On Tue, May 31, 2022 at 04:04:12PM -0700, Eric Wheeler wrote: > > * Write-through: write is done synchronously both to the cache and to > the backing store. > > * Write-back (also called write-behind): initially, writing is done only > to the cache. The write to the backing store is postponed until the > modified content is about to be replaced by another cache block. > [ https://en.wikipedia.org/wiki/Cache_(computing)#Writing_policies ] > > > So the kernel's notion of "write through" meaning "Drop FLUSH/FUA" sounds > like the industry meaning of "write-back" as defined above; conversely, > the kernel's notion of "write back" sounds like the industry definition of > "write-through" > > Is there a well-meaning rationale for the kernel's concept of "write > through" to be different than what end users have been conditioned to > understand? I think we all agree what "write through" vs "write back" mean. I'm just not sure what's the source of the disconnect with the kernel's behavior. A "write through" device persists data before completing a write operation. Flush/FUA says to write data to persistence before completing the operation. You don't need both. Flush/FUA should be a no-op to a "write through" device because the data is synchronously committed to the backing store automatically.
