Writethrough with write allocate versus no write allocate

http://semipublic.comp-arch.net/wiki/Writehrough_with_write_allocate_versus_no_write_allocate

Write-through caches can be write allocate or no-write allocate.

A no-write allocate write-through cache can only put data in the cache on a read.
A write that misses is written through, to the next cache level or memory, but not stored in the cache.

A write-through with write-allocate cache, if it does not read clean data,
must necessarily have valid bits for bytes or words within the cache line.
Unless the writes can only be cache line sized.

Actually, we can imagine a write-through with write allocate cache, that reads clean data,
and thereby avoids the need to have byte valid bits.
I was about to say that this somewhat misses the point...
and it certainly would on "write and forget" systems.
But, some systems must obtain ownership even on write-through caches.
E.g. the IBM z-series (descendants of the System 360)
must ensure that all other copies of a cache line are invalidated before
"performing" the write.
If you have to do that, you might almost as well obtain the clean data,
and not have to maintain byte dirty bits.

Contrast:
* P6 family: the write-through invalidates other caches, but the data can be read from the local cache before the remote invalidations. Do not need to get a reply from the invalidation.
* IBM family: must invalidate before forwarding locally; i.e. must wait for the invalidation to be complete.