On 2018年02月23日 00:31, Ellis H. Wilson III wrote: > On 02/21/2018 11:56 PM, Qu Wenruo wrote: >> On 2018年02月22日 12:52, 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> >> >> Hi Ellis, >> >> Would you please try this patch to see if it helps to speedup the mount >> of your large filesystem? > > I will try either tomorrow or over the weekend. I'm waiting on hardware > to be able to build and load a custom kernel on.
If you're using Archlinux, I could build the package for you. (For other distributions, unfortunately I'm not that familiar with) Thanks, Qu > > Thanks so much for taking a stab at this! > > ellis
signature.asc
Description: OpenPGP digital signature
