7 Tuning the WebLogic Persistent Store

The following section provides general information on tuning File Stores:

• For custom and default file stores, tune the Synchronous Write Policy.

  • There are three transactionally safe synchronous write policies: Direct-Write-With-Cache, Direct-Write, and Cache-Flush. Direct-Write-With-Cache is generally has the best performance of these policies, Cache-Flush generally has the lowest performance, and Direct-Write is the default. Unlike other policies, Direct-Write-With-Cache creates cache files in addition to primary files.

  • The Disabled synchronous write policy is transactionally unsafe. The Disabled write-policy can dramatically improve performance, especially at low client loads. However, it is unsafe because writes become asynchronous and data can be lost in the event of Operating System or power failure.

  • See "Guidelines for Configuring a Synchronous Write Policy" in Administering the WebLogic Persistent Store.

  Note:

  Certain older versions of Microsoft Windows may incorrectly report storage device synchronous write completion if the Windows default Write Cache Enabled setting is used. This violates the transactional semantics of transactional products (not specific to Oracle), including file stores configured with a Direct-Write (default) or Direct-Write-With-Cache policy, as a system crash or power failure can lead to a loss or a duplication of records/messages. One of the visible symptoms is that this problem may manifest itself in high persistent message/transaction throughput exceeding the physical capabilities of your storage device. You can address the problem by applying a Microsoft supplied patch, disabling the Windows Write Cache Enabled setting, or by using a power-protected storage device. See http://support.microsoft.com/kb/281672 and http://support.microsoft.com/kb/332023.

• When performing head-to-head vendor comparisons, make sure all the write policies for the persistent store are equivalent. Some non-WebLogic vendors default to the equivalent of Disabled.

• If disk performance continues to be a bottleneck, consider purchasing disk or RAID controller hardware that has a built-in write-back cache. These caches significantly improve performance by temporarily storing persistent data in volatile memory. To ensure transactionally safe write-back caches, they must be protected against power outages, host machine failure, and operating system failure. Typically, such protection is provided by a battery-backed write-back cache.

The Direct-Write-With-Cache synchronous write policy is commonly the highest performance option that still provides transactionally safe disk writes. It is typically not as high-performing as the Disabled synchronous write policy, but the Disabled policy is not a safe option for production systems unless you have some means to prevent loss of buffered writes during a system failure.

Direct-Write-With-Cache file stores write synchronously to a primary set of files in the location defined by the Directory attribute of the file store configuration. They also asynchronously write to a corresponding temporary cache file in the location defined by the CacheDirectory attribute of the file store configuration. The cache directory and the primary file serve different purposes and require different locations. In many cases, primary files should be stored in remote storage for high availability, whereas cache files are strictly for performance and not for high availability and can be stored locally.

When the Direct-Write-With-Cache synchronous write policy is selected, there are several additional tuning options that you should consider:

• Setting the CacheDirectory. For performance reasons, the cache directory should be located on a local file system. It is placed in the operating system temp directory by default.

• Increasing the MaxWindowBufferSize and IOBufferSize attributes. These tune native memory usage of the file store.

• Increasing the InitialSize and MaxFileSize tuning attributes. These tune the initial size of a store, and the maximum file size of a particular file in the store respectively.