Write-Behind (was Re: Record-access libraries)

[John Pope]

In article <14122 at mimsy.UUCP>, chris at mimsy (Chris Torek) writes:

>Why do you think DEC thought the 16 kB in the UDA50 was such a
>wonderful buffer? 

I guess because it was 16k more than they had before *:o) . My UDA-50
documentation has virtually nothing in it about controller
optimization - maybe it's in one of the parts with "this section
deliberately omitted" on it.

>[the UDA-50] does not report the transfer as done until it has in fact been
>written.  It *may* reorder writes, however (it is hard to tell for sure).

Most of the "smart" SMD and IPI controllers seem to do this now. I
believe most of them treat reads and writes the same within the
elevator (or whatever) algorithm, but I've seen at least one
implementation of disksort() in the operating system that optimized
reads ahead of writes on the theory that processes don't wait for
writes.  Such algorithms may also have made their way on board a disk
controller or two...
John Pope
	Sun Microsystems, Inc. 
		pope at sun.COM