硬盘Cache写机制 Write-through与Write-back的区别

http://en.wikipedia.org/wiki/cache#writing_policies上的说明，cache写机制分为write through和write back两种。

write-through- write is done synchronously both to the cache and to the backing store.
write-back (or write-behind) – writing is done only to the cache. a modified cache block is written back to the store, just before it is replaced.
write-through（直写模式）在数据更新时，同时写入缓存cache和后端存储。此模式的优点是操作简单；缺点是因为数据修改需要同时写入存储，数据写入速度较慢。


write-back（回写模式）在数据更新时只写入缓存cache。只在数据被替换出缓存时，被修改的缓存数据才会被写到后端存储。此模式的优点是数据写入速度快，因为不需要写存储；缺点是一旦更新后的数据未被写入存储时出现系统掉电的情况，数据将无法找回。

write-misses写缺失的处理方式

对于写操作，存在写入缓存缺失数据的情况，这时有两种处理方式：

write allocate (aka fetch on write) – datum at the missed-write location is loaded to cache, followed by a write-hit operation. in this approach, write misses are similar to read-misses.
no-write allocate (aka write-no-allocate, write around) – datum at the missed-write location is not loaded to cache, and is written directly to the backing store. in this approach, actually only system reads are being cached.
write allocate方式将写入位置读入缓存，然后采用write-hit（缓存命中写入）操作。写缺失操作与读缺失操作类似。

no-write allocate方式并不将写入位置读入缓存，而是直接将数据写入存储。这种方式下，只有读操作会被缓存。

无论是write-through还是write-back都可以使用写缺失的两种方式之一。只是通常write-back采用write allocate方式，而write-through采用no-write allocate方式；因为多次写入同一缓存时，write allocate配合write-back可以提升性能；而对于write-through则没有帮助。

处理流程图

write-through模式处理流程：



a write-through cache with no-write allocation

write-back模式处理流程：



a write-back cache with write allocation

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