Exadata的flashcache是不是只对读有好处？

Wavelet123

Oracle 11g的flashcache允许用户用SSD创建自己的flashcache,但是这种flashcache是local的,Exadata的flashcache是cellserver上的。可以在节点间共享。根据我自己的理解,oracle 11g的flashcache只对读有好处，对写没有任何帮助。但是Exadata是不是做了不同的处理呢？

谢谢。

Maclean Liu(刘相兵

FYI

The Exadata Smart Flash Cache is also used to reduce the latency of log write I/O eliminating
performance bottlenecks that might occur due to database logging. The time to commit user
transactions is very sensitive to the latency of log writes. Also, many performance critical
database algorithms such as space management and index splits are also very sensitive to log
write latency. Today Exadata storage speeds up log writes using the battery backed DRAM cache
in the disk controller. Writes to the disk controller cache are normally very fast, but they can
become slower during periods of high disk IO. Smart Flash Logging takes advantage of the flash
memory in Exadata storage to speed up log writes.
Flash memory has very good average write latency, but it has occasional slow outliers that can be
one or two orders of magnitude slower than the average. The idea of the Exadata Smart Logging
is to perform redo writes simultaneously to both flash memory and the disk controller cache, and
complete the write when the first of the two completes. This literally gives Exadata the best of
both worlds. The Smart Flash Logging both improves user transaction response time, and
increases overall database throughput for IO intensive workloads by accelerating performance
critical database algorithms.

Wavelet123

smart flash logging只提高了redo的速度。但是对写数据文件没帮助啊。看这个链接分析的挺有道理。不知道Exadata是不是一样。http://www.dbthink.com/archives/169

Maclean Liu(刘相兵

Exadata Smart Flash Cache software (11.2.2.3.2) implements automatic caching of database reads and writes increasing write IOPS 20x and data bandwidth by 33%, This software delivers 1,000,000 8K SQL flash write IOPS in a Full Rack X3 Database Machine. Combined with 22.4 TB of flash capacity, the new write caching technology ensures ultra high performance for even the most demanding OLTP databases.
The new Exadata Smart Flash Cache software is backward compatible to prior generations of Database Machines enabling write caching on these systems. Up to 500,000 8K SQL flash write IOs per second can be perforated on X2 and V2 Database Machines providing transparent performance improvement to existing customers. In addition, Exadata Smart Flash Cache is persistent across Exadata Storage Server restarts and will not require any warm up period.

需要具体测试