This configuration variable allows you to flush the disk buffers after a certain number of file updates has occurred. For example, if you set this configuration variable to 10, then the buffers will be flushed after every ten updates to disk files. Only indexed files are counted. When the buffers are flushed, the exact action depends on the operating system:

Windows Buffers are written to disk and the file's directory information is updated. This is roughly equivalent to the action that occurs when a file is closed.
UNIX The sync system routine is called. This causes all of UNIX's cache to be written to disk. This operation is only scheduled--it occurs when the system finds time to do it. Because the system does this every 30 seconds anyway, probably the only reason to request a call to sync is if you have unreliable power.
VMS VMS does not have a system cache, so this configuration variable has no effect.
Note: Setting this variable to a low non-zero value will improve the chances of recovering a file after a power failure, but will decrease performance. If FLUSH_COUNT is set to 0, then the system buffers are flushed only when a file is closed. The default setting is 0.