Module superblock 

The superblock, its two slots, the run length encoded shard table and the per shard checkpoint entries.

07 sections 2 and 3. The superblock is the only thing in the file that is overwritten in place, which is why there are two of them and why the choice between them is a sequence number guarded by a checksum. Everything else in a .yo file is append only.

One thing worth stating because it is easy to get wrong: the slot checksum covers all 16380 bytes before it, which means the shard table and the checkpoint entries are inside it. So the order is always write the header, write the table, write the entries, then seal, and only then hand the 16 KiB to the kernel.

Modules

superblock_flags

The flags word at offset 84.

Structs

CheckpointEntry

One shard’s position in its log, as of the last checkpoint.

Superblock

The header half of a superblock slot, decoded.

Constants

CHECKPOINT_ENTRY_LEN

A per shard checkpoint entry, in bytes.

CRC_OFFSET

Where the checksum lives, and therefore how much of the slot it covers.

DEFAULT_SHARD_TABLE_OFF

The default offset of the shard table, which is the end of the header.

Functions

decode_shard_table

Expands a run length encoded shard table into out.

encode_shard_table

Run length encodes a slot to shard mapping into out.

pick

Picks the live slot out of the two.

seal

Computes the slot checksum, stores it, and returns it.