Friday, December 5, 2014

Alignment issues

Before the last update of our NetApp filer,  Proximus asked us to check the alignment of the VM's.
Having read this great blog entry of Duncan Epping,  I already knew something about the problem.
It can have a huge impact on your performance so don't ignore it!

In this specific case, I looked up some NetApp documentation concerning the problem.  TR-3747 also explains the problem and solutions very well from a NetApp perspective.   

Alignment problems vary in complexity depending how you set up your VMware system.  If you use iSCSI you have to check the virtual disks, VMFS and the underlying storage array.  Obviously NFS does not need the VMFS layer so that's a bit easier. 

The best thing is to make sure your virtual machine is properly aligned right after you create it.   You can put these changes in a template for easy deployment.   If the virtual machine is already in production it can be a lot harder/more time consuming.  TR-3747 covers this.

Basically these alignment problems are quickly becoming a thing of the past for these reasons:
  • The OS on virtual machines is aware of the problem  Companies like Microsoft and Red Hat are aware of these problems and Windows 7, 8, 2008, 2008 R2, 2012, RHEL/CentOS 6, Debian 6, Ubuntu 10, 11, 12, SUSE 11 onwards are not affected .  For older OS like RHEL/CentOS 5 (or Windows 2003) you have a problem, though.
  • VMFS has become a lot smarter  If you did your Datastore configuration from vSphere 4 or later and don't use any legacy Datastores,  there are no alignment problems either.
NetApp makes the distinction between functionally aligned and actually aligned.
To make a virtual machine functionally aligned, it is moved to a special datastore which compensates the mis-alignment.  Everything is untouched.   Virtual machines with the same offset can be moved to the same datastore.  To make a virtual machine actually aligned you have to bring it offline and use a tool such as the VMware vCenter Converter, UberAlign or mbralign (NetApp specific) to align it.

NetApp has a built a tool in Virtual Storage Console (VSC) to detect and online (for people who have Storage vMotion) fix misalignments .    

This is how it works:

You scan your datastores.  Your VM's are then categorized into folders.
  • Aligned > Functionally Aligned and the Aligned > Actually Aligned folders :  a virtual machine is functionally (I/O only) or actually aligned. No steps required.
  • Misaligned > Online Migration folder:  a misaligned virtual machine can be aligned using the online feature
  • Offline Migration folder : When a virtual machine cannot use the online alignment (migration) feature.     
  • Misaligned > Other:  the VM cannot be aligned for some reason.  Most common cause :  the VM has multiple disks spanning multiple datastores.  But other causes exist. 
In the latest version of VSC,  mbralign  isn't supported anymore so NetApp probably thinks functional alignment is the preferred method. 

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