A little about Equallogic

Purchased by Dell in 2008 for a mere $1.4 billion, EqualLogic has continued to develop its technology under the new brand name ‘Dell EqualLogic’ and now offers SANs that utilise Solid State Disk (SSD) and 10GB Ethernet technologies.

The PS Series

Features are another area that Dell EqualLogic set themselves apart from the competition – other vendors will charge you more for adding replication, automation or integration. Not the case with Dell EqualLogic – all features are available out of the box with every single SAN model they produce, which makes selling them such a simple affair. Apart from the hardware and maintenance, there’s nothing else to sell and no hidden extras!

What Is The ‘Peer Storage Architecture’?

In a nutshell, the Peer Storage Architecture refers to the self-managing and scalability features of the Dell EqualLogic SAN range. It’s essentially a form of storage virtualisation – a clever software layer that sits between connecting systems and the hardware.

Simplicity is very high on Dell EqualLogic’s list of priorities and what you get presented with is a simple, logical, well designed interface that is nothing short of a joy to use – I can get one of these SANs up and running in 15 minutes with the remainder of the advanced configuration options defined in a further 10 minutes.

The Dell EqualLogic storage virtualisation layer is quite simply awesome. There are loads of automatic features that occur in the background too with no involvement from the administrator. I’m not going to go into all the features of these arrays, but I will list my favourites:

• No waiting for the SAN to initialise the RAID level before you can start defining LUNs (called ‘volumes’ in EqualLogic Land), connecting initiators or storing data – initialisation occurs in the background.
• Suddenly decide that you’d rather have the space instead of uber performance? No problem. 3 clicks and you can convert from RAID10 to RAID50 on the fly without dropping a beat.
• Running out of room on your SAN and want to expand the capacity using another Dell EqualLogic SAN? Absolute doddle – just add your new SAN to the existing SAN group and the storage is instantly added to the existing pool. Over time your volumes optimise themselves and spread over the SANs in the group, enhancing load balancing and increasing performance.
• Have multiple Dell EqualLogic SANs in a storage group using different RAID levels and disk speeds? The storage architecture monitors each volume over time and redistributes the volume to an appropriate location to optimise performance.
• If the SAN has a failure it automatically contacts Dell and reports the issue. You then get an email from Dell wanting to progress the fault and you can get your replacement parts within 4 hours any time, day or night.
• Replication between SANs is really easy to set up.
• Snapshot integration with SQL and Exchange is really nice – databases are quiesced when taking SAN snapshots, ensuring database consistency instead of hoping that a crash consistent snapshot will be recoverable.
• Update firmware on the fly without killing connectivity. (It does pause disk I/O for around 40 seconds though, so don’t so it during the working day!)
• The free SAN HQ software is excellent for monitoring the health and performance of your SAN.
  

How Dell EqualLogic Do Networking

While you can buy a Dell EqualLogic SAN with a single controller module in it, I suggest you don’t as the secondary module acts as failover for the primary. If you don’t buy a second module then you’ll have no resilience against a module failure.

Modules are configured in an primary/secondary setup, with the secondary controller taking over the iSCSI and/or management communications in the event of a primary controller module failover. In the case of such an event it takes around 10-15 seconds for the secondary controller to become active and start servicing clients. If you’re running Windows or VMware on the SAN then by default they can withstand up to 60 seconds of I/O interruption before throwing a wobbler – all your users will notice is a bit of a hang when they go to save or load a file/access or update a database/etc.

Each active network interface is assigned its own IP address and you define a virtual ‘group’ IP address that encompasses all of these. All client communication uses the group IP address to communicate with the SAN and the SAN itself transmit load balances its outbound traffic across its interfaces. When you have multiple SANs they are all managed and accessed as a single entity via the group IP address.

It’s probably worth being aware of a few points here:

• Single NIC port failures on the primary controller will not fail over to the secondary controller – failover only occurs if the primary module fails completely.
• You do not configure IP addresses for the ports on the secondary module – in the event of a controller module failure the port configuration from the failed module is presented over the secondary module once it becomes active.

Network communication can be further enhanced by implementing MPIO. In the case of Windows systems with multiple NICs connected to the storage network, Dell EqualLogic provide their ‘Host Integration Tool Kit’ or ‘HIT Kit’ software which is freely available to download from their web site. Once installed, the HIT Kit integrates with Microsoft’s iSCSI Initiator’s MPIO to load balance iSCSI traffic transmitted to the array.

Similar MPIO connectivity can be established using VMware. Hopefully you are already using multiple NICs to connect your ESX servers to the storage network, so it’s just a case of creating multiple VMKernel ports on your iSCSI vSwitch, forcing port bindings to specific NICs and enabling Round-Robin on the storage paths. It’s something I deploy with all vSphere/EqualLogic setups and works very well.

PS4000 vs PS6000

When buying EqualLogic , your initial choice is between a PS4000 and PS6000 series SAN. As explained earlier, both have the same enterprise features – the main difference between the two series is scalability, which are best described in a table:

	PS4000	PS6000
iSCSI Gigabit Ports	2 per controller	4 per controller
Max Group Members	2	12
Max Volumes	256	1024
Max Snapshots / Replicas	2048	10000
Max Snapshots Per Volume	128	512
Max Volumes For Replication	32	192
Max Replicas Per Volume	128	512
Max Replication Partners	16	16
Max Host Connections	128 per pool 256 per group	512 per pool 2048 per group

PS4000E / PS4000X / PS4000XV

The PS4000 comes in three flavours, each defining the spindle speeds of the hard drives:

PS4000E = 7,200rpm SATA2 drives
PS4000X = 10,000rpm SAS drives
PS4000XV = 15,000rpm SAS drives


Spindle speeds are directly related to the latency and IOP performance ceiling of the SAN – I’ll discuss this in more detail later on.

Eagle-eyed readers will note that there are 3 Ethernet connections on each module in the pic – two are dedicated to iSCSI traffic and one is a 10/100Mbit connection dedicated to management; you don’t need to use the management NIC if you don’t want to – the SAN can be managed via its iSCSI group IP address if required (but may be handy if you don’t want to create a routed connection to your storage network).

PS6000E / PS6000X / PS6000XV / PS6000S

The PS6000 comes in four flavours and as before, each model defines the hard drive types:

PS6000E = 7,200rpm SATA2 drives
PS6000X = 10,000rpm SAS drives
PS6000XV = 15,000rpm SAS drives
PS6000S = SSD drives

The PS6000 series SANs come with 4 Ethernet connections on each module and all 4 connections can be used for iSCSI connectivity. Optionally, eth3 on each module can be configured as a dedicated management port in a different subnet if required.

PS6500E / PS6500X

Dell EqualLogic also offer their “SUMO” high capacity arrays based on the PS6000 series in the following formats:

PS6500E = 7,200rpm SATA2 drives
PS6500X = 10,000rpm SAS drives

These 4U behemoths are equipped with 3 PSUs (compared to the 2 PSUs in the PS4000/PS6000 series SANs) and come equipped with 48 spindles installed! Yep, that’s 48TB in a 4U chassis using 1TB drives! I expect 2TB drives to be ratified soon allowing 96TB in the same footprint.

As with the PS6000 series, the PS6500 SANs are also equipped with 4 Ethernet connections on each controller module.

PS6010E / PS6010X / PS6010XV / PS6010E

Following the SATA2 / SAS drive type naming convention of the PS6000 series, Dell EqualLogic’s latest PS6010 models now incorporate 10GBit conectivity using 2x SFP+ connections on each module with an Ethernet interface dedicated to management.

I’ve not had the opportunity to play with a PS6010 yet, but the literature states up to10x the bandwidth per port and 2.5x the bandwidth per array compared to previous generations.

PS6510E / PS6510X

Finally, we see the logical combination of the SUMO and PS6010 SANs with the PS6510 series – that’s a 4U chassis equipped with 48 drives and 10GBit controller modules.

As with the PS6500 series, they come in the following drive configurations;

PS6510E = 7,200rpm SATA2 drives
PS6510X = 10,000rpm SAS drives

The reason why Dell EqualLogic don’t supply the PS6500/PS6510 series SANs with the 15K SAS drives is due to the challenge of cooling 48x 15K disks in this form factor. Personally I’m holding out for the SSD version anyway. 😉

Performance

It’s important to understand what performance means when talking about storage. To me, performance is about IOP capacity under pressure, ie. how hard can the array perform when suffering a heavy, random bout of usage?

When comparing fibre channel and iSCSI solutions it’s easy to say, “Fibre channel is 2GB or 4GB and iSCSI is 1GB so fibre channel must be faster, right?” but you’re actually talking about bandwidth, not latency or IOP performance of the SAN. From a network technology perspective, the latency of iSCSI and fibre channel are all but identical. Yes, a large file will copy faster on a fibre channel setup, but unless you’re storing excessively large files on your SAN you are unlikely to notice a difference during day to day usage.

Dell EqualLogic addresses any bandwidth concerns by load balancing connectivity over multiple NICs. Assuming your switched infrastructure is capable of gigabit saturation, has flow control and jumbo frames enabled, a single connection can easily max gigabit Ethernet out at 125MB/sec. Using MPIO and connections to multiple stores means you can easily pull a simultaneous 250MB/sec off a PS4000 and 500MB/sec off a PS6000. In short, bandwidth should not be a concern in a correctly configured environment.

SAN performance is measured in IOPs (Input/output Operations Per second) and is ultimately governed by hard disk spindle speeds and RAID level. But is a RAID10 SATA2 SAN faster than a RAID50 SAS SAN?

As a rule of thumb a SAN equipped with 10K SAS disks will perform twice as fast as its 7200rpm SATA counterpart at the same RAID level. A 15K SAS SAN will perform three times as fast as a SATA SAN at the same RAID level. One of the new SSD SANs will perform up to 8x the performance of the SATA. These estimates are slightly conservative too – it’s actually a bit more than 2x & 3x the performance with the SAS models.

When comparing RAID level performance, a RAID50 SAN will perform around two-thirds the performance of a RAID10 SAN.

So, according to the above, the answer is no – a RAID50 10K SAS SAN will perform better than a RAID10 SATA2 SAN. I would have to say my real world experience tallys with these figures – a SAS SAN is noticeably speedier once there’s quite a bit going on.

In my own experience with Dell EqualLogic SANs I’ve personally pulled over 5000 IOPs of random read/write I/O during testing of a 15K RAID10 SAS SAN with IOMeter, but IOP capabilites are infrastructure specific, so it’s hard to generalise with actual numbers. If you wish to discuss I/O performance for your specific needs then please contact me.

It’s also worth noting that these figures are the same for both PS4000 and PS6000 SANs, so don’t be worried about performance if opting for a PS4000.

Replication

As I’ve already mentioned, SAN to SAN replication is supported out of the box on all Dell EqualLogic models. All replication is asynchronous, meaning changes are stored (using SAN snapshots) and replicated according to a periodical schedule.

Replication is performed at the disk block level and disk block sizes on a Dell EqualLogic SAN are 512KB. This means that a 3 byte change to a file could potentially update 3 separate blocks on disk – so now you have 1.5MB to replicate for your 3 byte change! It’s also worth mentioning that if a block gets updated multiple times in between replication schedules, it will only replicate the block once.

For SAN replication, the capacity of the link between SAN groups is entirely related to the rate of change and how often you want to replicate. Setting up replication schedules and the required storage space can be something of an art and will need to be tweaked over time to get right. A replication schedule that runs every 15 minutes will mean there will be a reduced opportunity for changed blocks to receive additional changes. If your replication schedule is set to replicate every hour, changed blocks will be more likely to receive further updates, so you will probably replicate less data overall than 4x 15 minute schedules. However, the transfer will take longer and you’ll need to set aside more storage to hold the snapshots between replication schedules.

As you can see, configuration requires balance so expect to have to tweak the initial configuration over time.

It’s a shame that WAN optimisations such as caching and compression aren’t used as large WAN links can be out of reach of most companies – especially in today’s financial climate. That said, Dell EqualLogic are continually adding new features to their storage architecture, so should such a feature come along it will be free and implemented with nothing more than a firmware update!

One option to squeeze more out of your WAN for replication would be to implement WAN optimisation appliances at each end of the link such as the Riverbed, Cisco, or F5 products, but these can work out quite pricey too!

Conclusion

Dell EqualLogic have designed excellent, well featured storage appliances with the PS series SANs. Unless there’s a specific feature requirement that they don’t do (WAN optimisation / Campus Clustering / etc) I would recommend them over any other iSCSI storage appliance.

Despite their simplicity, it’s worth engaging in some consultancy with your favourite Channel Partner when deploying Dell EqualLogic SANs to ensure the SAN, network infrastructure and connecting systems are configured optimally. 🙂

Pro’s

• Easy to use
• Great performance
• Lots of useful features
• No hidden feature costs
• Excellent support
• Snapshot integration with VMware/Exchange/SQL is a nice touch
• Regular firmware updates

Con’s

• No WAN optimisation for replication
• Replication can be a little fiddly to maintain