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ADR Preallocation Factor (server configuration option)

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Applies to: SQL Server

Starting with SQL Server 2019 (15.x), this configuration setting is required for accelerated database recovery.

Accelerated database recovery (ADR) maintains versions of data for recovery purposes. These versions are generated as part of various data manipulation language (DML) operations. Versions are stored in an internal table called the persistent version store (PVS).

Remarks

Performance can degrade if pages are allocated for the PVS as part of foreground user DML operations. A background thread preallocates pages, and keeps them readily available for DML transactions. Performance is best when the background thread preallocates enough pages and the percentage of foreground PVS allocations is close to 0. The error log contains entires with the tag PreallocatePVS if the percentage goes high and is affecting performance.

The number of pages the background thread preallocates, is based on various workload heuristics, but largely allocates pages in chunks of 512 pages. The ADR preallocation factor is a multiple of the chunk. By default, the factor is 4, which means that it preallocates 2048 pages at once when required.

While the background thread takes workload patterns into consideration, this factor can be increased if necessary to improve performance.

Caution

If PVS preallocation is increased too much, it will contend with other allocations in the system and might actually reduce overall performance. Before you modify this setting, test the overall performance of the system.

Known issue

For SQL Server 2019 (15.x) CU 12 and previous versions, this value may be set to 0. We recommend that you reset the value to 4, which is the designed default, using the example in this article.

Examples

The following example sets the preallocation factor to 4.

sp_configure 'show advanced options', 1;
RECONFIGURE;
GO
sp_configure 'ADR Preallocation Factor', 4;
RECONFIGURE;
GO

See also