Excerpts from Arne Jansen's message of 2011-03-23 09:06:02 -0400: > While looking into the performance of scrub I noticed that a significant > amount of time is being used for loading the extent tree and the csum > tree. While this is no surprise I did some prototyping on how to improve > on it. > The main idea is to load the tree (or parts of it) top-down, order the > needed blocks and distribute it over all disks. > To keep you interested, some results first. > > a) by tree enumeration with reada=2 > reading extent tree: 242s > reading csum tree: 140s > reading both trees: 324s > > b) prefetch prototype > reading extent tree: 23.5s > reading csum tree: 20.4s > reading both trees: 25.7s
Very nice, btrfsck does something similar. > > The test setup consists of a 7 Seagate ES.2 1TB disks filesystem, filled > 28%. It is created with the current git tree + the round robin patch and > filled with > > fs_mark -D 512 -t 16 -n 4096 -F -S0 > > The 'normal' read is done by enumerating the leaves by btrfs_next_leaf() > with path->reada=2. Both trees are being enumerated one after the other. > The prototype currently just uses raw bios, does not make use of the > page cache and does not enter the read pages into the cache. This will > probably add some overhead. It also does not check the crcs. > > While it is very promising to implement it for scrub, I think a more > general interface which can be used for every enumeration would be > beneficial. Use cases that come to mind are rebalance, reflink, deletion > of large files, listing of large directories etc.. > > I'd imagine an interface along the lines of > > int btrfs_readahead_init(struct btrfs_reada_ctx *reada); > int btrfs_readahead_add(struct btrfs_root *root, > struct btrfs_key *start, > struct btrfs_key *end, > struct btrfs_reada_ctx *reada); > void btrfs_readahead_wait(struct btrfs_reada_ctx *reada); > > to trigger the readahead of parts of a tree. Multiple readahead > requests can be given before waiting. This would enable the very > beneficial folding seen above for 'reading both trees'. > > Also it would be possible to add a cascading readahead, where the > content of leaves would trigger readaheads in other trees, maybe by > giving a callback for the decisions what to read instead of the fixed > start/end range. > > For the implementation I'd need an interface which I haven't been able > to find yet. Currently I can trigger the read of several pages / tree > blocks and wait for the completion of each of them. What I'd need would > be an interface that gives me a callback on each completion or a waiting > function that wakes up on each completion with the information which > pages just completed. > One way to achieve this would be to add a hook, but I gladly take any > implementation hints. We have the bio endio call backs for this, I think that's the only thing you can use. -chris -- 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
