Description

Sync is whether to sync underlying writes from the OS buffer cache through to actual disk, if applicable. Setting Sync can result in slower writes.

If false, and the machine crashes, then some recent writes may be lost. Note that if it is just the process that crashes (and the machine does not) then no writes will be lost.

In other words, Sync being false has the same semantics as a write system call. Sync being true means write followed by fsync.

The default value is false.