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[CachePruning] Clarify the per-directory entry limit on Linux ext4. Summary: 508 root node entries (root_limit) 510 internal node entries (node_limit) For a filename with 40 bytes, its sizeof(ext4_dir_entry_2) = 48, a linear directory can contain at most floor(4096/48)=85 of them. The real per-directory entry limit should be 508*510*85 = 22021800 The limit varies with the average length of filenames. However, the Linux ext4 code does not try rebalancing the htree, so we will not be able to create filenames in a full leaf node. This is demonstrated with the following example, certain filenames cannot be used while others can: % touch d/0000000000000000000000000000000000816a6f touch: cannot touch 'd/0000000000000000000000000000000000816a6f': No space left on device % touch d/0000000000000000000000000000000000816a70 # succeeded Reviewers: pcc Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D45546 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329966 91177308-0d34-0410-b5e6-96231b3b80d8 Fangrui Song 2 years ago
1 changed file(s) with 5 addition(s) and 2 deletion(s). Raw diff Collapse all Expand all
5151 /// the number of files based pruning.
5252 ///
5353 /// This defaults to 1000000 because with that many files there are
54 /// diminishing returns on the effectiveness of the cache, and file
55 /// systems have a limit on total number of files.
54 /// diminishing returns on the effectiveness of the cache. Some systems have a
55 /// limit on total number of files, and some also limit the number of files
56 /// per directory, such as Linux ext4, with the default setting (block size is
57 /// 4096 and large_dir disabled), there is a per-directory entry limit of
58 /// 508*510*floor(4096/(40+8))~=20M for average filename length of 40.
5659 uint64_t MaxSizeFiles = 1000000;
5760 };