User guide

Technical white paper | HP Enterprise Virtual Array Storage and VMware vSphere 4.x and 5.x configuration best practices
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Table 12 shows the ESXi UNMAP recommendation usage.
Table 12. ESXi UNMAP usage recommendations
ESX Version
UNMAP Support
Execution Recommendation
ESXi 5.0
Not Supported
NA
ESXi 5.0 patch 2
Not Supported
NA
ESXi 5.0U1
Supported
Offline
ESXi 5.1
Supported
Offline
ESXi 5.5
Supported
Offline
In this paper, we will not explain in detail how the various versions of ESX perform UNMAP. This topic has been abundantly
discussed in other documents, papers and VMworld presentations. However we will contrast key features and highlight
important considerations when using UNMAP with EVA Storage arrays.
UNMAP in ESXi 5.0Ux and ESXi 5.x
There are many important considerations to keep in mind when performing space reclamation operations. However most
considerations can be summed up as follows:
• Reclaim efficiency consideration
• Capacity reservation consideration
• Performance consideration
In ESXi 5.0Ux and ESXi 5.1Ux, capacity is reclaimed by invoking the vmkfstools –y command with a percentage of free
capacity as the parameter. vmkfstools –y 50 for example, will attempt to reclaim 50% of the free capacity on the datastore.
However for a datastore that has areas of the disk that were never written, then the reclaim request with the 50% flag may
result in sending UNMAP requests to physical addresses that were never allocated. Thus leading to no visible capacity
returned to the user. Furthermore, it may take multiple reclaim iterations when using a small reclaim flag value before all of
the desired capacity to reclaim is effectively reclaimed. For this reason, users tend to want to reclaim with the 99% flag (max
supported) instead to guarantee effectiveness.
Reclaiming 99% of free capacity while the datastore is online however makes it subject to capacity reservations and also
performance issues. During the reclaim process, 99% of the free capacity on the datastore is exclusively reserved for access
by the UNMAP operation. While the capacity to be reclaimed is reserved, existing thin VMDKs that need capacity to grow or
new VMDKs that need to be created will fail to allocate space. Making use of the vmkfstools command online is not a very
desirable thing especially when factoring in the performance impact induced to the datastore from the UNMAP requests.
These considerations make space reclaiming on these versions of ESX more suitable during maintenance windows, which
essentially will require migrating running VMs to another datastore before reclaiming capacity. But it probably is much
simpler at that point to delete the volume at the array and recreate it.
When performing space reclamation offline (during maintenance window) on the EVA, it is recommended to use the 99%
flag with the vmkfstools –y command. Not only is this flag value more efficient and effective from an ESX point of view, it
will also allow the EVA to achieve the highest reclaim efficiency because larger contiguous blocks will be reclaimed.