[Info-vax] Databases versus RMS

[Johnny Billquist]

On 2012-04-18 22:35, glen herrmannsfeldt wrote:
> Johnny Billquist<bqt at softjar.se>  wrote:
>
> (snip, someone wrote)
>>> You are mixing read and write cache. Make sure you are talking
>>> about the same thing before arguing... :-)
>
>> Huh? They are not separate! If they were, you'd get some really bad results.
>
>> But even disregarding that, I do not understand what you were
>> commenting. Please explain.
>
> More specifically, the difference between write-through and write-back.
> A write-back cache always writes to the underlying (RAM or disk)
> storage system, though the data is also available for later read.

The data can be available for later read in both write-back and 
write-through. The difference is wether your write will actually pass 
straight through (write-through), or if your write will only go to the 
cache (write-back). In the case of write-back, the cache system itself 
will then at some later point initiate a write of the data to the 
backing store.

That has nothing to do with wether you will be able to read the data 
back from the cache. In a write-back cache, the data must be allowed to 
live in the cache after a write. With a write-through you can either 
invalidate the cache on a write, which is silly and hurts performance, 
but works, you can update the cache (the sensible thing to do), or leave 
the cache alone, which cause cache inconsistency and a broken system.

> It was an NFS tradition for write operations not to return (acknowledge)
> until the data was on the physical disk. It would do read-ahead caching
> to speed up read operations, though.

Right. But that is a different issue.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol