Starting an Oracle database on physical server using VMware VMDK volumes

Bart Sjerps Oracle, Virtualization

By now, we all know Oracle is fully supported on VMware. Anyone telling you it’s not supported is either lying to you, or doesn’t know what he is talking about (I keep wondering what’s worse).

VMware support includes Oracle RAC (if it’s version 11.2.0.2.0 or above).  However, Oracle may request to reproduce problems on physically deployed systems in case they suspect the problem is related to the hypervisor. The support note says:

Oracle will only provide support for issues that either are known to occur on the native OS, or can be demonstrated not to be as a result of running on VMware.

In case that happens, I recommend to contact VMWare support first because they might be familiar with the issue or can escalate the problem quickly. VMware support will take full ownership of the problem. Still, I have met numerous customers who are afraid of having to reproduce issues quickly and reliably on physical in case the escalation policy does not help. We need to get out of the virtual world, into reality, without making any other changes.  How do we do that?

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ZFS and Database fragmentation

Bart Sjerps Oracle, Performance
Disk Fragmentation
Disk Fragmentation – O&O technologies.
Hope they don’t mind the free advertising

Yet another customer was asking me for advice on implementing the ZFS file system on EMC storage systems. Recently I did some hands-on testing with ZFS as Oracle database file store so that I could get an opinion on the matter.

One of the frequent discussions comes up is on the fragmentation issue. ZFS uses a copy-on-write allocation mechanism which basically means, every time you write to a block on disk (whether this is a newly allocated block, or, very important, overwriting a previously allocated one) ZFS will buffer the data and write it out on a completely new location on disk. In other words, it will never overwrite data in place. Now a lot of discussions can be found in the blogosphere and on forums debating whether this is really the case, how serious this is, what the impact is on performance and what ZFS has done to either prevent, or, alternatively, to mitigate the issue (i.e. by using caching, smart disk allocation algorithms, etc).

In this post I attempt to prove how database files on ZFS file systems get fragmented on disk quickly. I will not make any comments on how this affects performance (I’ll save that for a future post). I also deliberately ignore ZFS caching and other optimizing features – the only thing I want to show right now is how much fragmentation is caused on physical disk by using ZFS for Oracle data files. Note that this is a deep technical and lengthy article so you might want to skip all the details and jump right to the conclusion at the bottom 🙂

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Why clone databases for firefighting

Bart Sjerps Oracle, Virtualization

clonesAs more and more customers are moving their mission-critical Oracle database workloads to virtualized infrastructure, I often get asked how to deal with Oracle’s requirement to reproduce issues on a physical environment (especially if they use VMware as virtualization platform – as mentioned in Oracle Support Note # 249212.1).

In some cases, database engineers are still reluctant to move to VMware for that specific reason. But the discussion is not new – I remember a few years ago I was speaking in Vienna to a group of customers and partners from Eastern Europe, and these were the days we still had VMware ESX 3.5 as state-of-the-art virtualization platform. Performance was a bit limited (4 virtual CPUs max, some I/O overhead and memory limitations) but for smaller workloads it was stable enough for mission critical databases. So I discussed the “reproduce on physical in case of problems” issue and I stated that I never heared of any customer who really had to do this because of some issues. Immediately someone in the audience raised his hand and said, “well, I had to do that once!” – Duh, so far for my story…

Let’s say that very often I learn as much from my audience as (hopefully) the other way around 😉

Later I heard of a few more occasions where customers actually were asked by Oracle support to “reproduce on physical” because of suspected problems with the VMware hypervisor. In all of the cases I am aware of, the root cause turned out to be elsewhere (Operating System or configuration) but having to create a copy in case of issues is a scary thought for many database administrators – as it could take a long time and if you have strict SLAs then this might bite back at you.

So what is my take on this?

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