1，Flash Cache簡介
Flash cache 本身是Facebook的一個開源項目，（準確的說是一個Linux的模塊），可以動態加載。Flashcache通過在文件系統（VFS）和設備驅動之間新增了一次緩存層，來實現對熱門的緩存。Flashcache是另一種緩存，一般用SSD作爲介質的緩存（一般的緩存用的是內存），通過將傳統硬盤上的熱門數據緩存到SSD上，然後利用SSD優秀的讀性能，來加速系統。這個方法較之內存緩存，沒有內存快，但是空間可以比內存大很多。如下圖：
現在很多硬件廠商也會在存儲設備中增加這個功能來，例如IBM XIV的Flash cache和 EMC的VFCache
2， XIV Flash Cache
從11.1.0開始，XIV Gen3開始支持可選的Flash caching feature，極大的提高小數據塊，隨機讀的性能，適用於Data patterns一直改變的環境。
XIV flash caching is implemented as an extension of the primary cache layer. 每個模塊400GB Cache，那一個系統就是6TB(400GB*15)，從11.4開始，每個Module能支持800GB。Flash cache 只用於讀操作，當不再需要Cache中的數據時，直接drop掉就行了。
XIV flash caching overview
Flash caching算法是嵌入在XIV Firmware中的，能自動適應相應的IO類型，對用戶透明，不需要管理員手動的做performance turning。
XIV系統中有2種類型的Cache：main 和 extended
--The main cache handles host write I/Os and then destages them directly to the disk drive. //Main cache主要處理主機寫IO然後直接Flush到Disk
--The extended cache handles the caching of random read miss operations less than 64 KB. //Extended cache主要處理小於64KB的隨機讀操作
Sequential read prefetches (larger than 64 KB) are handled in main dynamic random access memory (DRAM) cache. //大於64KB的順序讀操作是由main cache處理的
3，Flash Cache learning
A flash cache map is built as read misses occur in the DRAM cache. The process, known as flash cache learning
關於XIV中Main cache和Extended Cache（Flash Cache）是如何扮演各自角色的，請看下圖：
XIV flash cache learning
The cache node immediately checks the extended cache for the requested I/O. If the requested I/O exists in the extended flash cache, it is served to the host through the main cache. The I/O operation is now complete and is recorded as a flash cache read hit.
If the operation results in a true read miss (not in the DRAM cache and not in extended flash cache), the request is forwarded in an unmodified state to the disk drive (SAS layer). The I/O is retrieved from the disk drive and served to the host through the main cache. From a host perspective, the I/O operation is now complete and is recorded as a read miss. The related pages are copied into reserved buffers in the main cache.
Important: Any read larger than 64 KB bypasses the extended flash cache.
當Buffer中的data達到512KB時，會順序地寫到Flash cache中。這種方式延長了Flash cache中的壽命。
Note: XIV在系統重啓和Firmware升級中能保存Flash Cache中的數據
XIV Storage System software Version 11.2 introduced improved flash caching algorithms,providing a performance boost of up to 4.5 times over systems without flash cache for random database-type workloads. This boost is accomplished by storing and computing all flash cache-related data integrity checking tasks in DRAM rather than on the flash cache.
4，Approaches for Using SSDs in a Storage System
Approach of the tier. With policies that moved the data … or As a caching layer. This is the approach of XIV.
5，SSD failure
No redistribution in case of failure/phase-out
SSD Failure:
-Reinitialize the metadata of the SSD (so it is not used)
-The degraded module continue to server reads from its DRAM cache and large sequential reads from its disks
-Small read misses are redirected to the secondary
SSD Phase-out and not failed
- Its data is invalidated (on writes)
=> if phased-in not all data is lost
During a rebuild, following a SAS disk failure in a module, the data on SSD in that module is
not invalidated. Rather, it is gradually updated to contain the new data blocks (the same way
the DRAM does).
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後記：關於EMC VFCache和IBM Flash Cache，感興趣的可以看看比特網的下圖對比。
