Exadata Smart Flash Logging新特性

从Exadata Storage Software 11.2.2.4开始引入了Exadata Smart Flash Logging的新特性，该特性允许LGWR进程将redo同时并行写入flash cache 和 disk controller 中， 只要在flash cache 和 disk controller中有任意一个率先写完成就会通知RDBMS数据库继续工作， 该特性着眼于改善了Exadata对Redo写入的响应时间和吞吐量。(The feature allows redo writes to be written to both flash cache and disk controller cache, with an acknowledgement sent to the RDBMS as soon as either of these writes complete; this improves response times and thoughput.)；特别有利于改善log file sync等待事件是主要性能瓶颈的系统。

当频繁写redo重做日志的IO成为Exadata一体机的主要性能瓶颈的时候，Exadata开发部门自然想到了通过DBM上已有的flashcache来减少响应时间的办法。但是又要保证不会因为flashcache的引入而导致redo关键日志信息的丢失：

The main problem that number of writes in redo logs is very high, even there are no activity in database. Therefore Flash Cache disk will reach his write limit very fast – some days or months (I am not see exact test results). In this way you will lost flash cache disk and all data on them. But losing redo logs is very unpleasant case of database unavailability, which can lie in big downtime and possible data loss.
As you already know, 11.2.2.4.0 introduced the Smart Flash Log feature. For customers that are not in 11.2.2.4.0, don’t suggest putting the redo logs on a diskgroup that uses griddisks carved from flashdisks. There are different issues when using redo logs on the flashcache in previous versions and those should be avoided.

只要是安装过Exadata Storage Software 11.2.2.4补丁的系统都会隐式地启用该Exadata Smart Flash Logging特性，但是它同时也要求数据库版本要大于Database 11.2.0.2 Bundle Patch 11。

Metalink目前没有介绍如何在已经启用Exadata Smart Flash Logging的DBM上禁用(disable)该特性。

实际每个cell会分配512MB的flashcache用于Smart Flash Logging，因此现在每个cell的可用flash空间为 364.75Gb 。

不仅局限于Online Redo Log可以受益于Smart Flash Logging，Standby Redo Log 也可以从该特性中得到性能提升，前提是满足必要的软件版本组合cell patch 11.2.2.4 and Database 11.2.0.2 Bundle Patch 11 or greate。

可以通过CellCLI 命令行了解现有的Smart Flash Logging配置，若有输出则说明配置了Smart Flash Logging。

CellCLI> LIST FLASHLOG DETAIL

更多信息可以参考文档”Exadata Smart Flash Logging Explained”，引用如下:

Smart Flash Logging works as follows. When receiving a redo log write request, Exadata will do
parallel writes to the on-disk redo logs as well as a small amount of space reserved in the flash
hardware. When either of these writes has successfully completed the database will be
immediately notified of completion. If the disk drives hosting the logs experience slow response
times, then the Exadata Smart Flash Cache will provide a faster log write response time.
Conversely, if the Exadata Smart Flash Cache is temporarily experiencing slow response times
(e.g., due to wear leveling algorithms), then the disk drive will provide a faster response time.
Given the speed advantage the Exadata flash hardware has over disk drives, log writes should be
written to Exadata Smart Flash Cache, almost all of the time, resulting in very fast redo write
performance. This algorithm will significantly smooth out redo write response times and provide
overall better database performance.

The Exadata Smart Flash Cache is not used as a permanent store for redo data – it is just a
temporary store for the purpose of providing fast redo write response time. The Exadata Smart
Flash Cache is a cache for storing redo data until this data is safely written to disk. The Exadata
Storage Server comes with a substantial amount of flash storage. A small amount is allocated for
database logging and the remainder will be used for caching user data. The best practices and
configuration of redo log sizing, duplexing and mirroring do not change when using Exadata
Smart Flash Logging. Smart Flash Logging handles all crash and recovery scenarios without
requiring any additional or special administrator intervention beyond what would normally be
needed for recovery of the database from redo logs. From an end user perspective, the system
behaves in a completely transparent manner and the user need not be aware that flash is being
used as a temporary store for redo. The only behavioral difference will be consistently low
latencies for redo log writes.

By default, 512 MB of the Exadata flash is allocated to Smart Flash Logging. Relative to the 384
GB of flash in each Exadata cell this is an insignificant investment for a huge performance
benefit. This default allocation will be sufficient for most situations. Statistics are maintained to
indicate the number and frequency of redo writes serviced by flash and those that could not be
serviced, due to, for example, insufficient flash space being allocated for Smart Flash Logging.
For a database with a high redo generation rate, or when many databases are consolidated on to
one Exadata Database Machine, the size of the flash allocated to Smart Flash Logging may need
to be enlarged. In addition, for consolidated deployments, the Exadata I/O Resource Manager
(IORM) has been enhanced to enable or disable Smart Flash Logging for the different databases
running on the Database Machine, reserving flash for the most performance critical databases.

以及<Exadata Smart Flash Cache Features and the Oracle Exadata Database Machine>官方白皮书，公开的文档地址：

[gview file=”http://www.oracle.com/technetwork/database/exadata/exadata-smart-flash-cache-366203.pdf”]