Users Guide

PhysicalDiskRedundancy 1 -StorageTiers $ssd_tier, $hdd_tier -StorageTierSizes

20GB, 10TB

Write-back cache

Configuring write-back cache (WBC) allows Storage Spaces to use a portion of available disk space of the

SSDs to provide low latency cache for incoming ‘write’ commands. All incoming ‘write’ requests are

cached on SSDs and immediately respond back to the host with write completion status. Without write-

back cache, the physical disks operate in write-through mode where incoming ‘write’ requests hit slower,

traditional hard drives before responding with completion. Using WBC can make a significant

performance improvement to almost all workloads.

To use WBC, you must have SSDs in a storage pool. The default disk space of WBC is 1 GB and can be

changed when creating a VD. Dell recommends you to use the default WBC disk space.

Whenever you create a VD, as long as the SSD tier disk space is greater than either the default or specified

WBC disk space and you have the minimum number of SSDs in the storage pool to enable WBC for the

resiliency setting, then WBC is enabled for the VD.

Table 10. WBC Resiliency Setting

Resiliency

setting

Simple Two-way

mirroring

Three-way

mirroring

Single parity Dual parity

Minimum

number of SSDs

for WBC (per

pool)

1 SSD 2 SSDs 3 SSDs 2 SSDs 3 SSDs

Journal disks

Assigning dedicated journal disks to SSDs in the pool when using VDs with parity resiliency improves

sequential write performance. Without dedicated journal disks, the journal disks reside on the same

physical disks as the parity VD and this can cause increased seek times and reduce overall throughput of

a parity VD.

When using a dedicated journal disk, ensure that the quantity of journal disks scales with the number of

parity spaces in the pool. Dedicated journal disks can only be added using PowerShell.

PowerShell command for adding a dedicated journal disk to an existing pool.

Add-PhysicalDisk -StoragePoolFriendlyName <poolName> -PhysicalDisks

<physicalDiskObject> -Usage Journal

For example,

A dedicated journal disk which is an SSD labeled PhysicalDisk5 is assigned to variable $pd. The disk is

then added to an existing pool MyPool1 with the

Usage attribute set to Journal disk.

$pd = Get-PhysicalDisk –CanPool $true -FriendlyName PhysicalDisk5