- The data file is broken down into transactions, which are composed of
  one or more records.

- Atomic transactions are ensured by the initial "data flag" on each
  transaction.

- Each transaction consists of a control header followed by zero or more
  records, followed by a check code.

- The transaction immediately following the last one is marked as empty.

- Data from multiple records may be concatenated into one transaction.

- Check code is 64-bit CRC (8 bytes); polynomial is x64 + x4 + x3 + x1 + 1.

File Header Format:
- Constant string (8 bytes): "journald"
- Version (2)
- Page size (maximum of OS page size and FS block size)
- First global record number
- Option bytes
- Option data (variable size)
- Check code
- NUL padding to the end of the first page

Option Data:
- NUL seperated list of ASCII strings?

Options:
- Writer concurrency

Transaction Format:
- Zero or more records (empty transaction marks the end of journal)
- One NUL byte
- NUL padding to the end of the page (should be ignored by reader)

Record Format:
- Record type/flags:
  0xf TYPE: mask for valid type bits; all other bits are flags
  0x0 EOT: end of transaction; all flags must be zero
  0x1 INFO: stream information
  0x2 DATA: data record for the stream
  0x4 EOS: end of the records for the stream
  (both DATA and EOS may be set at once)
  0x8 ABORT: abort record
- Global record number
- Stream number
- Local (per-stream) record number
- Record data length
- Record data
- Check code on all the data from the record type flag to the last byte
  of the record data.

Stream Information Data Format:
- Current stream offset
- Identifier string

- All record types may contain data.  Data for ABORT records, and records
  without the DATA flag is ignored.

- The global record number is a sequential marker that is incremented on
  each record that does not mark the end of a transaction.

- The stream number is a sequential marker that starts at zero when the
  server starts up, and could be non-zero for the first record in the
  file.

- The record number is a sequential marker that starts at zero for each
  new stream.

- Possible sequences of record types within a stream:
  INFO DATA|EOS
  INFO DATA* EOS
  INFO DATA* ABORT

- Unless otherwise noted, all the above numbers are represented as a
  4-byte binary value, MSB first.

- All strings are prefixed with their length

- The stream information record always has a record number of zero, and
  do not affect the record numbering of other records.

- The stream information record for a given stream will always precede
  any data records for that stream within an individual journal file.

- The following "magic" entries can be used to identify journald files:
0	string		journald	Journald file
>8	belong		x		version %d
>12	belong		x		page size %d