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Volume creation and sizing
39 Dell EMC SC Series: Best Practices with VMware vSphere | 2060-M-BP-V
The most common indicator a datastore has too many virtual machines is if the queue depth of the datastore
is regularly exceeding set limits and increasing disk latency. Remember that if the driver module is set to a
queue depth of 256, the maximum queue depth of each datastore is also 256. Meaning that if there are 16
virtual machines on a datastore, all heavily driving a queue depth of 32 (16 x 32 = 512), they are essentially
overdriving the disk queues by double. The resulting high latency will most likely degrade performance. See
appendix A for more information about determining if the queue depth of a datastore is being correctly used.
In rare situations where (VAAI) hardware-assisted locking is unavailable or disabled, a far less common
indicator of overdriving the datastore would be the frequent occurrence of SCSI reservation conflicts. Within
esxtop, there is a field in the Disk Device screen for reserve stats (RESVSTATS). When monitoring
datastores, it is normal to see a few reservation (RESV/s) entries and even a few conflicts (CONS/s) from
time to time. However, when CONS/s happen frequently on a volume, it may be time to move some of the
virtual machines to a different datastore. The VAAI hardware-assisted locking primitive will help to alleviate
the performance degradation caused by these SCSI-2 reservations. If datastores are impacted, it is
recommended to upgrade to a version of SCOS that supports this VAAI primitive. See section 16.3 on VAAI
for more information.
Note: Many resources discuss VMware infrastructure design and sizing. The general rule discussed
previously may vary based on the needs of the environment.
8.3 VMFS partition alignment
Partition alignment is a performance-tuning technique used with traditional SANs to align the guest operating
system partitions and VMFS partitions to the physical spinning media. This alignment reduces the number of
disk transactions it takes to process an I/O.
Due to how dynamic block architecture virtualizes the blocks, manual partition alignment is not necessary. SC
Series automatically aligns its 512 KB, 2 MB, or 4 MB pages to the physical sector boundaries of spinning
drives. Since the largest percentage of performance gains are seen from aligning the SC Series pages to the
physical disks, manually tuning has a minor effect on performance.
As found in Dell EMC internal lab testing, any performance gains achieved by manually aligning partitions are
not substantial enough (±1%) to justify the extra effort. However, because all workloads are different,
performing testing is recommended to determine the impact of an aligned partition for applications.
To manually align the VMFS block boundaries to the SC Series page boundaries for performance testing, the
recommended offset when creating a datastore is 8,192 (4 MB).
Note: Using the SC Series vSphere client plug-in to create datastores will automatically align them to the
recommended offset. More information about how to obtain the plug-in is in appendix B.