Simplify hostid logic

There is plenty of compatibility code for a hw_hostid
that isn't used by anything. At the same time, there are apparently
issues with the current hostid logic. coredumb in #zfsonlinux on
freenode reported that Fedora 17 changes its hostid on every boot, which
required force importing his pool. A suggestion by wca was to adopt
FreeBSD's behavior, where it treats hostid as zero if /etc/hostid does
not exist

Adopting FreeBSD's behavior permits us to eliminate plenty of code,
including a userland helper that invokes the system's hostid as a
fallback.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #224
This patch add a CTASSERT macro for compile time assertion.

This macro makes the compile to spit "mixed definition and code"
warning, I can't find a way to avoid it.

This patch lays some groundwork for the persistent l2arc feature.
See https://www.illumos.org/issues/3525.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #303
Add ddi_time_after and friends

When comparing times gotten from ddi_get_lbolt, we have to take account of
wrap around of jiffies. Therefore, we cannot use 't1 < t2'. Instead we should
use 't1 - t2 < 0'.

This patch add ddi_time_after and friends to address this issue. They have
strict type restriction, clock_t for vanilla and int64_t for 64 version, to
prevent type conversion from screwing things.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #335
Expose max/min objs per slab and max slab size

By default maximal number of objects in slab can't exceed (16*2 - 1) and slab
size can't exceed 32M.
Today's high end servers having couple hundreds of RAM available for ARC may
run into a trouble with virtual memory because of the restriction mentioned
above.

Problem:
Reasons for very high number of virtual memory allocations:
	* Real slab size very small relative to the size of the entire RAM
	* Slabs allocated on virtual memory and fill entire ARC

The result is very high number of allocated virtual memory ranges (hundreds of
ranges). When virtual memory subsystem manages high number of ranges its
performance become so poor that it freezes from time to time.

Solution:
Number of objects per slab should be increased taking into account maximal
slab size which can also be increased if needed.

Signed-off-by: Andrey Vesnovaty <andrey.vesnovaty@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #337
Change spl_kmem_cache_expire default setting to 2

This behavior is more consistent with the way memory reclaim
is expected to work under Linux.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #349