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Configure Persistent Memory (PMEM) - Linux
Learn how to configure persistent memory (PMEM) for SQL Server on Linux, and how to create namespaces for PMEM devices
briancarrig
brcarrig
randolphwest
01/21/2025
sql
linux
how-to
linux-related-content
>=sql-server-linux-ver15 || >=sql-server-ver15

Configure persistent memory (PMEM) for SQL Server on Linux

[!INCLUDE SQL Server - Linux]

This article describes how to configure the persistent memory (PMEM) for [!INCLUDE sssql19-md] and later versions on Linux.

Overview

[!INCLUDE sssql19-md] introduced many in-memory features that use persistent memory. This article covers the steps required to configure persistent memory for [!INCLUDE ssnoversion-md] on Linux.

Note

The term enlightenment was introduced to convey the concept of working with a persistent memory aware file system. Direct access to the file system from user-space applications is facilitated using memory mapping (mmap()). When a memory mapping for a file is created the application can issue load/store instructions bypassing the I/O stack completely. This is considered an "enlightened" file access method from the perspective of the host extension application (which is the code that allows SQLPAL interact with the Windows or Linux OS).

Create namespaces for PMEM devices

Configure the devices

In Linux, use the ndctl utility.

  • Install ndctl to configure PMEM device. You can find it here.
  • Use ndctl to create a namespace. Namespaces are interleaved across PMEM NVDIMMs and can provide different types of user-space access to memory regions on the device. fsdax is default and desired mode for [!INCLUDE ssnoversion-md].
ndctl create-namespace -f -e namespace0.0 --mode=fsdax --map=dev

We have chosen fsdax mode and are using system memory to store per-page metadata. We recommend using --map=dev. This option stores the metadata on the namespace directly. Storing metadata in memory using --map=mem is experimental at this time.

Use ndctl to verify the namespace.

Sample output follows:

# ndctl list -N
{
  "dev":"namespace0.0",
  "mode":"fsdax",
  "map":"dev",
  "size":4294967296,
  "sector_size":512,
  "blockdev":"pmem0",
  "numa_node":0
}

Create and mount PMEM device

For example, with XFS

mkfs.xfs -f /dev/pmem0
mount -o dax,noatime /dev/pmem0 /mnt/dax
xfs_io -c "extsize 2m" /mnt/dax

For example, with EXT4

mkfs.ext4 -b 4096 -E stride=512 -F /dev/pmem0
mount -o dax,noatime /dev/pmem0 /mnt/dax

Technical considerations

  • Block allocation of 2 MB for either XFS/EXT4, as described previously
  • Misalignment between block allocation and mmap results in silent fallback to 4 KB
  • File sizes should be a multiple of 2 MB (modulo 2 MB)
  • Don't disable transparent huge pages (THP) (enabled by default on most distributions)

Once the device is configured with ndctl, created, and mounted, you can place database files in it or create a new database.

You can store the [!INCLUDE ssnoversion-md] data files (MDFS, NDFS) and tempdb files on a PMEM device when configured with the mode fsdax using the following command. Don't use this to store the [!INCLUDE ssnoversion-md] log (LDFS) files, as transaction log needs to be on storage that provides sector atomic guarantees:

ndctl create-namespace -f -e namespace0.0 --mode=fsdax --map=dev

Before you set the map option in the preceding command, keep the following points in mind:

  • For best performance at accessing and updating these NVDIMM page entries for this device, it's preferable to use -map=mem
  • If the capacity of the NVDIMM is too large (greater than 512 GB), set the –map=dev, which would impact the IO throughput and stymie the performance

For [!INCLUDE ssnoversion-md] log files on PMEM devices, con the PMEM device(s) to use sector/Block Translation Table (BTT). This provides the needed sector atomicity for [!INCLUDE ssnoversion-md] logs files for this technology of storage devices. We also recommend that you perform workload performance validations. You can compare the [!INCLUDE ssnoversion-md] log performance for your workload between this solution and best-in-class NVMe SSDs, and then select the solution that best meets your needs and provides better performance.

ndctl create-namespace -f -e namespace0.0 --mode= sector

Disable forced flush behavior

Because PMEM devices are O_DIRECT (direct I/O) safe, you can disable the forced flush behavior.

Note

A storage system can make sure that any cached or staged writes are considered safe and durable, by guaranteeing that writes issued to the device are kept on a medium that will persist across system crashes, interface resets and power failures, and the medium itself is hardware redundant.

  • Database (.mdf and .ndf) and transaction log (.ldf) files don't use writethrough and alternatewritethrough by default in [!INCLUDE sssql17-md] CU 6 and later versions, because they use the forced flush behavior. Trace Flag 3979 disables the use of the forced flush behavior for database and transaction log files, and uses the writethrough and alternatewritethrough logic.

  • Other files that are opened by using FILE_FLAG_WRITE_THROUGH in [!INCLUDE ssnoversion-md], such as database snapshots, internal snapshots for database consistency checks (DBCC CHECKDB), profiler trace files, and extended event trace files, use the writethrough and alternatewritethrough optimizations.

For more information about the changes introduced in [!INCLUDE sssql17-md] CU 6, see KB 4131496. For more information about forced unit access (FUA) internals, see FUA internals.

SQL Server and Forced Unit Access (FUA) I/O subsystem capability

[!INCLUDE linux-forced-unit-access]

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