Sunday, June 20, 2010

Virtualization and Postgres

It will be soon couple of months for me at VMware. I have been learning a lot of VMware and Virtualization. Often time I try to view it from the point of view of a database like Postgres. To say the least it has been a learning experience.

In the most simplest form of understanding, virtualization (not to be confused with simulation or emulation) is a set of resources. The primary resources are CPU, Memory, Disk and Network. Then there are others but most others are not that interested for majority of the database users (or alteast for me).  For a java programmer this is more like an interface which can be implemented by a class but retains its properties. One cannot use it directly but you use a class typically implementing that interface. Similarly for virtualization while you think only in terms of this interface resources it is actually implemented by the underlying physical resources of the system. Hence it is not to be confused with "emulation".

That said one would think that they need not think of it as a "special" case in their deployment. This is where expectations and reality start to diverge and we could have a case of finger pointing. To set the right expectations for virtualization one needs to understand the philosophy behind virtualization which I guess is what I have been doing recently. It is more like soul-searching in some sense. From what I get the philosophy behind virtualization is actually efficient and simplified utilization of resources. Again lets go back to our primary resources CPU, Memory, Disk, Network. What virtualization tries to achieve is efficient utilization of these resources beyond one operating system. I think everyone understands the efficient utilization part. The clause that becomes tricky is "beyond one operating system". This clause is like the last minute change included by some smart lawyer which no one will understand till some years letter when another legal review is done. :-)

This clause is what is causing many operating systems, applications like database to rethink their behavior of "My world is the only world here".  The details of these behaviour are actually too long to list but to give an idea, an idle operating system throws enough tick interrupts to keep the clock up to date that few of these operating systems while doing nothing can keep a core busy just to do these time updates. Occassionally it can happen that the interrupt that does not happen in time which can cause time drifting in operating systems.

Similarly my experience with databases and shared memory is often the databases just pin the max memory we expect the database will use at peak loads. So even though it may be say 10-20% of the case that peak utilization will occur but  memory has been  fully allocated and pinned (marked "unusable" for others) which may not be the most efficient way of using that resource. (Primary true in my past life where the immediate thing we do is use Intimate Shared Memory on Solaris so nothing else can use it.) Other thing that can come to mind is spin locks where CPU cylces are forced to spin to do nothing but wait, network polling to see if there is more incoming data and many other operations.

Plus with the isolation provided a person within an operating system now has no idea what other things could be using the same resources that the OS is using which makes it more difficult for both the administrator and the Virtualization professional to reach some common understanding. :-)

Anyway this is probably my first post on this topic in relation with Postgres. As I learn more things about Virtualization, you can expect me to share  my learning of the impact of Virtualization on Postgres and vice versa. In the mean while if you have other "bugging" questions related to Postgres and Virtualization do let me know and maybe I can test it out on my spare time.
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