Private strand flush not complete 说明

前几天一个朋友问了我一个问题。说她在alert log里面看到了如下信息：

Thread 1 cannot allocate new log,sequence 415

Private strand flush not complete

Current log# 4 seq# 414 mem# 0: /dev/rora_redotb04

Thread 1 advanced to log sequence 415

Current log# 5 seq# 415 mem# 0: /dev/rora_redotb05

Thu Nov 11 16:01:51 2010

我遇到的是：Checkpoint not complete。有关这方面的解释参考我的Blog：

Redo Log和Checkpoint not complete

http://blog.youkuaiyun.com/xujinyang/article/details/6832719

在oracle官网搜了一下：

Alert Log Messages: Private Strand Flush Not Complete [ID 372557.1]
	Modified01-SEP-2010TypePROBLEMStatusMODERATED

In this Document
Symptoms
Cause
Solution
References

---

Platforms: 1-914CU;

This document is being delivered to you via Oracle Support's Rapid Visibility (RaV) process and therefore has not been subject to an independent technical review.

Applies to:

Oracle Server - Enterprise Edition - Version: 10.2.0.1 to 11.1.0.7 - Release: 10.2 to 11.1
Information in this document applies to any platform.
"Checked for relevance on 04-Dec-2007"


Private strand flush not complete

Symptoms

"Private strand flush not complete" messages are being populated to the alert log for unknown
reasons.

ALERT LOG EXAMPLE:
>>>
Fri May 19 12:47:29 2006
Thread 1 cannot allocate new log, sequence 18358
Private strand flush not complete
Current log# 7 seq# 18357 mem# 0: /u03/oradata/bitst/redo07.log
Thread 1 advanced to log sequence 18358
Current log# 8 seq# 18358 mem# 0: /u03/oradata/bitst/redo08.log
<<<


>>

Cause

The message means that we haven't completed writing all the redo information to the log when we are trying to switch.It is similar in nature to a "checkpoint not complete"except that is only involves the redo being written to the log.The log switch can not occur until all of the redo has been written.

A "strand" is new terminology for 10g and it deals with latches for redo .

Strands are a mechanism to allow multiple allocation latches for processes to write redo more efficiently in the redo buffer and is related to the log_parallelism parameter present in 9i.

The concept of a strand is to ensure that the redo generation rate for an instance is optimal and that when there is some kind of redo contention then the number of strands is dynamically adjusted to compensate.

The initial allocation for the number of strands depends on the number of CPU's and is started with 2strands with one strand for active redo generation.

For large scale enterprise systems the amount of redo generation is large and hence these strands are *made active* as and when the foregrounds encounter this redo contention (allocated latch related contention) when this concept of dynamic strands comes into play.

There is always shared strands and a number of private strands .

Oracle 10g has some major changes in the mechanisms for redo (and undo), which seem to be aimed at reducing contention.

Instead of redo being recorded in real time, it can be recorded 'privately' and pumped into the redo log buffer on commit.

Similary the undo can be generated as 'in memory undo' and applied in bulk.

This affect the memory used for redo management and the possibility to flush it in pieces.

The message you get is related to internal Cache Redo File management.

You can disregard these messages as normal messages.

When you switch logs all private strands have to be flushed to the current log before the switch is allowed to proceed.

Solution

These messages are not a cause for concern unless there is a significant gap in seq# between the "cannot allocate new log" message and the "advanced to log sequence" message.

This issue is infact not a bug and is expected behavior.

In some cases, this message can be resolved by increasing db_writer_process value.

这里面涉及到一些Redo的机制问题。具体参考Blog：

Oracle Redo并行机制

http://blog.youkuaiyun.com/xujinyang/article/details/6831357

一个redo条目包含了相应操作导致的数据库变化的所有信息，所有redo条目最终都要被写入redo文件中去。Redo log buffer是为了避免Redo文件IO导致性能瓶颈而在sga中分配出的一块内存。一个redo条目首先在用户内存(PGA)中产生，然后由oracle服务进程拷贝到log buffer中，当满足一定条件时，再由LGWR进程写入redo文件。

由于log buffer是一块“共享”内存，为了避免冲突，它是受到redo allocation latch保护的，每个服务进程需要先获取到该latch才能分配redo buffer。因此在高并发且数据修改频繁的oltp系统中，我们通常可以观察到redo allocation latch的等待。

为了减少redo allocation latch等待，在oracle 9.2中，引入了log buffer的并行机制。其基本原理就是，将log buffer划分为多个小的buffer，这些小的buffer被成为Shared Strand。每一个strand受到一个单独redo allocation latch的保护。多个shared strand的出现，使原来序列化的redo buffer分配变成了并行的过程，从而减少了redo allocation latch等待。

为了进一步降低redo buffer冲突，在10g中引入了新的strand机制——Private strand。Private strand不是从log buffer中划分的，而是在shared pool中分配的一块内存空间。

Private strand的引入为Oracle的Redo/Undo机制带来很大的变化。每一个Private strand受到一个单独的redo allocation latch保护，每个Private strand作为“私有的”strand只会服务于一个活动事务。获取到了Private strand的用户事务不是在PGA中而是在Private strand生成Redo，当flush private strand或者commit时，Private strand被批量写入log文件中。如果新事务申请不到Private strand的redo allocation latch，则会继续遵循旧的redo buffer机制，申请写入shared strand中。事务是否使用Private strand，可以由x$ktcxb的字段ktcxbflg的新增的第13位鉴定：

对于使用Private strand的事务，无需先申请Redo Copy Latch，也无需申请Shared Strand的redo allocation latch，而是flush或commit是批量写入磁盘，因此减少了Redo Copy Latch和redo allocation latch申请/释放次数、也减少了这些latch的等待，从而降低了CPU的负荷。

看了这些理论知识，我们在来看一下之前的错误：

Private strand flush not complete

当我们flush或者commit的时候，必须先将buffer中的内容写入到redo中，才能去接收新的记录。这个错误就是发生在这个过程中。Oracle对这个问题提了2个方法：

（1）忽略，在使用之前，必须要等待buffer的信息flush完成。这时候进程是会短暂的hang住。

（2）增加db_writer_process的数据。

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