RAID configuration options for WSS and the Microsoft iSCSI Software Target

It’s very common for a server running Windows Storage Server (WSS) with the Microsoft iSCSI Software Target or Windows Unified Data Storage Server (WUDSS) to include a RAID controller and many hard drives in the box. I got a question on exactly how to configure RAID volumes for this kind of setup. The standard “it depends” engineering answer does apply here. However, allow me to elaborate on that :-).

First of all, it helps to understand a few details about the hardware. The main thing here is the types of RAID supported by the controller (RAID 0, 1, 5 and 10 are common, RAID 6 is sometimes available). It’s also important to know how many physical disks you have there, since this will impact you options (RAID 1 requires 2 disks, RAID 5 requires at least 3 disks).

Second, you need to be more specific about your high availability, capacity and performance requirements. You see, striping all the disks into a single RAID 0 volume with no fault tolerance will give your maximum capacity, but if you lose a single disk, you lose data. You can use RAID 5 to offer fault tolerance without much impact to capacity, but your performance will suffer, especially if a failed disk is replaced and you need to rebuild the parity. RAID 1 (mirror) offers good performance with disk fault tolerance, but you will basically slash your storage capacity in half.

A common strategy is to have a multiple RAID volumes with different characteristics, so you can choose where to place your iSCSI LUNs (which are basically VHD files stored in a volume). For instance, you could have one RAID 1 volume (let’s call it drive F:) and one RAID 5 volume (drive G:, for instance). If you’re working with SQL databases, for instance, you could store the LUNs that are used for the actual database as VHD files on the G: drive and the LUNs used for the database log as VHD files on drive F:. Keep in mind that, in order to do this, you will need at least 5 drives (2 for the mirror and 3 for the RAID 5 volume).

Also keep in mind that you probably do not want to put the OS itself on a RAID 5 volume, since that will slow down the entire system (the OS will need to do paging or write to the registry, for instance). When you have only 4 disks, you might need to create a volume using just part of a disk (15GB of the first 2 disks for a mirrored system volume, for instance). You usually want to avoid sharing spindles, but that is harder to do when you have a low number of physical disks.

Here are a few examples if you have 6 drives (70GB each) and you want to offer iSCSI LUNs for a couple of SQL Servers:

Option 1 – Single RAID 0 (highest capacity, no fault tolerance)
Striped volume (RAID 0, 6 drives, 420GB) for system, log LUNs and database LUNs.

Option 2 – Single RAID 5 (highest capacity with fault tolerance, worst performance)
RAID 5 volume (RAID 5, 6 drives, 350GB) for system, log LUNs and database LUNs.

Option 3 – RAID 1 + RAID 5 (balanced capacity/performance)
Mirrored volume (RAID 1, 2 drives, 70GB) for system and log LUNS
RAID5 volume (RAID 5, 4 drives, 210GB) for database LUNS

Option 4 – Single RAID 6
RAID 5 volume (RAID 6, 6 drives, 280GB) for system, log LUNs and database LUNs.

Option 5 – Single RAID 10 (lowest capacity, good performance)
Striped and mirrored volume (RAID 10, 6 drives, 210GB) for system, log LUNs and database LUNs.

Option 6 – RAID 1 + RAID 10 (highest performance with fault tolerance)
Mirrored volume (RAID 1, 2 drives, 70GB) for system and log LUNS
Striped and mirrored volume (RAID 10, 4 drives, 140GB) for database LUNS

Option 7 – RAID 1 + RAID 10 + RAID 5
RAID 1 volume, (RAID 1, 2 drives, 15GB) – For system only, uses only part of each disk (15GB out of 70)
RAID 10 volume (RAID 10, 4 drives, 30GB) – log LUNs, uses only part of the disk (15GB out of 70)
RAID 5 volume (RAID 5, 6 drives, 275GB) – database LUNs, uses only part of the disk  (55GB out of 70)

A few additional comments:

• All sizes are approximate and actual formatted capacity will be slightly different (but you already knew that, right?)
• Some of the configurations do use the same disks for multiple workloads (shared spindles), which will impact performance
• Not every RAID controller offers RAID 6, RAID 10 or the ability to use part of the disk in a RAID volume, so your choices might be limited
• There’s a lot of documentation already on how RAID 0, 1, 5, 6 and 10 work. My goal here was to tickle your brain on how to apply them together
• When using WSS or WUDSS, the Storage Provisioning Wizard needs a VDS hardware provider to manage the hardware RAID
• If the WSS or WUDSS computer does not have a RAID controller, you can still use the software RAID in Windows (with dynamic disks)
• When in doubt, consult a storage specialist to see what makes sense given your specific workload, configuration and budget

P.S.: The main target audience of this post are the Microsoft OEM Partners considering a WSS or WUDSS solution. It could also be useful for IT Professionals that want to understand the features of WSS or WUDSS before buying a server with it pre-installed from an OEM Partner. For more information on WSS, check http://www.microsoft.com/storageserver.

P.P.S.: Windows Storage Server 2008 is also available from MSDN or TechNet subscriber downloads for non-production use (see details at http://blogs.technet.com/josebda/archive/2009/05/13/windows-storage-server-2008-with-the-microsoft-iscsi-software-target-3-2-available-to-msdn-and-technet-plus-subscribers.aspx).