In multiple cases zio_buf_alloc() was used instead of kmem_alloc()
or vmem_alloc(). This was often done because the allocations
could be large and it was easy to use zfs_buf_alloc() for them.
But this isn't ideal for allocations which are small or short
lived. In these cases it is better to use kmem_alloc() or
vmem_alloc(). If possible we want to avoid the case where
we have slabs allocated for kmem caches which are rarely used.
Note for small allocations vmem_alloc() will be internally
converted to kmem_alloc(). Therefore as long as large
allocations are infrequent and short lived the penalty for
using vmem_alloc() is small.
Reviewed-by: Chunwei Chen <david.chen@osnexus.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5409