Retry reading from boot disk if first access fails (HP BIOS bug??)

[allanjude]

Summary

PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=144234

If you try to boot ZFS on certian HP servers, and apparently various other machines, gptzfsboot reports errors:

error 1 lba 32
error 1 lba 1
No ZFS pools located, can't boot

This can be worked around by reading the ZFS pool from a disk other than the one that was booted from.

This patch retries the boot disk, with 'secondary disk' settings, if reading from it fails the first time.

[smh]

Apart from the comment this looks good in principle, however I think we need to better understand why retrying the probe works as it feels like where just hiding the real error with this.

sys/boot/i386/zfsboot/zfsboot.c

609

I think we should make this comment describe why not what we're doing, as the what can be determined from the code, its the why that's important for future readers.