This message is from the T13 list server.
Hale,
There is a difference between read look ahead and read cache
Read Look ahead:
If the user requests lba 0, then if enabled the drive can read lba 1
before the user requests it
For Read cache:
If the user requests lba 0, and reads lba 0 again, if read cache is
enabled the drive does not
have to go to the media
One would think that if you don't have a read cache, doing a read look
would not mean much because where would you store the look ahead data
(or in the old days, the drive would read ahead and only if the next
access was the next lba would it serve up the data with out going to
media....
However, I can definitely see that just because you have a read cache,
does not mean you do read ahead or read behind, etc
Yes I agree, in today's world they may be very similar, but they are
still slightly different.
Jeff
Jeff Wolford Email: [EMAIL PROTECTED]
Master Architect
Storage Interface and Tools - Business PC Group
Voice: (281) 514-9465, Pager: (800) 973-5739
Hewlett-Packard Corporation
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
> Behalf Of Hale Landis
> Sent: Wednesday, June 29, 2005 11:27 AM
> To: [email protected]
> Subject: Re: [t13] Read Look Ahead
>
> This message is from the T13 list server.
>
>
> Mark Overby wrote:
> > This message is from the T13 list server.
> > That's interesting Hale, because as a host vendor that's NOT what I
> > expect to have happen.
> > If the reality is that some very large % of the drives implement it
> > this way, then I'd have no problem changing it. But, if
> it's the other
> > way around then I'd have a problem changing it because it doesn't
> > reflect reality.
> > The other way out of this going forward is to have a seperate bit
> > indicating if the read cache is completely disabled by this set
> > features or not.
>
> Are we talking about the same thing?
>
> I am talking about the "name" given to these things:
>
> SET FEATURES subcommands 55H and AAH
> - aka disable/enable "read look ahead"
>
> ID word 82 bit 6 and ID word 85 bit 6
> - the "read look ahead" supported/enabled bits
>
> The name "read look ahead" is a little out of date. The name
> "read look ahead" was introduced 15 years ago before "cache"
> became a more common name (back when some drives implemented
> a very simple read ahead function and before drives
> implemented the complex caching see see today).
>
> I think the less confusing name for these would be "read
> cache" - especially since "write cache" is used for the
> corresponding "write cache" feature.
>
> Hale
>
> --
>
> ++ Hale Landis ++ www.ata-atapi.com ++
>
>
