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VMware offers automated UNMAP capability at the VMFS 6 datastore level (VMFS 5 is not supported). The automation is enabled by a background process (UNMAP crawler) which tracks which LBAs that can be unmapped and then issues those UNMAPs at various intervals. It is an asynchronous process. VMware estimates that if there is dead space in a datastore, automated UNMAP should reclaim it in under 12 hours.
UNMAP is enabled by default when a VMFS 6 datastore is created through a vSphere Client wizard. There are two parameters available, one of which can be modified: Space Reclamation Granularity and Space Reclamation Priority. The first parameter, Space Reclamation Granularity, designates at what size VMware reclaims storage. It cannot be modified from the 1 MB value. The second parameter, Space Reclamation Priority, dictates how aggressive the UNMAP background process is when reclaiming storage. This parameter only has two values: None (off), and Low. It is set to Low by default. Dell Technologies recommends leaving it at this setting. These parameters are seen in Figure 101.
It may become necessary to either enable or disable UNMAP on a VMFS 6 datastore after creation. The setting can be modified through the vSphere Client or CLI. Within the datastore view in the Client, go to the Configure tab and the General subtab. Here there is a section on Space Reclamation where you can use the Edit button to make the pertinent change as shown in Figure 102.
Changing the parameter is also possible through CLI, demonstrated in Figure 103.
Manual UNMAP using esxcli is still supported and can be run on a datastore whether it is configured with automated UNMAP. Some customers may choose to use manual UNMAP to control when the commands are issued and avoid any unwanted impact.
VMware vSphere 6.7 and higher offers the ability to change the reclamation priority that is used for UNMAP, and change the method from priority to a fixed reclaim bandwidth. As explained above, before vSphere 6.7 automated UNMAP was either on, which meant low priority, or off. Now priority can be changed to medium or high, each with their own bandwidth. Medium translates into 75 MB/s while high is 256 MB/s (despite the bandwidth indicating 0 MB/s). Only the CLI is offered to change the priority.
By default, the “Low” priority setting translates into reclaiming at a rate of 25 MB/s. To change it to medium priority, issue the following command as shown in Figure 104.
esxcli storage vmfs reclaim config set -volume-label RECLAIM_TEST_6A -reclaim-priority medium
To take advantage of the power of all flash storage arrays, VMware also offers the ability to set a fixed rate of bandwidth. The user can set the value anywhere from 100 MB/s to a maximum of 2000 MB/s in multiples of 100. This capability, unlike priority, is offered in the UI and CLI. In the following example in Figure 105, the reclaim method is changed to “fixed” at a 1000 MB/s bandwidth. In this interface, it is clear that the priority cannot be changed as previously mentioned.
The CLI can also be used for the fixed method. For example, to change the reclaim rate to 200 MB/s on the datastore "RECLAIM_TEST_6A" issue the following:
esxcli storage vmfs reclaim config set -volume-label RECLAIM_TEST_6A -reclaim-method fixed -b 200
Here is the output in Figure 106 along with the config get command which retrieves the current value.
VMware has indicated that they do not advise changing the reclaim granularity.
Note: While an active UNMAP operation is underway, changing the bandwidth value takes longer than normal to complete.
You may notice that in Figure 104, the reclaim bandwidth shows as 0 MB/s, despite knowing the value is 75 MB/s. VMware does not show the correct value, however, unless the reclaim method is first set to fixed, then changed back to priority as in Figure 107.
The default of low priority for UNMAP is the preferred value for automation. Some customers with extremely active arrays have found the value of 26 MB/s to produce excessive load on the system. To help address these customer environments in vSphere 8, VMware added a lower fixed value of 10 MB/s. The new value is available in the drop-down options show in Figure 108.
NVMeoF on the PowerMax supports both datastore and in-guest UNMAP.
Dell conducted numerous tests on UNMAP capabilities but found little difference in how quickly data is reclaimed on the array based on size or priority. This finding is consistent with manual UNMAP block options. However, at higher bandwidths and priority, if the array is already operating at close to peak, performance was impacted. Based on the testing, Dell Technologies recommends continuing to use the priority method at the low setting. By default, VMware keeps the UNMAP rate set to low priority to minimize the impact on VM I/O across the array, without regard to the type of storage backing the datastores. If, for some reason, space must be reclaimed more quickly, a manual UNMAP can be issued.