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What’s your favorite fruit? EMC versus HDS

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Nigel has posted the age old question, which is best EMC or HDS? For those who watch Harry Hill - there’s only one way to sort it out – fiiiiight!

But seriously, I have been working with both EMC and HDS for the last 6 years on large scale deployments and you can bet I have my opinion – Nigel, more opinion than I can put on a comment on your site, so forgive me for hijacking your post.

Firstly, the USP and DMX have fundamentally the same architecture. Front end adaptor ports and processors, centralised and replicated cache and disks on back-end directors. All components are connected to each other providing a “shared everything” configuration. Both arrays use hard disk drives from the same manufacturers which have similar performance characteristics. Both offer multiple drive types, the DMX3 including 500GB drives.

From a scalability perspective the (current) USP scales to more front-end ports but can’t scale to the capacity of the DMX3. Personally, I think the DMX3 scaling is irrelevant. Who in their right mind would put 2400 drives into a single array (especially with only 64 FC ports)? The USP offers 4GB FC ports, I’m not sure if DMX3 offers that too. The USP scales to 192 ports, the DMX3 only 64 (or 80 if you lose some back-end directors).

The way DMX3 and USP disks are laid out is different. The USP groups disks into array groups depending on the RAID type – for instance a 6+2 RAID group has 8 drives. It’s then up to you how you carve out the LUNs – they’re completely customisable to your choice of size. Although a configuration file can be loaded (like an EMC binfile) its usually never used and LUNs are user-created through a web interface to the USP SVP called Storage Navigator. LUN numbering is also user configured, so it’s possible to carve all LUNs consecutively from the same RAID group – not desirable if you assign LUNs sequentially and put them on the same host. EMC split physical drives into hypers. Hypers are then recombined to create LUNs – two hypers for a RAID1 LUN, 4 hypers for a RAID5 LUN. The hypers are selected from different (and usually opposing) back-end FC loops to provide resiliency and performance. It is possible for users to create LUNs on EMC arrays (using Solutions Enabler), but usually not done. Customers tend to get EMC to create new LUNs via a binfile change which replaces the mapping of LUNs with a new configuration. This can be a pain as it has to go though EMC validation and the configuration has to be locked for new configurations until EMC implement the binfile.

For me, the main difference is how features such as synchronous replication are managed. With EMC, each LUN has a personality even before it is assigned to a host or storage port. This may be a source LUN for SRDF (an R1) or a target LUN (an R2). Replication is defined from LUN to LUN, irrespective of how the LUNs are then assigned out. HDS on the other hand, only allow replication for LUNs to be established once they are presented on a storage port and the pairing is based on the position of the LUN on the port. This isn’t easy to manage and I think prone to error.

Now we come to software. EMC wipe the floor with HDS at this point. Solutions Enabler, the tool used to interact with the DMX is slick, simple to operate and (usually) works with a consistent syntax. The logic to ensure replication and point-in-time commands don’t conflict or lose data is very good and it takes a certain amount of effort to screw data up. Solutions Enabler is a CLI and so quick to install and a “lite” application. There’s a GUI version (SMC) and then the full blown ECC.

HDS’s software still leaves a lot to be desired. Tools such as Tuning Manager and Device Manager are still cumbersome. There is CLIEX, which provides some functionality via the command line, but none of it is as slick as EMC. Anyone who uses CCI (especially earlier versions) will know how fraught with danger using CCI commands can be.

For reliability, I can only comment on my experiences. I’ve found HDS marginally more reliable than EMC, but that’s not to say DMX isn’t reliable.

Overall, I’d choose HDS for hardware. I can configure it more easily, it scales better, and – as Hu mentions almost weekly, it supports virtualisation (more on that in a moment). If I was dependent on a complex replication configuration, then I’d choose EMC.

One feature I’ve not mentioned earlier is virtualisation. HDS USP and NSC55 offer the ability to present externally connected arrays and present them as HDS storage. There are lots of benefits for this – migration, cost saving etc. I don’t need to list them all. It’s true that virtualisation is a great feature but it is *not* free and you have to look at the cost benefit of using it – or beat your HDS salesman up to give you it for free. Another useful HDS feature is partitioning. An array can be partitioned to look like up to 32 separate arrays. Great if you want to segment cache, ports and array groups to isolate for performance or security.

There are lots of other things I could talk about but I think if I go on much further I will start rambling…

About Chris M Evans

Chris M Evans has worked in the technology industry since 1987, starting as a systems programmer on the IBM mainframe platform, while retaining an interest in storage. After working abroad, he co-founded an Internet-based music distribution company during the .com era, returning to consultancy in the new millennium. In 2009 Chris co-founded Langton Blue Ltd (www.langtonblue.com), a boutique consultancy firm focused on delivering business benefit through efficient technology deployments. Chris writes a popular blog at http://blog.architecting.it, attends many conferences and invitation-only events and can be found providing regular industry contributions through Twitter (@chrismevans) and other social media outlets.
  • snig

    Just to clarify on your statement about HDS replication LUNs having to be defined by port and LUN in order to replicate it. Each LUN does have to be assigned to a port, but the horcm files now only have to contain the LDEV address and you can replicate it. This made things much simpler.

    I have been told that they are working on removing the requirement for a LUN to be assigned to a port.

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