Re: Potential to loose data in case of disk failure

[Dmitry Katsubo <dma_k@mail.ru>]

On 2015-11-12 13:47, Austin S Hemmelgarn wrote:
>> That's a pretty unusual setup, so I'm not surprised there's no quick and
>> easy answer. The best solution in my opinion would be to shuffle your
>> partitions around and combine sda3 and sda8 into a single partition.
>> There's generally no reason to present btrfs with two different
>> partitions on the same disk.
>>
>> If there's something that prevents you from doing that, you may be able
>> to use RAID10 or RAID6 somehow. I'm not really sure, though, so I'll
>> defer to others on the list for implementation details.
> RAID10 has the same issue.  Assume you have 1 block.  This gets stored
> as 2 copies, each with 2 stripes, with the stripes split symmetrically.
>  For this, call the first half of the first copy 1a, the second half 1b,
> and likewise for 2a and 2b with the second copy.  1a and 2a have
> identical contents, and 1b and 2b have identical contents.  It is fully
> possible that you will end up with this block striped such that 1a and
> 2a are on one disk, and 1b and 2b on the other.  Based on this, losing
> one disk would mean losing half the block, which would mean based on how
> BTRFS works that you would lose the whole block (because neither copy
> would be complete).

Does it equally apply to RAID1? Namely, if I create

mkfs.btrfs -mraid1 -draid1 /dev/sda3 /dev/sda8

then btrfs will "believe" that these are different drives and mistakenly
think that RAID pre-condition is satisfied. Am I right? If so then I
think this is a trap, and mkfs.btrfs should at least warn (or require
--force) if two partitions are on the same drive for raid1/raid5/raid10.
In other words, the only scenario when this check should be skipped is:

mkfs.btrfs -mraid0 -draid0 /dev/sda3 /dev/sda8

-- 
With best regards,
Dmitry