Re: About the md-bitmap behavior

On 2022/6/20 15:48, Wols Lists wrote:

On 20/06/2022 08:29, Qu Wenruo wrote:

quoted

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