bcachefs: Document disk accounting keys and conuters

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

diff --git a/fs/bcachefs/disk_accounting_format.h b/fs/bcachefs/disk_accounting_format.h
index 09198ffb57b188ddd8855b67a038970cca819c42..8269af1dbe2a094454f780194f4ece33c4a4e461 100644 (file)
--- a/fs/bcachefs/disk_accounting_format.h
+++ b/fs/bcachefs/disk_accounting_format.h
@@ -119,22 +119,57 @@ enum disk_accounting_type {
        BCH_DISK_ACCOUNTING_TYPE_NR,
 };
 
+/*
+ * No subtypes - number of inodes in the entire filesystem
+ *
+ * XXX: perhaps we could add a per-subvolume counter?
+ */
 struct bch_acct_nr_inodes {
 };
 
+/*
+ * Tracks KEY_TYPE_reservation sectors, broken out by number of replicas for the
+ * reservation:
+ */
 struct bch_acct_persistent_reserved {
        __u8                    nr_replicas;
 };
 
+/*
+ * device, data type counter fields:
+ * [
+ *   nr_buckets
+ *   live sectors (in buckets of that data type)
+ *   sectors of internal fragmentation
+ * ]
+ *
+ * XXX: live sectors should've been done differently, you can have multiple data
+ * types in the same bucket (user, stripe, cached) and this collapses them to
+ * the bucket data type, and makes the internal fragmentation counter redundant
+ */
 struct bch_acct_dev_data_type {
        __u8                    dev;
        __u8                    data_type;
 };
 
+/*
+ * Compression type fields:
+ * [
+ *   number of extents
+ *   uncompressed size
+ *   compressed size
+ * ]
+ *
+ * Compression ratio, average extent size (fragmentation).
+ */
 struct bch_acct_compression {
        __u8                    type;
 };
 
+/*
+ * On disk usage by snapshot id; counts same values as replicas counter, but
+ * aggregated differently
+ */
 struct bch_acct_snapshot {
        __u32                   id;
 } __packed;
@@ -143,10 +178,27 @@ struct bch_acct_btree {
        __u32                   id;
 } __packed;
 
+/*
+ * inum counter fields:
+ * [
+ *   number of extents
+ *   sum of extent sizes - bkey size
+ *     this field is similar to inode.bi_sectors, except here extents in
+ *     different snapshots but the same inode number are all collapsed to the
+ *     same counter
+ *   sum of on disk size - same values tracked by replicas counters
+ * ]
+ *
+ * This tracks on disk fragmentation.
+ */
 struct bch_acct_inum {
        __u64                   inum;
 } __packed;
 
+/*
+ * Simple counter of the amount of data (on disk sectors) rebalance needs to
+ * move, extents counted here are also in the rebalance_work btree.
+ */
 struct bch_acct_rebalance_work {
 };
 
@@ -155,7 +207,7 @@ struct disk_accounting_pos {
        struct {
                __u8                            type;
                union {
-               struct bch_acct_nr_inodes               nr_inodes;
+               struct bch_acct_nr_inodes       nr_inodes;
                struct bch_acct_persistent_reserved     persistent_reserved;
                struct bch_replicas_entry_v1    replicas;
                struct bch_acct_dev_data_type   dev_data_type;