[#LUDOC-317] document OST reserved space tunables

osp_pre_status=-ENOSPC is used to skip OST from object allocation.
The error was set when OST available space is less than 0.1% of total
OST size. This value is not configurable, so procfs files was
added:
reserved_mb_low - low watermark, if available space is less
		  than it, object allocation is stopped.
reserved_mb_high - highw watermark, if available space is more
		   than it, object allocation is enabled.

By default ~0.1% is reserved as low watermark. The high watermark
is twice bigger than the low by default.
High and low watermark could be changed by:
lctl set_param osp.lustre-OST0000-osc-MDT0000.reserved_mb_high=1024

When object allocation is disabled, a clients could appened to
existing files. And 0.1% is too low for them. For example, OST size
is 8TB, 0.1% is 8GB, if cluster has 1k clients, reserved space is
~8MB per client. The main reason of the patch is ability to increase
reserved space.

[LUDOC-317] document OST reserved space tunables Created: 11/Dec/15 Updated: 05/Oct/16 Resolved: 05/Oct/16
Status:	Resolved
Project:	Lustre Documentation
Component/s:	None
Affects Version/s:	None
Fix Version/s:	None

[LUDOC-317] document OST reserved space tunables Created: 11/Dec/15 Updated: 05/Oct/16 Resolved: 05/Oct/16