On 02/22/18 05:52, Qu Wenruo wrote: > btrfs_read_block_groups() is used to build up the block group cache for > all block groups, so it will iterate all block group items in extent > tree. > > For large filesystem (TB level), it will search for BLOCK_GROUP_ITEM > thousands times, which is the most time consuming part of mounting > btrfs. > > So this patch will try to speed it up by: > > 1) Avoid unnecessary readahead > We were using READA_FORWARD to search for block group item. > However block group items are in fact scattered across quite a lot of > leaves. Doing readahead will just waste our IO (especially important > for HDD). > > In real world case, for a filesystem with 3T used space, it would > have about 50K extent tree leaves, but only have 3K block group > items. Meaning we need to iterate 16 leaves to meet one block group > on average. > > So readahead won't help but waste slow HDD seeks. > > 2) Use chunk mapping to locate block group items > Since one block group item always has one corresponding chunk item, > we could use chunk mapping to get the block group item size. > > With block group item size, we can do a pinpoint tree search, instead > of searching with some uncertain value and do forward search. > > In some case, like next BLOCK_GROUP_ITEM is in the next leaf of > current path, we could save such unnecessary tree block read. > > Cc: Ellis H. Wilson III <ell...@panasas.com> > Signed-off-by: Qu Wenruo <w...@suse.com>
Decided to give this a try & got interesting results! The scene of the crime: -- 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
