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Started prototyping best-effort fsinfo.known_free/inuse fields

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The idea here was that we could provide best-effort known in-use/free
block info (and eventually pre-erased and bad block info) as a cheaper
alternative to lfs3_fs_usage. It's probably still not what users expect
from lfs3_fs_stat, but may be useful as debug/diagnostic info:

- fsinfo.known_free - Number of known free blocks
- fsinfo.known_inuse - Number of known in-use blocks
- fsinfo.known_preerased* - Number of pre-erased blocks
- fsinfo.known_bad* - Number of bad blocks
- fsinfo.block_count-(all of the above) - Number of unknown blocks

But while known_free/known_inuse is easy enough to find from the
lookahead buffer, it's surprisingly tricky from the gbmap. The best
option I can think of requires scanning the gbmap in O(d log_b d) either
(1) during mount, (2) during mkconsistent, or (3) during lookahead
scans. And that much extra work for debug/diagnostic info seems like a
poor tradeoff.

Note, though, that after scanning once, in theory the info would be
~free to maintain during gbmap rebuilds.

Will revert.

Code changes:

                 code          stack          ctx
  before:       35160           2136          660
  after:        35220 (+0.2%)   2136 (+0.0%)  660 (+0.0%)

                 code          stack          ctx
  gbmap before: 38020           2152          772
  gbmap after:  38116 (+0.3%)   2152 (+0.0%)  776 (+0.5%)
This commit is contained in:
Christopher Haster
2025-12-02 21:21:05 -06:00
parent 1abd9d732f
commit 0b48faf898
2 changed files with 43 additions and 1 deletions
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lfs3.h
+12 -1
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@@ -687,6 +687,16 @@ struct lfs3_fsinfo {
// Upper limit on the size of files in bytes.
lfs3_off_t file_limit;
// Number of known free blocks
//
// Note this is limited by lookahead/gbmap
lfs3_block_t known_free;
// Number of known in-use blocks
//
// Note this is limited by lookahead/gbmap
lfs3_block_t known_inuse;
};
// Traversal info structure
@@ -1221,6 +1231,7 @@ typedef struct lfs3_grm {
typedef struct lfs3_gbmap {
lfs3_block_t window;
lfs3_block_t known;
lfs3_sblock_t known_free;
lfs3_btree_t b;
lfs3_btree_t b_p;
} lfs3_gbmap_t;
@@ -1276,7 +1287,7 @@ typedef struct lfs3 {
struct lfs3_lookahead {
lfs3_block_t window;
lfs3_block_t off;
lfs3_block_t known;
lfs3_size_t known;
lfs3_block_t ckpoint;
uint8_t *buffer;
} lookahead;