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

[Michael Watterson]

paul at clubi.ie wrote:
> 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..
true... but I'm not convinced a *basic*  SATA -IDE convvertor is any 
more than a serial / parallel bridge. I'm sure there are fancier ones.
>
>> 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.
That's not how I understand it. You do need a SATA driver for a basic 
SATA port. An IDE driver won't work.
>
>> 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).
1.5Gbps SATA is same speed as (P)ATA / E-IDE 133MHz interface. It's only 
later faster revisions that  are faster (though  in all  cases it's 
burst  speed  to  cache.  Sustained  transfer on  7200rpm drive at that 
speed is unlikely.)
>
> 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.
>
Which proves my point that all SATA drives are not the same thing. The 
"consumer" Mobo and single drive has no advanatge over Parallel ATA 
other than less obstruction from the cable.
> 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..).
Unless the Mobo/Controller, Driver and Drive all support AHCI, it's just 
IDE. Unless the drive and controller *BOTH* support SAS, it's just AHCI 
SATA, not SCSI.

So I think we have possibly 3 different products/ protocols that can use 
the same physical serial cable.
>
> 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).
>
SCSI was invented for Minicomputers, hence "*S*mall Computer System 
Interface" where  "small" means smaller that mainframe. It's still 
popular for scanners part of Document Management Systems (where firewire 
could be used). SCSI was never a consumer standard though some early 
consumer CD writers and Flatbed scanners used "brain dead" dumb SCSI 
hosts.  SAS is not for consumer gear and I doubt much consumer PCs 
/laptops use AHCI

It's similar to the smoke and mirrors around the dying Express Card 
format. The laptop makers put it in before existing PCMCIA/Cardbus 
devices had been migrated. BUT it is TWO standards, one is a 3.3V 
version of USB and the other PCIe on the same connector. Of course the 
Express Card devices than can use USB do and are also in USB dongle 
format. The Express card devices did not materialise. Makers of new 
modems etc wanted older Cardbus and newer Express Card so produced 
external USB dongles.

So hardly any "true" Express cards exist. We have Express card format 
USB devices. The super duper PCIe speed not realised and the 32bit 
Cardbus is faster than USB 2.0 (16bit PCMCIA/Cardbus is faster than 1.1 
but slower than 2.0).

So don't buy an Express Card thinking it is faster than USB 2.0. It 
might *be* USB 2.0, or even really only USB1.1! Similarly I suspect that 
SATA is not "automatically" better/faster than E-IDE/PATA on typical 
consumer gear.



-- 
Mike