Aligning disks in virtual world
The alignment of partitions of the underlying storage infrastructure is a theme that returns very often in the analysis and implementation of virtual architecture. In fact, the problem also exists in the physical world virtually wherever you have a RAID controller, but is mitigated by the fact that access to records is limited to a single server (so there is only in the case of I / O very intense). In the virtual world, where multiple hosts are accessing the same LUN to manage multiple virtual machines, the alignment problem can become a bottleneck in the overall performance of our virtual infrastructure.
It 's important to understand exactly what the problem is the alignment of the disk partition of a Windows system. This helps an old document titled VMware Recommendations for Aligning VMFS Partitions : NTFS disk is formatted in blocks of variable size. The operating system optimizes the I / O grouping many areas in the so-called cluster (allocation unit).
At the lowest level SANs work aggregating multiple disks in one array, whose smallest unit is called a chunk or stripe. To optimize the I / O the chunks are usually much larger than a single sector, and then read operations of a sector imply the reading of an entire chunk.
Between these two layers is the filesystem used by the hypervisor, which in the case of VMware is the vmfs.
Tom Hirt has published a simple drawing that explains very well what may be the impact of a disc is not properly aligned. 
From this design is difficult to understand as the reading of a single cluster from a Windows machine may require the reading of two locks VMFS which in turn can be coated on the chunk of even three SAN.
The mismatch arises automatically when installing a Microsoft operating system because Windows "signature" of records occupying the first 63 blocks (typically 512 bytes each). SAN side, the chunks are usually 32, 64, or multiples of 128KB, but with 63 blocks 512B are always misaligned.
On a Windows Server 2003, you can check the alignment of a partition with the following command:
wmic partition get BlockSize, StartingOffset, Name, Index
The output of this command usually returns something like:
BlockSize Index Name StartingOffset
512 0 Disk #0, Partition #0 32256
32256/512 = 63 blocks, which is exactly the space occupied by the signatures of Windows disks! If you have never paid attention to this problem is likely that all your Windows machines return the same value. What to do?
In most cases the answer is: absolutely nothing. The requests for I / O of a normal servers are not such as to create problems and the reduction of I / O physical host does not lead bebefici directly proportional on the VM. In general, on all servers with a second disc, you can add a new virtual disk, create the partition aligned by hand (using diskpart) and copy / clone the data and then replace the original disk. This procedure is highly recommended for file servers, databases, Exchange Server and in general for all those cases in which the I / O is very high.
The speech becomes more complicated when we reason on the boot disk. In this case, as I hinted, it is appropriate to assess the real benefits: if the boot disk contains only the operating system might not be as crucial to align the partition. To align the partition is still "just" move, but this must be done with special instruments, most of them commercial. There is also a procedure based on GParted.
In addition, some manufacturers of SAN / NAS (like NetApp, through its portal Now) provide procedures and / or tools to align the partitions for different specific hypervisor.
For more information:
Other articles on similar subjects:
- Extending a system disk has never been easier ...
- IDC publishes data on the server virtualization for Q4 2009
- New benchmark ESX, Hyper-V, XenServer
- Virtual Reality Check: new hardware, new benchmark
- Partition Manager 10 for Virtual Machines
- VMware ESX, Citrix XenServer, Microsoft Hyper-V: the benchmarck Virtual Reality Check
- Fujitsu and DataCore together to deliver advanced storage
The biggest problem for me is whether the performance of my subsystem are correct (such as range and what tests should I take as a reference)?
And especially how to improve them ...
Among other things, Win2k8 no longer has the alignment problem:
Windows Server 2008 Attempts to align new partitions out-of-the-box, yet disk partition alignment remains a Relevant technology for partitions created on prior versions of Windows.
http://msdn.microsoft.com/en-us/library/dd758814.aspx