Performance best practices and configuration guidelines for SQL Server on Linux

Artikkeli
11/18/2024

Applies to: SQL Server - Linux

This article provides best practices and recommendations to maximize performance for database applications that connect to SQL Server on Linux. These recommendations are specific to running on the Linux platform. All normal SQL Server recommendations, such as index design, still apply.

The following guidelines contain recommendations for configuring both SQL Server and the Linux operating system (OS). Consider using these configuration settings to experience the best performance for a SQL Server installation.

Storage configuration recommendation
Kernel and CPU settings for high performance
SQL Server configuration

Storage configuration recommendation

The storage subsystem hosting data, transaction logs, and other associated files (such as checkpoint files for in-memory OLTP) should be capable of managing both average and peak workload gracefully.

Use storage subsystem with appropriate IOPS, throughput, and redundancy

Normally, in on-premises environments, the storage vendor supports appropriate hardware RAID configuration with striping across multiple disks to ensure appropriate IOPS, throughput, and redundancy. Though, this can differ across different storage vendors and different storage offerings with varying architectures.

For SQL Server on Linux deployed on Azure Virtual Machines, consider using software RAID to ensure appropriate IOPS and throughput requirements are achieved. When configuring SQL Server on Azure virtual machines with similar storage considerations, see Configure storage for SQL Server on Azure VMs.

The following example shows how to create software RAID in Linux on Azure Virtual Machines. Keep in mind that you should use the appropriate number of data disks for the required throughput and IOPS for volumes based on the data, transaction log, and tempdb I/O requirements. In the following example, eight data disks were attached to the Azure Virtual Machine; 4 to host data files, 2 for transaction logs, and 2 for tempdb workload.

To locate the devices (for example /dev/sdc) for RAID creation, use the lsblk command.

# For Data volume, using 4 devices, in RAID 5 configuration with 8KB stripes
mdadm --create --verbose /dev/md0 --level=raid5 --chunk=8K --raid-devices=4 /dev/sdc /dev/sdd /dev/sde /dev/sdf

# For Log volume, using 2 devices in RAID 10 configuration with 64KB stripes
mdadm --create --verbose /dev/md1 --level=raid10 --chunk=64K --raid-devices=2 /dev/sdg /dev/sdh

# For tempdb volume, using 2 devices in RAID 0 configuration with 64KB stripes
mdadm --create --verbose /dev/md2 --level=raid0 --chunk=64K --raid-devices=2 /dev/sdi /dev/sdj

Disk partitioning and configuration recommendations

For SQL Server, you should use a RAID configuration. The deployed filesystem stripe unit (sunit) and stripe width should match the RAID geometry. For example, this is an XFS-based example for a log volume.

# Creating a log volume, using 6 devices, in RAID 10 configuration with 64KB stripes
mdadm --create --verbose /dev/md3 --level=raid10 --chunk=64K --raid-devices=6 /dev/sda /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf

mkfs.xfs /dev/md3 -f -L log
meta-data=/dev/md3               isize=512    agcount=32, agsize=18287648 blks
         =                       sectsz=4096  attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=1, rmapbt=0
         =                       reflink=1
data     =                       bsize=4096   blocks=585204384, imaxpct=5
         =                       sunit=16     swidth=48 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=285744, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

The log array is a 6-drive RAID-10 with a 64-KB stripe. As you can see:

For sunit=16 blks, 16 * 4096 block size = 64 KB, matches the stripe size.
For swidth=48 blks, swidth / sunit = 3, which is the number of data drives in the array, excluding parity drives.

filesystem configuration recommendation

SQL Server supports both ext4 and XFS filesystems to host the database, transaction logs, and additional files such as checkpoint files for in-memory OLTP in SQL Server. Microsoft recommends using XFS filesystem for hosting the SQL Server data and transaction log files.

Format the volume with the XFS filesystem:

mkfs.xfs /dev/md0 -f -L datavolume
mkfs.xfs /dev/md1 -f -L logvolume
mkfs.xfs /dev/md2 -f -L tempdb

It's possible to configure the XFS filesystem to be case insensitive when creating and formatting the XFS volume. It isn't the frequently used configuration in the Linux ecosystem, but can be used for compatibility reasons.

For example, you can run the following command. -n version=ci is used to configure the XFS filesystem to be case insensitive.

mkfs.xfs /dev/md0 -f -n version=ci -L datavolume

Persistent memory filesystem recommendation

For the filesystem configuration on Persistent Memory devices, the block allocation for the underlying filesystem should be 2 MB. For more information on this article, review the article Technical considerations.

Open file limitation

Your production environment might require more connections than the default open file limit of 1,024. You can set soft and hard limits of 1,048,576. For example, in RHEL, edit the /etc/security/limits.d/99-mssql-server.conf file to have the following values:

mssql - nofile 1048576

Note

This setting doesn't apply to SQL Server services started by systemd. For more information, see How to set limits for services in RHEL and systemd.

Disable last accessed date/time on filesystems for SQL Server data and log files

To ensure that the drive(s) attached to the system remount automatically after a restart, add them to the /etc/fstab file. You should also use the UUID (Universally Unique Identifier) in /etc/fstab to refer to the drive, rather than just the device name (such as /dev/sdc1).

Use the noatime attribute with any filesystem that stores SQL Server data and log files. Refer to your Linux documentation on how to set this attribute. An example of how to enable noatime option for a volume mounted in Azure Virtual Machine follows.

The mount point entry in /etc/fstab:

UUID="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" /data1 xfs rw,attr2,noatime 0 0

In the previous example, UUID represents the device that you can find using the blkid command.

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

Certain versions of supported Linux distributions provide support for FUA I/O subsystem capability, which provides data durability. SQL Server uses the FUA capability to provide highly efficient and reliable I/O for SQL Server workloads. For more information on FUA support by Linux distribution and its effect on SQL Server, see SQL Server On Linux: Forced Unit Access (FUA) Internals.

SUSE Linux Enterprise Server 12 SP5, Red Hat Enterprise Linux 8.0, and Ubuntu 18.04 introduced support for FUA capability in the I/O subsystem. If you're using SQL Server 2017 (14.x) CU 6 and later versions, you should use following configuration for high performing and efficient I/O implementation with FUA by SQL Server.

Use this recommended configuration if the following conditions are met.

SQL Server 2017 (14.x) CU 6 and later versions
Linux distribution and version that supports FUA capability (starting with Red Hat Enterprise Linux 8.0, SUSE Linux Enterprise Server 12 SP5, or Ubuntu 18.04)
XFS file system for SQL Server storage
Storage subsystem and/or hardware that supports and is configured for FUA capability

Recommended configuration:

Enable Trace Flag 3979 as a startup parameter.
Use mssql-conf to configure control.writethrough = 1 and control.alternatewritethrough = 0.

For almost all other configuration that doesn't meet the previous conditions, the recommended configuration is as follows:

Enable Trace Flag 3982 as a startup parameter (which is the default for SQL Server in the Linux ecosystem), and make sure that Trace Flag 3979 isn't enabled as a startup parameter.
Use mssql-conf to configure control.writethrough = 1 and control.alternatewritethrough = 1.

FUA support for SQL Server containers deployed in Kubernetes

The SQL Server must use persisted mounted storage, and not overlayfs.
The storage must use the XFS filesystem and should support FUA. Before enabling this setting, you should work with your Linux distribution and storage vendor, to ensure that the OS and storage subsystem supports FUA options. On Kubernetes, you can query for the filesystem type using the following command, where <pvc-name> is your PersistentVolumeClaim:
```
kubectl describe pv <pvc-name>
```
In the output, look for the fstype that is set to XFS.
The worker node hosting the SQL Server pods, should be using a Linux distribution and version that supports FUA capability (starting with Red Hat Enterprise Linux 8.0, SUSE Linux Enterprise Server 12 SP5, or Ubuntu 18.04).

If the above conditions are met, then you can use the following recommended FUA settings.

Enable Trace Flag 3979 as a startup parameter.
Use mssql-conf to configure control.writethrough = 1 and control.alternatewritethrough = 0.

Kernel and CPU settings for high performance

The following section describes the recommended Linux OS settings related to high performance and throughput for a SQL Server installation. See your Linux distribution's documentation for the process to configure these settings. You can use TuneD as described, to configure many CPUs and kernel configurations, described in the next section.

Use TuneD to configure kernel settings

For Red Hat Enterprise Linux (RHEL) users, the TuneD throughput-performance profile configures some kernel and CPU settings automatically (except for C-States). Starting with RHEL 8.0, a TuneD profile named mssql was codeveloped with Red Hat and offers finer Linux performance-related tunings for SQL Server workloads. This profile includes the RHEL throughput-performance profile, and we present its definitions in this article for your review with other Linux distributions and RHEL releases without this profile.

For SUSE Linux Enterprise Server 12 SP5, Ubuntu 18.04, and Red Hat Enterprise Linux 7.x, the tuned package can be installed manually. It can be used to create and configure the mssql profile as described in the following section.

Proposed Linux settings using a TuneD `mssql` profile

The following example provides a TuneD configuration for SQL Server on Linux.

[main]
summary=Optimize for Microsoft SQL Server
include=throughput-performance

[cpu]
force_latency=5

[sysctl]
vm.swappiness = 1
vm.dirty_background_ratio = 3
vm.dirty_ratio = 80
vm.dirty_expire_centisecs = 500
vm.dirty_writeback_centisecs = 100
vm.transparent_hugepages=always
# For multi-instance SQL deployments, use
# vm.transparent_hugepages=madvise
vm.max_map_count=1600000
net.core.rmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_default = 262144
net.core.wmem_max = 1048576
kernel.numa_balancing=0

If you use Linux distributions with kernel versions greater than 4.18, comment the following options as shown; otherwise, uncomment the following options if you use distributions with kernel versions earlier than 4.18.

# kernel.sched_latency_ns = 60000000
# kernel.sched_migration_cost_ns = 500000
# kernel.sched_min_granularity_ns = 15000000
# kernel.sched_wakeup_granularity_ns = 2000000

To enable this TuneD profile, save these definitions in a tuned.conf file under the /usr/lib/tuned/mssql folder, and enable the profile using the following commands:

chmod +x /usr/lib/tuned/mssql/tuned.conf
tuned-adm profile mssql

Verify that the profile is active, with the following command:

tuned-adm active

Or:

tuned-adm list

CPU settings recommendation

The following table provides recommendations for CPU settings:

Setting	Value	More information
CPU frequency governor	performance	See the cpupower command
ENERGY_PERF_BIAS	performance	See the x86_energy_perf_policy command
min_perf_pct	100	See your documentation on Intel p-state
C-States	C1 only	See your Linux or system documentation on how to ensure C-States is set to C1 only

Using TuneD as described earlier automatically configures CPU frequency governor, ENERGY_PERF_BIAS, and min_perf_pct settings appropriately due to the throughput-performance profile being used as base for the mssql profile. C-States parameter must be configured manually according to the documentation provided by Linux or the system distributor.

Disk settings recommendations

The following table provides recommendations for disk settings:

Setting	Value	More information
Disk `readahead`	4096	See the `blockdev` command
sysctl settings	`kernel.sched_min_granularity_ns = 15000000` `kernel.sched_wakeup_granularity_ns = 2000000` `vm.dirty_ratio = 80` `vm.dirty_background_ratio = 3` `vm.swappiness = 1`	See the sysctl command

Description

vm.swappiness: This parameter controls relative weight given to swapping out runtime process memory as compared to filesystem cache. The default value for this parameter is 60, which indicates swapping runtime process memory pages as compared to removing filesystem cache pages at ratio of 60:140. Setting the value 1 indicates strong preference for keeping runtime process memory in physical memory at expense of filesystem cache. Since SQL Server uses buffer pool as a data page cache and strongly prefers to write through to physical hardware bypassing filesystem cache for reliable recovery, aggressive swappiness configuration can be beneficial for high performing and dedicated SQL Server. You can find additional information at Documentation for /proc/sys/vm/ - #swappiness
vm.dirty_*: SQL Server file write accesses are uncached, satisfying its data integrity requirements. These parameters allow efficient asynchronous write performance and lower the storage I/O effect of Linux caching writes by allowing large enough caching while throttling flushing.
kernel.sched_*: These parameter values represent the current recommendation for tweaking the Completely Fair Scheduling (CFS) algorithm in the Linux Kernel, to improve throughput of network and storage I/O calls with respect to inter-process preemption and resumption of threads.

Using the mssql TuneD profile configures the vm.swappiness, vm.dirty_* and kernel.sched_* settings. The disk readahead configuration using blockdev command is per device and must be performed manually.

Kernel setting auto NUMA balancing for multi-node NUMA systems

If you install SQL Server on a multi-node NUMA system, the following kernel.numa_balancing kernel setting is enabled by default. To allow SQL Server to operate at maximum efficiency on a NUMA system, disable auto NUMA balancing on a multi-node NUMA system:

sysctl -w kernel.numa_balancing=0

Using the mssql TuneD profile configures the kernel.numa_balancing option.

Kernel settings for virtual address space

The default setting of vm.max_map_count (which is 65536) might not be high enough for a SQL Server installation. For this reason, change the vm.max_map_count value to at least 262144 for a SQL Server deployment, and refer to the Proposed Linux settings using a TuneD mssql profile section for further tunings of these kernel parameters. The maximum value for vm.max_map_count is 2147483647.

sysctl -w vm.max_map_count=1600000

Using the mssql TuneD profile configures the vm.max_map_count option.

Leave Transparent Huge Pages (THP) enabled

Most Linux installations should have this option on by default. We recommend for the most consistent performance experience to leave this configuration option enabled. However, if there's high memory paging activity in SQL Server deployments with multiple instances, for example, or SQL Server execution with other memory demanding applications on the server, we suggest testing your applications performance after executing the following command:

echo madvise > /sys/kernel/mm/transparent_hugepage/enabled

Or modify the mssql TuneD profile with the line:

vm.transparent_hugepages=madvise

And make the mssql profile is active after the modification:

tuned-adm off
tuned-adm profile mssql

Using the mssql TuneD profile configures the transparent_hugepage option.

Network setting recommendations

Like there are storage and CPU recommendations, there are Network specific recommendations as well listed below for reference. Not all settings in the following examples are available across different NICs. Refer and consult with NIC vendors for guidance for each of these options. Test and configure this on development environments before applying them on production environments. The following options are explained with examples, and the commands used are specific to NIC type and vendor.

Configuring network port buffer size. In the example below, the NIC is named eth0, which is an Intel-based NIC. For Intel based NIC, the recommended buffer size is 4 KB (4096). Verify the preset maximums and then configure it using the following example:

Check the pre-set maximums with the following command. Replace eth0 with your NIC name:
```
ethtool -g eth0
```
Set both the rx (receive) and tx (transmit) buffer size to 4 KB:
```
ethtool -G eth0 rx 4096 tx 4096
```
Check that the value is properly configured:
```
ethtool -g eth0
```
Enable jumbo frames. Before enabling jumbo frames, verify that all the network switches, routers, and anything else essential in the network packet path between the clients and the SQL Server support jumbo frames. Only then, enabling jumbo frames can improve performance. After jumbo frames are enabled, connect to SQL Server and change the network packet size to 8060 using sp_configure as shown below:
```
# command to set jumbo frame to 9014 for a Intel NIC named eth0 is
ifconfig eth0 mtu 9014
# verify the setting using the command:
ip addr | grep 9014
```
```
EXECUTE sp_configure 'network packet size', '8060';
GO

RECONFIGURE WITH OVERRIDE;
GO
```
By default, we recommend setting the port for adaptive RX/TX IRQ coalescing, meaning interrupt delivery is adjusted to improve latency when packet rate is low and improve throughput when packet rate is high. This setting might not be available across all the different network infrastructure, so review the existing network infrastructure and confirm that this is supported. The example below is for the NIC named eth0, which is an Intel-based NIC:
1. Set the port for adaptive RX/TX IRQ coalescing:
```
ethtool -C eth0 adaptive-rx on
ethtool -C eth0 adaptive-tx on
```
2. Confirm the setting:
```
ethtool -c eth0
```
Note

For a predictable behavior for high-performance environments, like environments for benchmarking, disable the adaptive RX/TX IRQ coalescing and then set specifically the RX/TX interrupt coalescing. See the example commands to disable the RX/TX IRQ coalescing and then specifically set the values:

Disable adaptive RX/TX IRQ coalescing:
```
ethtool -C eth0 adaptive-rx off
ethtool -C eth0 adaptive-tx off
```
Confirm the change:
```
ethtool -c eth0
```
Set the rx-usecs and irq parameters. rx-usecs specifies how many microseconds after at least 1 packet is received before generating an interrupt. The irq parameter specifies the corresponding delays in updating the status when the interrupt is disabled. For Intel bases NICs, you can use the following settings:
```
ethtool -C eth0 rx-usecs 100 tx-frames-irq 512
```
Confirm the change:
```
ethtool -c eth0
```
We also recommend receive-side scaling (RSS) enabled and by default, combining the RX and TX side of RSS queues. There have been specific scenarios, when working with Microsoft Support, where disabling RSS has improved the performance as well. Test this setting in test environments before applying it on production environments. The following example is for Intel NICs.

Get the preset maximum values:
```
ethtool -l eth0
```
Combine the queues with the value reported in the preset "Combined" maximum value. In this example, the value is set to 8:
```
ethtool -L eth0 combined 8
```
Verify the setting:
```
ethtool -l eth0
```
Working with NIC port IRQ affinity. To achieve expected performance by tweaking the IRQ affinity, consider few important parameters like Linux handling of the server topology, NIC driver stack, default settings, and irqbalance setting. Optimizations of the NIC port IRQ affinities settings are done with the knowledge of server topology, disabling the irqbalance, and using the NIC vendor-specific settings.

The following example of Mellanox specific network infrastructure helps to explain the configuration. For more information, and to download the Mellanox mlnx tools, see Performance Tuning tools for Mellanox Network Adapters. The commands change based on the environment. Contact the NIC vendor for further guidance.

Disable irqbalance, or get a snapshot of the IRQ settings and force the daemon to exit:
```
systemctl disable irqbalance.service
```
Or:
```
irqbalance --oneshot
```
Make sure that common_irq_affinity.sh is executable:
```
chmod +x common_irq_affinity.sh
```
Display IRQ affinity for Mellanox NIC port (for example, eth0):
```
./show_irq_affinity.sh eth0
```
Optimize for best throughput performance with a Mellanox tool:
```
./mlnx_tune -p HIGH_THROUGHPUT
```
Set hardware affinity to the NUMA node hosting physically the NIC and its port:
```
./set_irq_affinity_bynode.sh `\cat /sys/class/net/eth0/device/numa_node` eth0
```
Verify the IRQ affinity:
```
./show_irq_affinity.sh eth0
```
Add IRQ coalescing optimizations
```
ethtool -C eth0 adaptive-rx off
ethtool -C eth0 adaptive-tx off
ethtool -C eth0  rx-usecs 750 tx-frames-irq 2048
```
Verify the settings:
```
ethtool -c eth0
```
After the above changes are done, verify the speed of the NIC to ensure it matches the expectation using the following command:
```
ethtool eth0 | grep -i Speed
```

Advanced kernel and OS configuration

For best storage I/O performance, use Linux multiqueue scheduling for block devices, which enables the block layer performance to scale well with fast solid-state drives (SSDs) and multi-core systems. Check the documentation if it's enabled by default in your Linux distribution. In most other cases, booting the kernel with scsi_mod.use_blk_mq=y enables it, though documentation of the Linux distribution in use might have further guidance on it. This is consistent with the upstream Linux kernel.
As multipath I/O is often used for SQL Server deployments, configure the device mapper (DM) multi-queue target to use the blk-mq infrastructure, by enabling the dm_mod.use_blk_mq=y kernel boot option. The default value is n (disabled). This setting, when the underlying SCSI devices are using blk-mq, reduces locking overhead at the DM layer. For more information on how to configure multipath I/O, refer to your Linux distribution's documentation.

Configure swapfile

Ensure you have a properly configured swapfile to avoid any out of memory issues. Consult your Linux documentation for how to create and properly size a swapfile.

Virtual machines and dynamic memory

If you're running SQL Server on Linux in a virtual machine, make sure you select options to fix the amount of memory reserved for the virtual machine. Don't use features like Hyper-V Dynamic Memory.

SQL Server configuration

Perform the following configuration tasks after you install SQL Server on Linux to achieve best performance for your application.

Best practices

Use PROCESS AFFINITY for node and/or CPUs

Use ALTER SERVER CONFIGURATION to set PROCESS AFFINITY for all the NUMANODEs and/or CPUs you're using for SQL Server (which is typically for all NODEs and CPUs) on a Linux OS. Processor affinity helps maintain efficient Linux and SQL Scheduling behavior. Using the NUMANODE option is the simplest method. Use PROCESS AFFINITY even if you have only a single NUMA Node on your computer. For more information on how to set PROCESS AFFINITY, see the ALTER SERVER CONFIGURATION article.

Configure multiple `tempdb` data files

Because a SQL Server on Linux installation doesn't offer an option to configure multiple tempdb files, we recommend that you consider creating multiple tempdb data files after installation. For more information, see the guidance in the article, Recommendations to reduce allocation contention in SQL Server tempdb database.

Advanced configuration

The following recommendations are optional configuration settings that you might choose to perform after installation of SQL Server on Linux. These choices are based on the requirements of your workload and configuration of your Linux OS.

Set a memory limit with mssql-conf

In order to ensure There's enough free physical memory for the Linux OS, the SQL Server process uses only 80% of the physical RAM by default. For some systems with large amount of physical RAM, 20% might be a significant number. For example, on a system with 1 TB of RAM, the default setting would leave around 200 GB of RAM unused. In this situation, you might want to configure the memory limit to a higher value. See the documentation on the mssql-conf tool and the memory.memorylimitmb setting that controls the memory visible to SQL Server (in units of MB).

When changing this setting, be careful not to set this value too high. If you don't leave enough memory, you could experience problems with the Linux OS and other Linux applications.

Jaa

Performance best practices and configuration guidelines for SQL Server on Linux

Storage configuration recommendation

Use storage subsystem with appropriate IOPS, throughput, and redundancy

Disk partitioning and configuration recommendations

filesystem configuration recommendation

Persistent memory filesystem recommendation

Open file limitation

Disable last accessed date/time on filesystems for SQL Server data and log files

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

FUA support for SQL Server containers deployed in Kubernetes

Kernel and CPU settings for high performance

Use TuneD to configure kernel settings

Proposed Linux settings using a TuneD `mssql` profile

CPU settings recommendation

Disk settings recommendations

Description

Kernel setting auto NUMA balancing for multi-node NUMA systems

Kernel settings for virtual address space

Leave Transparent Huge Pages (THP) enabled

Network setting recommendations

Advanced kernel and OS configuration

Configure swapfile

Virtual machines and dynamic memory

SQL Server configuration

Best practices

Use PROCESS AFFINITY for node and/or CPUs

Configure multiple `tempdb` data files

Advanced configuration

Set a memory limit with mssql-conf

Palaute

Lisäresursseja

Jaa

Performance best practices and configuration guidelines for SQL Server on Linux

Storage configuration recommendation

Use storage subsystem with appropriate IOPS, throughput, and redundancy

Disk partitioning and configuration recommendations

filesystem configuration recommendation

Persistent memory filesystem recommendation

Open file limitation

Disable last accessed date/time on filesystems for SQL Server data and log files

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

FUA support for SQL Server containers deployed in Kubernetes

Kernel and CPU settings for high performance

Use TuneD to configure kernel settings

Proposed Linux settings using a TuneD mssql profile

CPU settings recommendation

Disk settings recommendations

Description

Kernel setting auto NUMA balancing for multi-node NUMA systems

Kernel settings for virtual address space

Leave Transparent Huge Pages (THP) enabled

Network setting recommendations

Advanced kernel and OS configuration

Configure swapfile

Virtual machines and dynamic memory

SQL Server configuration

Best practices

Use PROCESS AFFINITY for node and/or CPUs

Configure multiple tempdb data files

Advanced configuration

Set a memory limit with mssql-conf

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Palaute

Lisäresursseja

Proposed Linux settings using a TuneD `mssql` profile

Configure multiple `tempdb` data files