The cost of a call to sync() adds up quickly and especially for
HDDs. Playing around generating chunks for a while warranted a
3 min save time on a HDD. This is unacceptable default behaviour
and now the behaviour is hidden behind a flag for server owners
who are OK with taking a hit on saves (although SSDs will not have
this issue remotely as bad, since most of the time was spent seeking).
Previously we would write to header before writing our chunk data,
which opens a window for corruption (or we would overwrite entirely).
Now the saving process has been changed to follow this chain of events:
1. We always allocate a new space to write so we do not potentially
overwrite and corrupt the current data
2. Write the chunk data first (the order of the fields in
the chunk data isn't relevant though)
3. Flush to disk
4. Write to the region header last
5. Flush to disk
6. Then we free the previous space allocated
With this chain of events it is impossible for a chunk write to corrupt
a region file, unless the operating system has lied and we have NOT flushed
to disk.
However server administrators are still recommended to continue performing
regular backups.
Note that when Mojang finally decides to change their region format
to deal with oversized chunks this patch must be changed to deal with
whatever system they decide to impose.
We also make use of two flushes to disk per chunk save (to ensure
ordering and ensure data has gone to disk), so this will negatively
affect save performance.
