I discovered a serious problem in 5.5 related to fsync (5.3 and below appears
not affected).

By default, we do not run any fsync at all. This is because the
--debug-no-sync option is incorrectly enabled by default.

This problem was introduced here:

revid:sergii@pisem.net-20111212225840-8aotf02a1alw1gta

• {"sync_sys", 0, - "Enable system sync calls. Disable only when running tests or debugging!", - &opt_sync, &opt_sync, 0, GET_BOOL, NO_ARG, 1, 0, 0, 0, 0, 0}

  ,
  +

  {"debug-no-sync", 0, + "Disables system sync calls. Only for running tests or debugging!", + &my_disable_sync, &my_disable_sync, 0, GET_BOOL, NO_ARG, 1, 0, 0, 0, 0, 0}

  ,

The option was renamed and the meaning was reversed - but the default was left
unchanged, effectively reversing the default.

We need to fix this ASAP, as it causes data loss / silent corruption when the
host machine crashes.

There is an additional problem that even with --debug-no-sync=0, we are using
fsync(), not fdatasync() as we should. There is code to use fdatasync(), but
apparently availability of fdatasync() is not correctly detected by
cmake. This is also a quite serious issue; one one machine I tested with a
good RAID controller, fdatasync() is 2.5 times faster than fsync().

Work-around: pass --debug-no-sync=0 in config or on command line (but this will not fix fdatasync() performance problem).