size of a delete request sent to the providing device performed by g_dev_ioctl.
This allows the kernel and apps via ioctl e.g. newfs -E to request large LBA
deletes which siginificantly improves performance.
Previously this was hard coded to 65536 sectors, the new default is 262144
which doubles the throughput of deletes on commonly available SSD's.
In tests on a Intel 520 120GB FW: 400i disk it improved the delete throughput
from 1.6GB/s to over 2.6GB/s on a full disk delete such as that done via
newfs -E
For some SSD's where delete time is pretty much constant, no matter what
the request, setting this to 0 will provide significantly better throughput
e.g. Samsung 840 240GB FW DXT07B0Q @ 262144 = 79G/s, @ 0 = 2259G/s
Reviewed by: mav
Approved by: pjd (mentor)
MFC after: 2 weeks
6f926c0b82