Compellent SAN

We recently took delivery of a Dell/Compellent iSCSI SAN to replace an existing iSCSI SAN. (Dell now own Compellent).

At an architectural level, the Compellent is the first virtualised SAN I’ve used. With a virtualised SAN, you don’t allocated discs to RAID groups. Instead, you give the SAN some guidance on what RAID levels to use and the SAN sorts it all out, moving data blocks between the differing RAID levels based on usage.

Remember I said you don’t allocate discs to RAID groups? The SAN dynamically spreads data across discs using the required RAID level. It’s completely possible for a physical disc to hold RAID 10, 5 & 6 data blocks all at the same time. If you’re used to tightly controlling which discs in which shelf are used, you may have problems letting go!

Another issue of a virtualised SAN is that you can’t just read a simple spec sheet and it tell you the specs of the unit. It all depends on the drives you install in the system. Dell ran a tool against my current SAN to view its usage patterns, then came back with a configuration to at least match its current usage.

Before you go ahead and install your SAN, Dell strongly recommend that you go through their “Pro Deploy” service. This service aims to ensure your SAN is installed & configured correctly for your environment. This includes them sending engineers to site to do the unpacking and racking of the unit. I had two engineers who said they’d been scheduled six hours to do the racking. They did it in about 15 minutes. (I could have done it on my own in under 30 minutes)

It’s in pre-deploy questionnaire they send you that you start to find out the differences with the Compellent.

With my old iSCSI SAN, it had two iSCSI IP addresses and a single management IP address. The Compellent requires a few more addresses. In fact, it requires 10 iSCSI IP addresses and five management IP addresses!

But that’s not all the Compellent wants. It wants internet connectivity back to the Dell mothership. If the Compellent finds a fault it automatically opens a support call with Dell. I can confirm this works as the installing engineer forgot to put the system into maintenance mode when doing a test reboot of the system. Within five minutes I had Dell on the phone asking what was wrong as they’d seen an alert that one of the controllers had gone offline.

The system’s connectivity goes further than a simple email alert, though. If you enable it, Dell can remote into your system. No need for firewall rules. No giving them passwords or stressing how to give them a 2FA token. Just tick the box and the Dell engineer can get straight into your system.

To assign the device’s management IP address, you need to have a Windows machine on the same subnet as the Complement’s management interface. No DHCP here, just good old fashioned broadcasting. (Actually, they say it uses multicast rather than broadcast. Not sure why…) But once you’ve set the device’s IP address you can administer it from outside its subnet.

Then there’s the Dell Storage Manager. It does statistics/log collecting, and also allows acts as a central configuration platform for multiple arrays.

Dell Storage Manager needs a SQL Server instance – and this instance can NOT be stored on the Compellent. The docs say the monitoring system can use either:

  • An in built database engine (suspected to be just a simple flat-file system)
  • MS SQL Express
  • MS SQL Server
  • Oracle MySQL.

The first option is not supported at all other than for testing/demo of the management system.

The SQL Express option is technically possible, but Dell really don’t want you to use it for a live system. (My installation engineer wasn’t happy with me using it and muttered several times about me upgrading to SQL Server proper) Which leaves you needing either a SQL Server or MySQL database.

In addition to the database, you need either a Windows VM or an appliance VM which talks to the database and the Compellent. (Yep, that’s another IP address)

There’s also an add-on to VMWare vCentre for the Compellent. This comes as an appliance (which needs its own IP address!) Unfortunately, the add-on only works with the Flash version of the vCentre interface. (In testing, I also found it unreliable.) This add-on talks to your Dell Storage Manager.

To me, one downside of the Compellent is that you can only configure & manage it from Windows client. Yes, we’re in the 21st contrary but Dell/Compellent still haven’t heard of web browsers yet.

The Complement’s only just gone in. I’ll report back later on what it’s like in anger.

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