than stripes and mirrors across this pool. Is it 
therefore correct, that a raid10 layout consisting of 4 devices a,b,c,d is 
_not_

              raid0
       |---------------|
------------      -------------
|a|  |b|      |c|  |d|
   raid1            raid1

Rather, there is no clear distinction of device level between two devices 
which form a raid1 set which are than paired by raid0, but simply, each bit is 
mirrored across two different devices. Is this correct?

2) Recover raid10 from a failed disk
Raid10 inherits its redundancy from the raid1 scheme. If I build a raid10 from 
n devices, each bit is mirrored across two devices. Therefore, in order to 
restore a raid10 from a single failed device, I need to read the amount of 
data worth this device from the remaining n-1 devices. In case, the amount of 
data on the failed disk is in the order of the number of bits for which I can 
expect an unrecoverable read error from a device, I will most likely not be 
able to recover from the disk failure. Is this conclusion correct, or am I am 
missing something here.

Thanks,
Wolfgang

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