Thursday, May 26, 2011

Onboard Sata type - Ahci or IDE - which best

If you are a Windows user then you will have to go search based on your particular version - advice varies per version, and read up about install time drivers.

For GNU / Linux there is just a single set of current advice.
However if you are installing with a motherboard that has secure boot / Uefi, then you need to get familiar with these gatekeepers or experiment (some suggestions below )

This post has been edited since it was first published, and this 4 point summary is added:
  1. Use Ahci rather than Ide mode where you are doing a new install
  2. Existing installs should not switch from Ahci to Ide or vice versa once the operating system is already configured
  3. The main thing that Ahci will give you is support for Native Command Queuing which is a hard drive feature.
  4. If you are unable to install GNU/Linux with secure boot / uefi or other restrictive gatekeeper software with Ahci setting, then try ide mode and see if that allows the disk to be recognised (Later you can rebuild initrd and flick the bios switch if you feel you desperately need NCQ support)
If your problem is that the drive (hard drive / usb drive) is not being recognised so you cannot install Linux, then you can either become a master of secure boot zzzzzz, or just switch the thing off (there is usually a secure boot Yes/No in bios)


Choose at the outset - don't try changing it later:


The reasons for choosing Ahci or IDE will be covered next. But first you should make a commitment now to 'get it right' and stick with that setting.

To enable fast booting, grub (by default) will only includes the drivers it really needs, based on your initial system configuration.*

If you set your system up with 'Onboard Sata type' as Ahci, then later try and switch that mode in your bios, then you are asking for trouble.

If you set your system up with 'Onboard Sata type' as IDE, then later try and switch that mode in your bios, then you are asking for trouble.

Read on to make sure you 'get it right' at the outset.

*Sometimes this minimal approach is termed 'targeted' grub.


SATA and IDE emulation:

Do not use 'IDE mode' for Sata for any new install unless you are installing operating systems which had an initial release prior to 2007.

Use Ahci for current Debian and current Red Hat and similar.

The whole point of having a 'IDE mode' for SATA controllers was to help then current software ( Red Hat 5 & Windows XP ) cope with the change in disk standards.

The change to a newer version of Windows from Windows XP is not to be taken lightly for organisations with > 50 employees, which is why some IT departments continue to support that stable release.

Moving versions of Debian (Lenny -> Squeeze -> Wheezy) is much less of an issue, which is why most Debian installs were migrated from Etch long ago.

Moving point release of Red Hat Enterprise Linux (6.2 -> 6.3 -> 6.4) is also not a big deal, however there is just one caveat.

Red Hat is extremely popular as a base system for lots of virtualisation 'hosting containers'. If this is your situation, then that might require a little more planning  as hosting 40->200 VPS atop of Red Hat does make a migration a bit more involved.


Grub (initrd) and Ahci:

What will happen if you have 'Onboard Sata Type' set Ahci, and then do a GNU / Linux install, is that the boot mechanism (grub and initrd) will be built during install with --preload=ahci

To keep boot times down and loading from disk optimal, GNU / Linux installs keep the initrd minimal (only what your system needs*).

This approach gives sub 30 second boot times on my laptop (which I appreciate), however it does put the onus on you to choose the right bios setting and stick with it.

If 'Onboard Sata Type' was IDE mode when you installed, and then later you switched bios to be 'Onboard Sata Type' as Ahci, then ...
the workaround is to build a new initrd containing the AHCI module.
Source: Wikipedia

There is a great Fedora specific posting here describing in more detail why changing the bios setting after your install was done, will lead to issues.

*The latest Debian installer, specifically asks you if you want a 'targetted' boot setup or the more bloated but future proofed version - the choice is yours :)


Initially bios option was set 'Ahci' but for some reason now want IDE:

Hey maybe this is a little dose of nostalgia...what you are saying is in 2011 my system will be set to act like a 2006 / 2007 system.

Have no idea why you might choose to do this, however the kernel boot time option all_generic_ide might be what you require.


Ahci is not listed as an option in my bios - what should I do:

The bios menu selection might be titled 'Sata Controller Mode' or 'Onboard Sata Type' and should list two or three options.

Some bios have a selectable option named 'Compatible (sata only)', which is I guess just another way of saying Ahci



My operating system is very early version of Red Hat Enterprise Linux 5:

Check your kernel version. Kernel 2.6.18 is 5 years old now. Kernel 2.6.19 and newer, all support Ahci.

If you are thinking about doing an install today of a version of GNU / Linux that uses Kernel 2.6.18 then think again. Justify your choice to yourself.

The current 'stable' of Debian uses 2.6.32 and that is pretty conservative.

RHEL6 kernel is RHEL 6.0, Linux 2.6.32-71.29.1.el6.x86_64

Any kernel configured from the current kernel tree in the last 5 years,
should be > kernel 2.6.19 and therefore have Ahci support.

Kernel 2.6.32 and newer and Kernel 3.2 and newer have excellent support for Ahci.



There are other reasons why Kernel 2.6.18 for an OS acting as host container, might not be ideal.


Notes and further reading:


If you are working with bios that make necessary choice between Ahci and IDE today, then perhaps rather than being nostalgic, you are installing GNU / Linux on second hand machines.

Plenty of non-profits and charities are using donated hardware, and this might be your work.

If so, you might also find this link regarding AC97 audio, handy for new installs of Debian Squeeze, on that legacy hardware.

For native English speakers a translation here
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