[Info-vax] Databases versus RMS

[glen herrmannsfeldt]

Johnny Billquist <bqt at softjar.se> wrote:

(snip, I wrote)
>> 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.

I believe that there is also write-around, where the data isn't
written to the cache at all. Presumably rarely used, though.

> 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.

There might be some cases where you know that the data will never
be needed again, and it is better not to write it to the cache.
More often that not, though, you write to the cache.

-- glen