但是flashcache在使用中很多人會有個誤區，導致效能很低。首先我們看下flashcache的設計背景和適用場景：

Introduction :
============
Flashcache is a write back block cache Linux kernel module. This
document describes the design, futures ideas, configuration, tuning of
the flashcache and concludes with a note covering the testability
hooks within flashcache and the testing that we did. Flashcache was
built primarily as a block cache for InnoDB but is general purpose and
can be used by other applications as well.

它是為資料庫這樣的應用的離散讀寫優化。如果你用在了順序讀寫，就有非常大的效能問題。
那麼為什麼呢？我來分析下：

flashcache把內部的cache空間分成很多set, 是以set而不是整體為單位提供cache以及flush後備操作. 也就是說當一個set裡面的dirty page達到一個預設的值的時候，就需要把這麼dirty page 淘汰並且flush到後備裝置去，以便騰出空間給更熱的資料使用。
那麼每個set多大呢？

To compute the target set for a given dbn
target set = (dbn / block size / set size) mod (number of sets)
Once we have the target set, linear probe within the set finds the
block. Note that a sequential range of disk blocks will all map onto a
given set.

set預設是 512*4k = 2M大小，也就是說如果你的這個set剛好是一個檔案所在的塊，而且每次這個檔案都不停的順序寫，很快這個set都變成dirty, 那麼flashcache就選擇馬上刷，這樣加速效果就沒有了。

幸好作者Mohan認識到了這個問題，提供瞭解決方案：

見https://github.com/facebook/flashcache/blob/master/doc/flashcache-sa-guide.txt 中的章節Tuning Sequential IO Skipping for better flashcache performance

引入了配置引數來解決這個問題：

dev.flashcache..skip_seq_thresh_kb:
Skip (don’t cache) sequential IO larger than this number (in kb).
0 (default) means cache all IO, both sequential and random.
Sequential IO can only be determined ‘after the fact’, so
this much of each sequential I/O will be cached before we skip
the rest. Does not affect searching for IO in an existing cache.

這樣你可以把太大的順序操作給過濾掉了，大大提升效能。

祝玩得開心！

Post Footer automatically generated by wp-posturl plugin for wordpress.