Cache写机制:Write-through与Write-back

2015-03-18 476 0

from: http://witmax.cn/cache-writing-policies.html

Cache写机制

参考[http://en.wikipedia.org/wiki/Cache#Writing_Policies](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-allocateWrite 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](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/Write_through_with_no-write_allocation.png/445px-Write_through_with_no-write_allocation.png "A Write-Through cache with No-Write Allocation")](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/Write_through_with_no-write_allocation.png/445px-Write_through_with_no-write_allocation.png "A Write-Through cache with No-Write Allocation")A Write-Through cache with No-Write Allocation

Write-back模式处理流程:

 

 

[![A Write-Back cache with Write Allocation](http://upload.wikimedia.org/wikipedia/commons/thumb/a/ad/Write_back_with_write_allocation.png/468px-Write_back_with_write_allocation.png "A Write-Back cache with Write Allocation")](http://upload.wikimedia.org/wikipedia/commons/thumb/a/ad/Write_back_with_write_allocation.png/468px-Write_back_with_write_allocation.png "A Write-Back cache with Write Allocation")A Write-Back cache with Write Allocation

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