====== Disk cache ======


  * **writeback** - Small block are aggregated in qemu memory buffer and flushed once in a big transaction. Uses host page cache and flushes are performed according to host settings. When guest flushes, data is written to OS journal.
  * **writethrough** - When guest flushes, data is written to psyhical medium.
  * **none** - When guest use O_DIRECT it directly writes data to disk.
  * **unsafe** - Guest flushes are ignored.