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
1.CPU读Cache时：
●若hit，则CPU直接从Cache中读取数据即可。
●若miss，有两种处理方式：
>Read through，即直接从内存中读取数据；
>Read allocate，先把数据读取到Cache中，再从Cache中读数据。
2.CPU写Cache时：
●若hit，有两种处理方式：
>Write through：把数据同时写到Cache和内存中；
>Write back：先把数据写到Cache中，再通过flush方式写入到内存中。
●若miss，有两种处理方式：
>Write allocate：先把要写的数据载入到Cache中，写Cache，然后再通过flush方式写入到内存中；
>No write allocate：直接把要写的数据写入到内存中。
Cache写机制：Write-through与Write-back
Cache写机制
参考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