Dell Storage with Microsoft Storage Spaces Best Practices Guide

When planning to add physical disk space to existing storage pools and VDs there are several considerations to be made before adding the additional physical disks or storage enclosures.

After verifying all newly added disks are available to the cluster, you can now create new or expand an existing storage pool or VD. Dell recommends when expanding a storage pool to add physical disks in a quantity equal to the column count multiplied by the number of data copies plus any additional disks required for automatic rebuilds. For example, for a Two-way mirror VD, if the column count is four, you would add a minimum of eight disks to the pool to expand the VD.

To find out the number of columns used by a specific VD, run the PowerShell command.

Get-VirtualDisk –FriendlyName <vdName> | FL NumberOfColumns

The reasoning for the recommendation is to safeguard against expansion of VDs, which are already very low on usable disk space. In order for a write to a VD to be successful, all data must be striped across the number of disks indicated by the column count. If you add fewer disks, there may be new free disk space now on the VD, there is still not enough disks available for a full stripe to be written.

For example,

A 2x3 configuration with three Dell Storage MD1400 storage enclosures each with eight HDDs and four SSDs. For example, all 24 HDDs and 12 SSDs are added to one storage pool called MyPool1. One VD is created using storage tiers in the pool — 2wayVD1, with Two-way mirroring and a column count of 5. The plan is to add an extra Dell Storage MD1400 enclosure with eight new HDDs and four new SSDs.

For this example, 2wayVD1 had data added so that almost no usable disk space is remaining in the HDD tier. In an effort to add new disk space, all eight of the new HDDs are added to MyPool1. 2wayVD1 has a column count of five, which means a total of 10 disks are required for a full stripe, one stripe across five disks for first data copy and one stripe across another five disks for the second data copy. However, because only eight HDDs were added to the pool, after the original disks in the pool are completely out of disk space there will now only be eight disks, and not the 10 required to write to 2wayVD1.

There is one more factor to consider when adding physical disks or storage enclosures with the intent of expanding existing VDs which were created with enclosure awareness. Enclosure awareness spreads the data copies of VDs across three or more storage enclosures. However, when adding new physical disks or storage enclosures, depending on the existing VD layout, VDs created on the newly added storage disk space may not be enclosure aware. This occurs if there is not sufficient free disk space or there are not enough physical disks in existing storage enclosures to spread the new data copies in a method which satisfies the enclosure awareness requirements.

PowerShell command for adding a new physical disks to an existing pool.

Add-PhysicalDisk -StoragePoolFriendlyName <poolName> -PhysicalDisks <physicalDiskObject> -Usage AutoSelect

PowerShell command for extending a VD.

Resize-VirtualDisk –FriendlyName <vdName> -Size <newVDSize>

PowerShell command for extending a volume.

Resize-Partition –DiskNumber <diskNumber> -Size <newVolumeSize>

For example,

A physical disk which is labeled PhysicalDisk13 is assigned to variable $pd. The disk is then added to an existing pool MyPool1 with the Usage attribute set to AutoSelect. A VD 2wayVD1 is 30 GB and currently exists in the pool. The VD is extended to 60 GB. Finally, the volume must be extended to match the new VD size.

$pd = Get-PhysicalDisk –CanPool $true -FriendlyName PhysicalDisk13

Add-PhysicalDisk -StoragePoolFriendlyName "MyPool1" -PhysicalDisks $pd -Usage AutoSelect

Resize-VirtualDisk –FriendlyName 2wayVD1 –Size 60GB

$vd = Get-VirtualDisk -FriendlyName 2wayVD1

$diskNum = Get-Disk –VirtualDisk $vd

$partNum = Get-Partition –DiskNumber $diskNum.Number

$size = Get-PartitionSupportedSize –DiskNumber $diskNum.Number

Resize-Partition –DiskNumber $diskNum.Number –PartitionNumber $partNum.PartionNumber –Size $size.SizeMax