Saw this interesting post today, almost dated a year ago.

Q: Why does EqualLogic not support having active/active controllers?

A: This is a very good question. EqualLogic runs the active and passive controllers connected by a thick I/O pipe that effectively maintains the passive controller as a mirror of the active controller, this allows for near instantaneous failover in the event of a RAID controller failure – there is no need for the controller having to seize ownership of the failed controllers disks. This is supported by write cache mirroring and the write cache is cached to flash memory.

Note: The process of controller failover uses MAC spoofing and needs portfast and rapid spanning tree enabled on switches.

Q: Does EqualLogic support a Thin Provisioned LUN Space Reclaimer?

A: Not yet, this is in the pipeline.

Also came across this reply in Dell’s Forum.

Some terminology might be helpful here. Equallogic embeds their controller/filer/and disk shelves into one unit. The controllers are active/passive meaning only one controller is ever usable. The filer itself is tied directly to the disks. Other vendors handle this in different ways. Dell has chosen with the Equallogic system to do this.

Some vendors implement “raid” across the filers themselves (HP LeftHand’s network raid). Other vendors offer active/active controllers, or NetApp metrocluster functionality. Dell Equallogic does not.

We operate three Equallogic arrays in production use, and have never suffered a controller failure. When we preform firmware updates the unit reboots twice, taking it offline for 15 seconds. We do this during ‘quiet’ activity hours on our VMware, SQL Server, and Exchange clusters. They seem to handle the 15 second downtime without issue.

We have not seen the Active/Passive controller layout of the Dell Equallogic as a negative. The failure of an entire Equallogic filer (both controllers and both power supplies) is extremely extremely rare. There are no shared components between the controllers, they are functionally separate filers. The unit is right-sized for our organization and provides enterprise functionality at a fraction of the cost of a similar product that wold allow Active/Active enterprise level controllers, or Metrocluster functionality.

In summary:
> Dell Equallogic does not allow Active/Active Controllers, or full ‘raid’ between discrete units.
> Dell does not offer ‘Metrocluster’ or ‘Network Raid’ functionality like DRDB.
> Reboots of the entire SAN take 15 seconds (yes, really, as a customer, not Dell marketing) and do not cause any issue for us.