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f2fs: introduce large directory support

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This patch introduces an i_dir_level field to support large directory.

Previously, f2fs maintains multi-level hash tables to find a dentry quickly
from a bunch of chiild dentries in a directory, and the hash tables consist of
the following tree structure as below.

In Documentation/filesystems/f2fs.txt,

----------------------
A : bucket
B : block
N : MAX_DIR_HASH_DEPTH
----------------------

level #0   | A(2B)
           |
level #1   | A(2B) - A(2B)
           |
level #2   | A(2B) - A(2B) - A(2B) - A(2B)
     .     |   .       .       .       .
level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)

But, if we can guess that a directory will handle a number of child files,
we don't need to traverse the tree from level #0 to #N all the time.
Since the lower level tables contain relatively small number of dentries,
the miss ratio of the target dentry is likely to be high.

In order to avoid that, we can configure the hash tables sparsely from level #0
like this.

level #0   | A(2B) - A(2B) - A(2B) - A(2B)

level #1   | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)

With this structure, we can skip the ineffective tree searches in lower level
hash tables.

This patch adds just a facility for this by introducing i_dir_level in
f2fs_inode.

Signed-off-by: Jaegeuk Kim <[email protected]>
This commit is contained in:
Jaegeuk Kim 2014-02-27 18:20:00 +09:00
commit 3843154598
5 changed files with 20 additions and 12 deletions
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6
Documentation/filesystems/f2fs.txt
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@ -444,9 +444,11 @@ The number of blocks and buckets are determined by,
# of blocks in level #n = |
`- 4, Otherwise
,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2,
,- 2^ (n + dir_level),
| if n < MAX_DIR_HASH_DEPTH / 2,
# of buckets in level #n = |
`- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise
`- 2^((MAX_DIR_HASH_DEPTH / 2 + dir_level) - 1),
Otherwise
When F2FS finds a file name in a directory, at first a hash value of the file
name is calculated. Then, F2FS scans the hash table in level #0 to find the