Issue
=====
When creating a new file (like the log file or the file for a new table),
InnoDB flushes the write buffer to the disk after writing the entire file.
This causes problems on SSD disks when several instances are running and a
newly created instance sends a large stream of writes to the flash device,
causing other instances to stall significantly.

Fix
===
This revision adds a new global variable called innodb_fsync_threshold which
when specified determines the size of the write buffer after which innodb will
flush the contents of the buffer to disk.

This patch is a contribution from Facebook Inc.

RB : 19412
Reviewed-by : Mayank Prasad <mayank.prasad@oracle.com>