Fri Oct 17 19:59:51 2014

Thread 1 cannot allocate new log, sequence 4722

Private strand flush not complete

Current log# 1 seq# 4721 mem# 0: /oradata/sgomp5/redo01.log

Thread 1 advanced to log sequence 4722 (LGWR switch)

Current log# 2 seq# 4722 mem# 0: /oradata/sgomp5/redo02.log

在MOS社区中找到了一篇对于这个问题的文章：

Historically, Every user session wrote the changes to redo log buffer andwww.sangpi.com changes from redo log buffer are flushed to redo logs on disk by lgwr. As number of users increased, the race and the need to get latch for redo allocation and redo copy on the public redo buffer increased.

So, starting from 10g, Oracle came up with concept ofprivate redo (x$kcrfstrand) and in-memory undo (x$ktifp). Every session has private redo where session writes to and then a (small) batch of changes is written to public redo and finally from public redo log buffer to redo log files on disk. This mechanismreduces the gets/sleeps on redo copy and redo allocation latches on the public redo buffer and hence makes the architecture more scalable.

It is also worth noting that oracle falls back to old redo mechanism in case transaction is too big (with lots of changes) and if changes done by that transaction can't fit into private redo buffers.

当数据库切换日志时，所有private strand都必须刷新到以后日志，而后能力持续。此信息示意咱们在尝试切换时，还没有齐全将所有 redo信息写入到日志中。这有点相似于“checkpoint not complete”，不同的是，它仅波及到正在被写入日志的redo。在写入所有redo前，无奈切换日志。

Private Strands是10gR2才有的，它用于解决redo的latch（redo allocation latch）。是一种容许过程利用多个allocation latch更高效地将redo写入redo buffer cache的机制，它与9i中呈现的log_parallelism参数相干。提出Strand的概念是为了确保实例的redo生成率达到最佳，并能确保在呈现某种redo争用时，能够动静调整strand的数量进行弥补。初始调配的strand数量取决于CPU的数量，起码两个strand，其中一个strand用于active的redo生成。

对于大型的oltp零碎，redo生成量十分大，因而以后台过程遇到redo争用时，这些游戏strand会被激活。shared strand总是与多个private strand共存。Oracle 10g的redo（和undo）机制有一些重大变动，目标是为了缩小争用。此机制不再实时记录redo，而是先记录在一个private area，并在commit时flush到redo log buffer中去。在这种新机制引入后，一旦用户过程申请到private strand，redo不再保留到pga中，因而不再须要redo copy latch这个过程。

如果新事务申请不到private strand的redo allocation latch，则会持续遵循旧的redo buffer机制，申请写入shared strand中。对于这个新的机制，在进行redo被写出到logfile时，LGWR须要将shared strand与private strand的内容写出。当redo flush产生时，所有的public strands的redo allocation latch须要被获取，所有的public strands的redo copy latch须要被查看，所有蕴含流动事务的private strands须要被持有。

其实，对于这个景象也能够疏忽，除非“cannot allocate new log”信息和“advanced to log sequence”信息之间有显著的时间差。

如果想要在alert.log中避免出现Private strand flush not complete事件，那么能够通过减少参数db_writer_processes的值来实现，因为DBWn会触发LGWR将redo写入到logfile，如果有多个DBWn过程一起写，能够减速redo buffer cache写入redo logfile。

能够应用以下命令批改：

SQL> alter system set db_writer_processes=4 scope=spfile; --该参数时动态参数，必须重启数据库后失效

留神，DBWR过程数应该与逻辑CPU数相当。另外地，当oracle发现一个DB_WRITER_PROCESS不能实现工作时，也会主动减少其数量，前提是曾经在初始化参数中设定过最大容许的值。

对于DB_WRITER_PROCESSES和DBWR_IO_SLAVES参数的一些阐明：

DB_WRITER_PROCESSES replaces the Oracle7 parameter DB_WRITERS and specifies the initial number of database writer processes for an instance. If you use DBWR_IO_SLAVES, only one database writer process will be used, regardless of the setting for DB_WRITER_PROCESSES

DB_WRITER_PROCESSES参数就是在Oracle 7中的DB_WRITERS参数，用来指定数据库实例的DBWR过程个数，当零碎中还配置了DBWR_IO_SLAVES参数时（默认为0），则只能利用到一个DBWn过程，而疏忽其余的。

DBWR_IO_SLAVES

If it is not practical to use multiple DBWR processes, then Oracle provides a facility whereby the I/O load can be distributed over multiple slave processes. The DBWR process is the only process that scans the buffer cache LRU list for blocks to be written out. However, the I/O for those blocks is performed by the I/O slaves. The number of I/O slaves is determined by the parameter DBWR_IO_SLAVES.

当应用繁多DBWR过程时，Oralce提供了应用多个I/O slave过程来实现模仿异步IO，去实现全本应该由DBWR做的事件（写LRU上的数据块到磁盘文件），这个slave的数量是通过DBWR_IO_SLAVES参数来指定的

DBWR_IO_SLAVES is intended for scenarios where you cannot use multiple DB_WRITER_PROCESSES (for example, where you have a single CPU). I/O slaves are also useful when asynchronous I/O is not available, because the multiple I/O slaves simulate nonblocking, asynchronous requests by freeing DBWR to continue identifying blocks in the cache to be written. Asynchronous I/O at the operating system level, if you have it, is generally preferred.

DBWR_IO_SLAVES参数通常被用在单CPU的场景中，因为单CPU即便设置了多DBWR过程数也是没有成果的。无论操作系统是否反对异步IO，应用多个I/O slaves都是无效的，能够分担DBWR的工作。如果应用了异步IO，那就更加举荐设置了

DBWR I/O slaves are allocated immediately following database open when the first I/O request is made. The DBWR continues to perform all of the DBWR-related work, apart from performing I/O. I/O slaves simply perform the I/O on behalf of DBWR. The writing of the batch is parallelized between the I/O slaves.

DBWR的I/O slaves当数据库open时产生第一次I/O申请时被调配，DBWR过程持续实现与本身相干工作，而拆散出局部I/O解决工作给I/O slaves，各个I/O slaves之间的I/O解决都是并行的

Choosing Between Multiple DBWR Processes and I/O Slaves

Configuring multiple DBWR processes benefits performance when a single DBWR process is unable to keep up with the required workload. However, before configuring multiple DBWR processes, check whether asynchronous I/O is available and configured on the system. If the system supports asynchronous I/O but it is not currently used, then enable asynchronous I/O to see if this alleviates the problem. If the system does not support asynchronous I/O, or if asynchronous I/O is already configured and there is still a DBWR bottleneck, then configure multiple DBWR processes.

对于如何抉择多个DBWR过程和I/O slaves过程

当繁多的DBWR过程无奈胜任大量的写工作负载，配置多个DBWR过程是无效的。然而在配置多个DBWR过程前，须要先查看OS上是否反对异步I/O，如果反对但未开启，那么先开启；如果零碎不反对或曾经配置了异步IO后，依然有DBWR瓶颈，那么就能够配置多个DBWR过程

Using multiple DBWRs parallelizes the gathering and writing of buffers. Therefore, multiple DBWn processes should deliver more throughput than one DBWR process with the same number of I/O slaves. For this reason, the use of I/O slaves has been deprecated in favor of multiple DBWR processes. I/O slaves should only be used if multiple DBWR processes cannot be configured.

开启多个DBWR过程就意味着能够并行写更多的脏缓存（dirty buffer）到数据文件，而多个DBWR的吞吐量，也要比1个DBWR+相当数量的I/O slaves的要高，因而，当开启了多个DBWR过程时，就不应该再配置DBWR_IO_SLAVES（如果原来是非零的话），能够把这个参数设置为0

总结：

DBWR_IO_SLAVES次要用于模仿异步环境，在不反对异步操作的OS上，能够进步IO的读写速度。

多个DBWR过程能够并行地从data buffer中获取dirty block并且并行地写入磁盘。然而，在单DBWR+多个I/O slaves的场景下，只能是一个DBWR负责从data buffer中获取，而多个I/O slaves并行写入。如果零碎反对AIO（disk_async_io=true），个别不必设置多dbwr 或io slaves。

如果在有多个cpu的状况下倡议应用DB_WRITER_PROCESSES，因为这样的状况下不必去模仿异步模式，但要留神过程数量不能大于cpu数量。而在只有一个cpu的状况下倡议应用DBWR_IO_SLAVES来模仿异步模式，以便进步数据库性能。

----官网倡议：

Default value 1 or CPU_COUNT / 8, whichever is greater 默认值为1或CPU个数的1/8

[oracle@orcl ~]$ vmstat 5 ----对CPU以后信息进行采集，每5秒一次

SQL> show parameter cpu_count; ----查看以后cpu_count数

SQL> show parameter db_writer_processes; ----查看以后数据写过程数