VMware ESX offers several options for creating virtual disks (i.e., VMDK files) for your virtual machines (VMs). This article covers options for thin and thick VM disk provisioning, along with general recommendations and guidelines for storing VMDK files on ESX DAS and SANs. It’s especially important to consider the block size when formatting an ESX storage group; otherwise, you might have to back up any existing VMs and reformat the storage group. These disk configuration parameters have a significant effect on functionality, performance, and manageability.
Creating a Disk
When you add a disk to a VM in VMware ESX, you’re prompted with options to create a new virtual disk, use an existing virtual disk, or create a raw device mapping. The first two options are self-explanatory. When a raw device mapping is stored on a SAN, the files aren’t encapsulated inside a VMDK file. If you browse the files on the SAN LUN, the individual files will appear, not just a single VMDK file that encapsulates all the files inside the VM’s disk.
If you have a SAN, you can create a raw device mapping that maps storage to a LUN on the SAN. Raw device mapping can give you better throughput for sequential disk I/O and about the same performance for random disk I/O compared with non–raw device mapping disks. However, the main reason to use raw device mapping is when you need to expose the physical device properties for SAN management software.
Raw device mapping can be configured in either virtual or physical mode. Virtual mode presents the raw device mapping just like any other VMDK file, with all of the same functionality of a regular VMDK file, such as snapshots and cloning. Physical mode exposes all the physical hardware, with the exception of the REPORT LUNS command, which is virtualized so that ESX can track which VM is using the LUN.
Some SAN vendors’ VSS writers that are used for SAN snapshots, backup, or replication require the use of a raw device mapping in physical mode. However, raw device mappings in physical mode prevent you from using VMware snapshots, cloning, and VMotion on the VM. Physical mode is required by some SAN vendors to ensure that the VMs are properly quiesced before a SAN snapshot is taken.
Quiescing is important for VMs running any type of logged data changes, such as Microsoft Exchange Server and Microsoft SQL Server. Quiescing temporarily stops the transaction flow on the VM so that when a SAN snapshot is taken, only fully complete transactions reside in the database. If a VM isn’t properly quiesced, you run the risk of snapping a VM with a database in a “dirty” (partial transactions) state.
Raw device mapping disks can be used instead of a large VMDK file (over 1TB) if you don’t feel comfortable storing that much data in a single VMDK file. Check with your SAN vendor for more information about raw device mapping.
Disk Provisioning
You can choose either thick or thin disk provisioning when you create a VM disk. If you select thick provisioning, ESX will allocate the entire size of the disk on the storage group when it’s created. If you thin-provision a drive, only the space that’s used in the VM disk is allocated. Thin provisioning saves storage space, but you’ll take a performance hit and increase the probability of running out of space on the ESX storage group.
The performance hit of thick versus thin provisioning is based on many factors, including disk speed, array configuration, and degree of disk fragmentation. As a general rule, thick provisioning gives the best performance and reduces the chance of running out of disk space. Although it requires more disk space, it’s a more conservative approach.
If you plan to thin-provision your VM disks, I suggest using it only for VM base images rather than data drives for VMs. If you create several VMs with 1TB thin-provisioned data drives, and users start saving data on these drives, you might very quickly run out of disk space on the ESX storage group. If you’re concerned about disk fragmentation on the storage group, check out Diskeeper’s V-locity or Raxco Software’s PerfectDisk 11 vSphere Bundle disk defragmenting software for ESX.
Disk and Controller Types
When you create a VM, you have the option of creating either IDE or SCSI disks. You should use SCSI disks whenever possible, because they provide better performance than IDE disks. You might have to use IDE disks if the VM’s application doesn’t support SCSI disks.
You must select the SCSI controller type when creating disks. The Windows OS determines the type of SCSI controller to use, based on compatibility and performance. Table 1 lists controller types by Windows OS.
Table 1: SCSI Controller Type Based on Windows OS