Re: No space on empty, degraded raid10

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2014-09-07 16:38, Or Tal wrote:
> Hi,
> 
> I've created a new raid10 array from 4, 4TB drives in order to migrate
> old data to it.
> As I didn't have enough sata ports, I:
> - disconnected one of the raid10 disks to free a sata port,
> - connected an old disk I wanted to migrate,
> - mounted the array with -o degraded
> - copied the data it it.
> 
> After about 2MB I got a "no space left on device message."
> btrfs fi df showed strange things - much less space in every category
> (about 8GB?) and none of then was full.
> 
> Ubuntu 14.10 beta - linux 3.16.0-14
Yeah, RAID10 doesn't really work in degraded mode (even if you have two
disks that have stripes from the same copy).  The approach that would be
needed for what you want to do is:
 1. Make a BTRFS RAID1 filesystem with _3_ new drives
 2. Connect one of the old disks
 3. Transfer data from old disk to new filesystem
 4. After repeating steps 2 and 3 for each old disk, connect the final
new disk, add it to the filesystem, and rebalance with '-dconvert=raid10
-mconvert=raid10'

Also, I've found out the hard way that system chunks really should be
RAID1, _NOT_ RAID10, otherwise it's very likely that the filesystem
won't mount at all if you lose 2 disks.


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