What:           /sys/module/xen_blkback/parameters/max_buffer_pages
Date:           March 2013
KernelVersion:  3.11
Contact:        Roger Pau Monné <roger.pau@citrix.com>
Description:
                Maximum number of free pages to keep in each block
                backend buffer.

What:           /sys/module/xen_blkback/parameters/max_persistent_grants
Date:           March 2013
KernelVersion:  3.11
Contact:        Roger Pau Monné <roger.pau@citrix.com>
Description:
                Maximum number of grants to map persistently in
                blkback. If the frontend tries to use more than
                max_persistent_grants, the LRU kicks in and starts
                removing 5% of max_persistent_grants every 100ms.