Alice and Bob in I/O Land

journals are not committed immediately to disk - they are batched in memory.
On ext3 - the journal is flushed to disk every 5 seconds (configurable)
- This avoids too many seeks...
- ...at the price of potential transactions loss on system crash.
Journaling can be of several types:
- full data journal - the data will be written to disk twice - to the journal and to the file system. Simpler to implement.
- meta-data only - data won't be written to disk before entered into the journal - yet will be written to disk before the journal is written to disk.
Having an 'fsync()' call - requires flushing file-system blocks and journal blocks:
- will cause seeks...
- ...unless the journal is kept on a separate disk (separate spindle).

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