MFC r260138: MFV r242733:
3306 zdb should be able to issue reads in parallel
3321 'zpool reopen' command should be documented in the man page
and help message
illumos/illumos-gate@31d7e8fa33fae995f558673adb22641b5aa8b6e1
FreeBSD porting notes: the kernel part of this changeset depends
on Solaris buf(9S) interfaces and are not really applicable for
our use. vdev_disk.c is patched as-is to reduce diverge from
upstream, but vdev_file.c is left intact.