On Thu, Sep 03, 2015 at 12:44:27PM -0700, Omar Sandoval wrote: > Now we can finally hook up everything so we can actually use free space > tree. On the first mount with the free_space_tree mount option, the free > space tree will be created and the FREE_SPACE_TREE read-only compat bit > will be set. Any time the filesystem is mounted from then on, we will > use the free space tree. > > Having both the free space cache and free space trees enabled is > nonsense, so we don't allow that to happen. Since mkfs sets the > superblock cache generation to -1, this means that the filesystem will > have to be mounted with nospace_cache,free_space_tree to create the free > space trees on first mount. Once the FREE_SPACE_TREE bit is set, the > cache generation is ignored when mounting. This is all a little more > complicated than would be ideal, but at some point we can presumably > make the free space tree the default and stop setting the cache > generation in mkfs.
I have objections against introducing another options to do something with space cache. As you write, it does not make sens to have 'space_cache' and 'free_space_tree' enabled, and I agree. The b-tree approach is an "implementation detail", an improved version of space caching. Because of that I propose to do the following: * use space_cache mount option, and add a value denoting the used implementation, eg. space_cache=btree or space_cache=v2 etc * keep space_cache for backward compatibility for the current implementaion * clear_cache should reset state for both * nospace_cache prevents using any of the two versions of space cache On the mkfs side, we can add new incompat feature to the -O option that will set the incompat bit to the superblock. Mounting such filesystem would use the v2 cache automatically. I'd like to see the b-tree space cache default in the future, until then it'll be mkfs-time option or mount-time option. For backward compatibility, mounting a free space v2 filesystem on older kernel can be done with support of userspace tools: reset the cache generation (as if clear_cache was used), drop all the free-space-tree structures and unset the incompat bit. I think this kind of fallback is desirable. Other than that, I like the series and the improvements it's supposed to bring. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
