[dm-devel] dm-thin: Several Questions on dm-thin performance.

[Joe Thornber]

On Sun, Dec 15, 2019 at 09:44:49PM +0000, Eric Wheeler wrote:
> I was looking through the dm-bio-prison-v2 commit for dm-cache (b29d4986d) 
> and it is huge, ~5k lines.  Do you still have a git branch with these 
> commits in smaller pieces (not squashed) so we can find the bits that 
> might be informative for converting lv-thin to use dm-bio-prison-v2?
> 
> For example, I think that, at least, the policy changes and 
> btracker code is dm-cache specific and just a distraction when trying to 
> understand the dm-bio-prison-v2 conversion.

To be honest I would hold off for a couple of months.  I've been working
on the design for thinp 2 and have got to the point where I need to write
a userland proof of concept implementation.  In particular I've focussed on
packing more into btree nodes, and separating transactions so IO to different
thins has no locking contention.  The proof of concept will tell me just how
small I can get the metadata.  If the level of metadata compression is ~1/10th
we'll plug the new btrees into the existing design and switch to bio prison v2.
If it's greater, say 1/50th, then I'll rewrite the whole target to
use write-ahead logging for transactionality and ditch all metadata sharing altogether.
When the metadata is that small we can copy entire btrees to implement snapshots.

- Joe