illumos gate

This change introduces a new weighting algorithm to improve metaslab selection. The new weighting algorithm relies on the SPACEMAP_HISTOGRAM feature. As a result, the metaslab weight now encodes the type of weighting algorithm used (size-based vs segment-based).

This also introduce a new allocation tracing facility and two new dcmds to help debug allocation problems. Each zio now contains a zio_alloc_list_t structure that is populated as the zio goes through the allocations stage. Here's an example of how to use the tracing facility:

> c5ec000::print zio_t io_alloc_list | ::walk list | ::metaslab_trace
  MSID    DVA    ASIZE      WEIGHT             RESULT               VDEV
     -      0      400           0    NOT_ALLOCATABLE           ztest.0a
     -      0      400           0    NOT_ALLOCATABLE           ztest.0a
     -      0      400           0             ENOSPC           ztest.0a
     -      0      200           0    NOT_ALLOCATABLE           ztest.0a
     -      0      200           0    NOT_ALLOCATABLE           ztest.0a
     -      0      200           0             ENOSPC           ztest.0a
     1      0      400      1 x 8M            17b1a00           ztest.0a

> 1ff2400::print zio_t io_alloc_list | ::walk list | ::metaslab_trace
  MSID    DVA    ASIZE      WEIGHT             RESULT               VDEV
     -      0      200           0    NOT_ALLOCATABLE           mirror-2
     -      0      200           0    NOT_ALLOCATABLE           mirror-0
     1      0      200      1 x 4M            112ae00           mirror-1
     -      1      200           0    NOT_ALLOCATABLE           mirror-2
     -      1      200           0    NOT_ALLOCATABLE           mirror-0
     1      1      200      1 x 4M            112b000           mirror-1
     -      2      200           0    NOT_ALLOCATABLE           mirror-2

If the metaslab is using segment-based weighting then the WEIGHT column will display the number of segments available in the bucket where the allocation attempt was made.