[ILUG] Re: When is SATA SAS?, was ITX advice

[paul at clubi.ie]

On Tue, 11 Sep 2007, Michael Watterson wrote:

> About 20 euro buys an adaptor for SATA motherboard/cable to Parallel IDE.
>
> £25 in Maplin
> http://www.maplin.co.uk/module.aspx?ModuleNo=36036&doy=11m9

What's your point exactly?

You can get adapters to hook IDE drives to USB (which uses the SCSI 
command set). Yet it should be obvious that USB is not IDE..

> If AHCI is not enabled by the motherboard and chipset, SATA controllers 
> typically operate in "IDE emulation" mode which does not allow features of 
> devices to be accessed that are not supported by the ATA/IDE standard.

This means the /controller/ works in a mode where regular, old IDE
*drivers* can operate the controller. I.e. it's /software/ 
compatibility (in hardware).

It doesn't affect how the controller talks to the drive.

> So unless you do extra stuff (which needs to be Mobo, drive and OS 
> supported) SATA is just a cable width saving.

And higher-bandwidth (1.5Gbps), and more ports. etc. And it's SATA 
twixt controller and drive, so fully CRC protected (unlike PATA).

> It's my understanding that a SAS controller may operate a regular 
> SATA drive

It's a common capability, I believe.

> (no doubt by IDE commands),

No, by SATA transport commands. SATA simply is not IDE.

> but that a typical SATA port on a motherboard will not have a clue 
> what to do with a real SAS drive.

This is correct.

There's no functional difference though for most users, TTBOMK. It's 
more to support extant arrangements (SCSI mid-layers, which standards 
body a vendor prefers to work with, etc..).

There may be noticeable differences for more esoteric features, 
high-end stuff which has been developed for SCSI but never was needed 
in mass-market (I'm not a storage guru, so i don't know off-hand of 
examples).

regards,
-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Fortune:
While there's life, there's hope.
 		-- Publius Terentius Afer (Terence)