AHCI driver

b. f. bf1783 at googlemail.com
Tue Aug 10 12:46:36 UTC 2010

>Is it really better to enable AHCI driver?

Almost certainly, yes.  If your BIOS and SATA controller use AHCI, and
are recognized by the ahci(4), mvs(4),  or siis(4) drivers (I think
that these drivers are built as kernel modules by default in the
recent versions of FreeBSD, and don't require the use of a custom
kernel with the non-default ATA_CAM option -- all you have to do is
load them at boot time, either manually or via loader.conf(5)), then
you will be able to use features like NCQ and better power management
with disk drives that support those features.  This can give you
substantial benefits.

If your BIOS and/or SATA controller don't support AHCI, in order to
use cam(4) you must build a custom kernel with the ATA_CAM option.  In
that case you may still see some benefits, but they won't be as
dramatic as in the AHCI case.  If I recall correctly, the only
disadvantage to this option is that it prevents the use of ataraid(4)
-- everything else has a (usually slightly better) counterpart with
the option, and it is only a matter of configuring your system to use
it and learning how to use the new management tools (like
camcontrol(8)), rather than the old tools (like atacontrol(8)).

And yes, if you use the new drivers or the ATA_CAM option, some of
your disks will probably show up as /dev/adaX, rather than the old
/dev/adX. So make sure that you adjust fstab(5) and device.hints(5) as
necessary before rebooting.


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