Instance store support for the tempdb database on Amazon RDS for SQL Server - Amazon Relational Database Service

Instance store support for the tempdb database on Amazon RDS for SQL Server

An instance store provides temporary block-level storage for your DB instance. This storage is located on disks that are physically attached to the host computer. These disks have Non-Volatile Memory Express (NVMe) instance storage that is based on solid-state drives (SSDs). This storage is optimized for low latency, very high random I/O performance, and high sequential read throughput.

By placing tempdb data files and tempdb log files on the instance store, you can achieve lower read and write latencies compared to standard storage based on Amazon EBS.

Note

SQL Server database files and database log files aren't placed on the instance store.

Enabling the instance store

When RDS provisions DB instances with one of the following instance classes, the tempdb database is automatically placed onto the instance store:

  • db.m5d

  • db.r5d

To enable the instance store, do one of the following:

The instance store is available in all AWS Regions where one or more of these instance types are supported. For more information on the db.m5d and db.r5d instance classes, see DB instance classes. For more information on the instance classes supported by Amazon RDS for SQL Server, see DB instance class support for Microsoft SQL Server.

File location and size considerations

On instances without an instance store, RDS stores the tempdb data and log files in the D:\rdsdbdata\DATA directory. Both files start at 8 MB by default.

On instances with an instance store, RDS stores the tempdb data and log files in the T:\rdsdbdata\DATA directory.

When tempdb has only one data file (tempdb.mdf) and one log file (templog.ldf), templog.ldf starts at 8 MB by default and tempdb.mdf starts at 80% or more of the instance's storage capacity. Twenty percent of the storage capacity or 200 GB, whichever is less, is kept free to start. Multiple tempdb data files split the 80% disk space evenly, while log files always have an 8-MB initial size.

For example, if you modify your DB instance class from db.m5.2xlarge to db.m5d.2xlarge, the size of tempdb data files increases from 8 MB each to 234 GB in total.

Note

Besides the tempdb data and log files on the instance store (T:\rdsdbdata\DATA), you can still create extra tempdb data and log files on the data volume (D:\rdsdbdata\DATA). Those files always have an 8 MB initial size.

Backup considerations

You might need to retain backups for long periods, incurring costs over time. The tempdb data and log blocks can change very often depending on the workload. This can greatly increase the DB snapshot size.

When tempdb is on the instance store, snapshots don't include temporary files. This means that snapshot sizes are smaller and consume less of the free backup allocation compared to EBS-only storage.

Disk full errors

If you use all of the available space in the instance store, you might receive errors such as the following:

  • The transaction log for database 'tempdb' is full due to 'ACTIVE_TRANSACTION'.

  • Could not allocate space for object 'dbo.SORT temporary run storage: 140738941419520' in database 'tempdb' because the 'PRIMARY' filegroup is full. Create disk space by deleting unneeded files, dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup.

You can do one or more of the following when the instance store is full:

  • Adjust your workload or the way you use tempdb.

  • Scale up to use a DB instance class with more NVMe storage.

  • Stop using the instance store, and use an instance class with only EBS storage.

  • Use a mixed mode by adding secondary data or log files for tempdb on the EBS volume.

Removing the instance store

To remove the instance store, modify your SQL Server DB instance to use an instance type that doesn't support instance store, such as db.m5 or db.r5.

Note

When you remove the instance store, the temporary files are moved to the D:\rdsdbdata\DATA directory and reduced in size to 8 MB.