This topic applies to XInsight-managed systems running QSM 4.2 or later.

SSD cache allows an SSD to function as a read cache or write buffer for an HDD volume. In SSD read cache, it is a secondary cache that improves performance by keeping frequently accessed data on SSDs, where it is read far more quickly than from the HDD volume. When reads or writes are performed, the data from the HDDs is copied into the SSD cache. The SSD read cache does not contain the only copy of any data. If the read cache fails or becomes corrupted, the original data remains available on the HDD volume.

In the SSD write cache, SSDs are a write buffering storage that improves performance by storing the write data in SSDs temporarily, where it is written far more quickly than to the HDD volume. Then the write data will be flushed to the HDD volume at the appropriate time. When enabling write acceleration, users are strongly advised to implement additional data protection mechanisms to ensure data integrity.

SSD Write Cache Flushing #

In the SSD read-write cache, write data is temporarily stored in the SSD cache and later flushed to the HDD volume. Write cache data is flushed in the following situations:

• Before the SSD cache pool is deleted.
• Before a related volume is disabled.
• When the system is idle.
• When an SSD failure causes the SSD cache pool to enter a degraded state. In this condition, write caching stops and cached write data is flushed to the HDD volumes.

Flushing cached data to HDD volumes may take time. Do not remove SSDs until the flush operation is complete. When deleting an SSD cache pool, avoid heavy front-end I/O traffic to reduce the time required to complete the flush.

Note: SSD read-write cache provides the best performance when the amount of frequently accessed data is smaller than the available SSD cache capacity. Plan sufficient SSD cache capacity according to the expected workload.

The available capacity of the SSD cache is proportional to the size of the controller’s system memory. The following is a mapping table of SSD cache pool memory and capacity.

SYSTEM MEMORY PER CONTROLLER	MAXIMUM SSD CACHE POOL CAPACITY PER SYSTEM
8 GB	2 TB
16 GB	4 TB
32 GB	8 TB
64 GB	16 TB
128 GB	32 TB
256 GB	64 TB
512 GB	128 TB
1,024 GB	256 TB

For more information, see the SSD Cache White Paper from the QSAN website.

Select the Storage menu and SSD Cache item to manage the SSD cache pools. In this menu, you can create, modify, delete, or view the status of all SSD cache pools.

Create an SSD Cache Pool #

The Create SSD Cache wizard configures the cache properties, assigns SSDs, and selects the volumes that will use the cache.

Restriction: SSDs installed in the controller’s onboard M.2 slots can be used for Read Cache only. They cannot be used to create a Read/Write SSD cache.

Click the + icon in the lower-left corner to open the wizard. The wizard includes the following steps:

1. SSD Cache Property
2. Select SSD
3. Select Volume
4. Ready to complete

View SSD Cache Information #

Select an SSD cache, and then click SSD Cache Info to view its configuration in the right pane.

Manage SSD Cache Disks #

Click SSD List to view the SSDs assigned to the selected cache. The SSD Cache Disks pane displays the model, vendor, capacity, slot, and operating status of each SSD.

The number beside SSD List indicates how many SSDs are currently assigned to the cache.

Add an SSD to the SSD Cache #

1. Click the + icon in the lower-left corner of the SSD Cache Disks pane.
2. Select one or more SSDs from the Available SSD List. The selected drives appear in the Selected SSD List.
3. Review the selected SSDs, and then click Finish.

Only SSDs available for cache use are displayed in the wizard.

Remove an SSD from the SSD Cache #

1. Select the SSD that you want to remove.
2. Click the – icon in the lower-left corner of the SSD Cache Disks pane.

Attention: Removing an SSD may affect the capacity and operation of the SSD cache. Review the cache status and displayed warnings before confirming the operation.

Manage SSD Cache Volumes #

Click Applied Volumes to view the volumes assigned to the selected SSD cache. The SSD Cache Volumes pane displays information such as health, capacity, allocation unit, volume type, and volume usage.

The number beside Applied Volumes indicates how many volumes are currently using the SSD cache.

Add a Volume to the SSD Cache #

1. Click the + icon in the lower-left corner of the SSD Cache Volumes pane.
2. Select one or more volumes to add to the cache.
3. Review the selected volumes, and then click Finish.

Remove a Volume from the SSD Cache #

1. Select the volume that you want to remove.
2. Click the – icon in the lower-left corner of the SSD Cache Volumes pane.
3. Review the confirmation message, and then confirm the operation.

Removing a volume from the SSD cache stops the volume from using the cache.

Delete an SSD Cache #

Delete an SSD cache when it is no longer required.

1. Select the SSD cache that you want to delete.
2. Click the Delete button under SSD Cache Deletion.
3. Review the warning message, and then click Delete to confirm.

Warning: Deleting an SSD cache permanently removes the SSD cache and its data, which cannot be recovered. Back up all required data before deleting the SSD cache.