Re: [dm-crypt] btrfs with raid5/6

[Markus Krainz <ldm@gmx.at>]

> It's up to you. I'd prefer LUKS/dmcrypt.
So do I. But I have a related concern:

I currently do ext4 <-> dm-crypt <-> linux-software-raid <-> hdds

However Linux Software Raid (mdadm) does not protect you from silent 
data loss, because it does not store checksums in it's metadata.
So what mdadm does is, it compensates for failed devices, or blocks, if 
the device reports them as bad. But I can NOT correct data, that was 
e.g. corrupted by the SATA controller or backplane.
See http://neil.brown.name/blog/20100211050355 "Smart or simple RAID 
recovery??" for more information.

Btrfs (like ZFS) can be the solution here, as it stores CRC checksums 
with all data and metadata. RAID5 with protection from silent data loss 
is promised to be implemented in the future.
A setup on top of dm-crypt would look as follows:

btrfs <-> multiple dm-crypt partitions <-> multiple devices

However in the past I experienced very disappointing results, if raid is 
running on top of dm-crypt partitions.
If I remove hdds from such a setup the dm-crypt partition wont 
disappear. It won't propagate the error to the software raid on top of 
it (not familiar with the implementation details, maybe mdadm is to 
blame here).
And finally it won't even let itself be deleted without restart (I think 
the last issue has been fixed in the meantime).

I urge the dm-crypt developers to improve this situation with raid on 
top of dm-crypt, be it with mdadm or btrfs.
Solutions for other long-time annoying issues like insufficient 
threading support, and the lack of discard support, would be appreciated 
too. :P
I know the OSS pradigm is provide patches yourself. Would you be 
accepting patches?

Sincerly,
Markus