Optimizing local SSD performance

Linux

The performance by disk type chart describes the maximum achievable performance for local SSD partitions. To optimize your apps and VM instances to achieve these speeds, use the following best practices:

Use guest environment optimizations for Local SSDs

By default, most Compute Engine-provided Linux images automatically run an optimization script that configures the instance for peak Local SSD performance. The script enables certain queue sysfs settings that enhance the overall performance of your machine and mask interrupt requests (IRQs) to specific virtual CPUs (vCPUs). This script only optimizes performance for Compute Engine local SSD partitions.

Ubuntu, SLES, and other earlier images might not be configured to include this performance optimization. If you are using any of these images or an image that is earlier than v20141218, you can install the guest environment to enable these optimizations.

Choose an interface to connect your local SSDs

You can connect Local SSDs to your VMs using either the NVMe interface or the SCSI interface. The best choice depends on the operating system (OS) you are using. For most workload configurations involving Local SSDs, using the NVMe interface leads to better performance.

If you need to use a specific OS, choose an interface for your local SSD partitions that works best with your boot disk image.
If you have an existing setup that requires using a SCSI interface, use an image that supports multi-queue SCSI to achieve better performance over the standard SCSI interface.

Enable multi-queue SCSI

Some public images support multi-queue SCSI. To use multi-queue SCSI on custom images that you import to your project, you must enable it yourself. Your imported Linux images can use multi-queue SCSI only if they include kernel version 3.19 or later.

To enable multi-queue SCSI on a custom image, import the image with the VIRTIO_SCSI_MULTIQUEUE guest OS feature enabled and add an entry to your GRUB config:

CentOS

For CentOS7 only.

Import your custom image using the API and include a guestOsFeatures item with a type value of VIRTIO_SCSI_MULTIQUEUE.
Create an instance using your custom image and attach one or more local SSDs.
Connect to your instance through SSH.
Check the value of the /sys/module/scsi_mod/parameters/use_blk_mq file
```
$ cat /sys/module/scsi_mod/parameters/use_blk_mq
```
If the value of this file is Y, then multi-queue SCSI is already enabled on your imported image. If the value of the file is N, include scsi_mod.use_blk_mq=Y in the GRUB_CMDLINE_LINUX entry in your GRUB config file and restart the system.
1. Open the /etc/default/grub GRUB config file in a text editor.
```
$ sudo vi /etc/default/grub
```
2. Add scsi_mod.use_blk_mq=Y to the GRUB_CMDLINE_LINUX entry.
```
GRUB_CMDLINE_LINUX=" vconsole.keymap=us console=ttyS0,38400n8 vconsole.font=latarcyrheb-sun16 scsi_mod.use_blk_mq=Y"
```
3. Save the config file.
4. Run the grub2-mkconfig command to regenerate the GRUB file and complete the configuration.
```
$ sudo grub2-mkconfig -o /boot/grub2/grub.cfg
```
5. Reboot the instance.
```
$ sudo reboot
```

Ubuntu

Import your custom image using the Compute Engine API and include a guestOsFeatures item with a type value of VIRTIO_SCSI_MULTIQUEUE.
Create an instance using your custom image and attach one or more local SSDs using the SCSI interface.
Connect to your instance through SSH.
Check the value of the /sys/module/scsi_mod/parameters/use_blk_mq file.
```
$ cat /sys/module/scsi_mod/parameters/use_blk_mq
```
If the value of this file is Y, then multi-queue SCSI is already enabled on your imported image. If the value of the file is N, include scsi_mod.use_blk_mq=Y in the GRUB_CMDLINE_LINUX entry in your GRUB config file and restart the system.
1. Open the sudo nano /etc/default/grub GRUB config file in a text editor.
```
$ sudo nano /etc/default/grub
```
2. Add scsi_mod.use_blk_mq=Y to the GRUB_CMDLINE_LINUX entry.
```
GRUB_CMDLINE_LINUX="scsi_mod.use_blk_mq=Y"
```
3. Save the config file.
4. Run the update-grub command to regenerate the GRUB file and complete the configuration.
```
$ sudo update-grub
```
5. Reboot the instance.
```
$ sudo reboot
```

Disable write cache flushing

File systems, databases, and other apps use cache flushing to ensure that data is committed to durable storage at various checkpoints. For most storage devices, this default makes sense. However, write cache flushes are fairly slow on local SSDs. You can increase the write performance for some apps by disabling automatic flush commands in those apps or by disabling flush options at the file system level.

Local SSDs always flush cached writes within two seconds regardless of the flush commands that you set for your file systems and apps, so temporary hardware failures can cause you to lose only two seconds of cached writes at most. Permanent hardware failures can still cause loss of all data on the device whether the data is flushed or not, so you should still back up critical data to Persistent Disks or Cloud Storage buckets.

To disable write cache flushing on ext4 file systems, include the nobarrier setting in your mount options or in your /etc/fstab entries. For example:

$ sudo mount -o discard,defaults,nobarrier /dev/[LOCAL_SSD_ID] /mnt/disks/[MNT_DIR]

where: [LOCAL_SSD_ID] is the device ID for the local SSD that you want to mount and [MNT_DIR] is the directory in which to mount it.

Disable VM automatic restart

Local SSD firmware includes a capability that unlocks faster write performance for sync-heavy workloads. When you enable this performance enhancement, the local SSD disk is more likely to lose all of its data if the VM that it's attached to fails.

To enable this performance enhancement, set your VM's automaticRestart policy to false. The automaticRestart field is a VM scheduling policy that specifies whether Compute Engine should should automatically restart the VM if it fails. This means that if you enable the write performance enhancement on a local SSD and the VM that it's attached to fails, Compute Engine does not restart the VM and all data on the local SSD is lost.

You might need to make sure that your workload is set up to detect and restart terminated VMs before setting the automaticRestart field to false.