On 2022/6/20 15:48, Wols Lists wrote:
On 20/06/2022 08:29, Qu Wenruo wrote:
Hi,
Recently I'm trying to implement a write-intent bitmap for btrfs to
address its write-hole problems for RAID56.
Is there any reason you want a bit-map? Not a journal?
For btrfs, it's a tradeoff here
Bitmap is a little easier, and way less data to writeback.
And since btrfs already has all of its metadata, and quite some of its
data protected by COW (and checksum), a btrfs write-intent bitmap is
enough to close the write-hole already.
Although we may want to implement journal later, mostly to be able to
address combined cases, like powerloss followed by a missing device at
recovery time.
The write-hole has been addressed with journaling already, and this will
be adding a new and not-needed feature - not saying it wouldn't be nice
to have, but do we need another way to skin this cat?
I'm talking about the BTRFS RAID56, not md-raid RAID56, which is a
completely different thing.
Here I'm just trying to understand how the md-bitmap works, so that I
can do a proper bitmap for btrfs RAID56.
Thanks,
Qu
Cheers,
Wol
