Friday, October 30, 2009

On Virtualizing your Storage

Way back, I went over what's virtualization on Unix, and what's not. Well now it's time to hit storage over the head with the debunking hammer!

So, let's start with defining virtualization in the storage context. Storage virtualization is taking a homogenous or heterogenous set of storage resources, and distributing data over multiple arrays or controllers. That's the simplified version. There's a complicated set of requirements to actually qualify as storage virtualization. I'll break it down in a list format.
  • It must support more than one controller and more than one storage subsystem.
  • It must support more than one vendor's storage subsystem(s).
  • It must support more than one model of storage subsystem.
  • It must support at least one protocol of: Fiber Channel, iSCSI, or NAS.
  • It may or may not include it's own disk storage controllers.
  • Data must be capable of being spanned across two or more attached storage subsystems while being presented to hosts as a single LUN.

So we've got a good list of what is required to qualify, in my world, as storage virtualization. So, let's do the list! What IS Storage Virtualization? (In reverse alphabetical order, and not a complete list.)

Actually Virtualization

  • NetApp V-Series
  • LSI StoreAge SVM
  • IBM System Storage SAN Volume Controller
  • Hitachi Data Systems USP-V
  • Hitachi Data Systems USP-VM

Definitely Not Virtualization

  • HP LeftHand P4000 - scale-out is not virtualization!
  • EMC V-Max - does not attach to ANY other vendor or controller.
  • EMC Invista - does not support to ANY vendor except EMC.
  • Coraid ATA-over-Ethernet products - single vendor chassis with storage built in!

Don't Know Enough, So Might Be!

  • Incipient Network Storage Platform
    They hide all their documentation and technical specifications, so I can't tell if it's just a tool for mirroring and copying between different storage subsystems or it's actually virtualizing.

All that said, there's the argument that could be made that if the product hides the storage behind it and presents a single point of management for your storage, then it's virtualization. But, it's not. It's a gateway.
The EMC people will whine about V-Max being defined as "Not Virtualization." TOUGH LUCK. IT ISN'T. The V-Max is a storage subsystem, which spans data across multiple arrays and multiple controllers within itself. The V-Max does NOT support any externally attached arrays from any other vendors. The people who want to whine about Invista? TOUGH LUCK. IT STILL ISN'T. The Invista slid into supported EMC array cabinets and only worked with EMC.
It's NOT virtualization if it only works with one storage vendor. Period. The point of storage virtualization is to enable heterogenous environments. Be it tiering by applications, saving money by using multiple vendors, or increasing performance by using multiple arrays and subsystems.

So why would you virtualize your storage? There's dozens of good reasons. The one I hear the most frequently is the IBM SVC owning SPC benchmarks. They want that level of performance out of their storage, and they think virtualization is a magic wand. It isn't. Virtualization is, like all things, a piece of the puzzle. No more, no less. Can you rock your world with SPC record breaking performance by going to virtualization? Sure, if you pay for it. Just like everything else.
Virtualization is still a front end to storage subsystems. That means that just because the virtualization engine can do 9,500MB/s random, you're still limited by the arrays behind it. The counteracting component is the use of multiple controllers and arrays. One array does 250MB/s random, but with virtualization you can span the LUN across two, which gets you to 500MB/s random in theory. In reality, it'd likely be closer to 400MB/s, but that's still way up from 250MB/s. Need more performance? Add more controllers. It doesn't hold 100% true, and there's a scaling point where it stops helping, but that's the theory.
From the administration side, virtualization is very appealing. This is also how many gateways claim to be virtualization. A key tenet of storage virtualization is that it must provide a single point of management for your storage, regardless of what's behind it. When you virtualize, all your disk to host provisioning is done in the virtualization engine. You no longer need to slice things out on each controller after looking at loads, what space you have, etcetera. It consolidates the vast majority of your storage management into a single pane of glass.
So far, we've established that virtualization can turn multiple low-performance arrays into solid performing LUNs for your hosts, and your administration nightmares can be drastically reduced.

Now I'm going to tell you why both of those are bad too. First, administration nightmares only go down if you're capable of configuring things that way. You have to be ready to let go of individual arrays for individual applications on your storage subsystem. You create a bunch of large, high performance arrays and give them to the storage virtualization engine to manage. It writes blocks across those arrays. Stop managing spindles, start managing performance. Group arrays by performance and by capacity in your storage subsystem, not by application. Group by application in your storage virtualization engine.

Low performance arrays are STILL low performance arrays. If your issue is seek performance, virtualization will not help you. Seek performance is dependent on the arrays behind the virtualization layer, and the virtualization adds seek penalty - anywhere from 40us to 5ms depending. Putting two SATA arrays behind an SVC will not get you FC. It will get you 1.5 times SATA. Virtualization is not a replacement or workaround for baseline array performance. It's a way to enhance performance. And the most common configuration error? People tier their storage by controller. Tier 1 is this storage subsystem, tier 2 is that storage subsystem, and tier 3 is yet another. You won't realize significant performance increases from this configuration. You'll dramatically increase spindle count, but you become hard limited by controller performance, and unbalance controller loading. The optimal configuration is to span tiers across multiple controllers. 3 Controllers with 10 arrays of 5 SAS15k disks, 10 arrays of 5 SAS10k disks and 10 arrays of 5 SATA disks will typically perform better than 1 controller with 30 arrays of each type of disk. The load becomes more balanced across all three controllers, rather than having one controller sitting idle while one is begging for mercy.

So, how do you determine if virtualization is right for you? If you said "I have more than two storage subsystems and I need better performance and ease of management," I want that gold star back right now. First and foremost, storage virtualization is not for everyone, it's not always appropriate, and it's not a cheap solution to an expensive problem. Storage virtualization allows you to consolidate multiple heterogenous resources into a single point of management and allocation. That's it. Performance benefits should not be at the top of your list of reasons. Does that mean it can't be? No, but it shouldn't be the primary reason you're taking your first looks at storage virtualization, or even your second looks. Many environments can realize performance gains for far less money by analyzing and optimizing their storage configuration to suit their environment. And no, I don't mean best practices. Best practices are a starting point - not an end point. Reconfiguration of existing storage might buy a lot more than you expect.

Storage Virtualization fits most easily in rapidly or frequently changing environments, medium to large environments looking for increased scalability or flexibility on existing or new hardware, or environments with large amounts of administrative overhead. I'll get shot for saying it, but I'm going to anyways - in some environments, storage virtualization can change the staffing requirement from 5 to just 2 or even 1 Storage Admin. That doesn't mean it isn't suited to other environments, just that these environments are the most likely to receive immediate benefit from storage virtualization.
Unlike server virtualization and partitioning, storage virtualization is still rather immature. If a vendor starts telling you they solve everything including the kitchen sink, be wary. Every business should look at the costs and benefits of storage virtualization on a case by case basis, with detailed analysis not just from the vendor, but from internal staff as well. Can you use your existing storage subsystems? Can you expand with new subsystems? Will you have to forklift upgrade the storage behind it to expand further? Does it support your preferred vendors? Who's using it for what application with what results?

Storage virtualization has amazing potential in many, many environments but it also has the capability to burn you just as badly.

Come back later, where I openly mock every VTL on the market for being the utter crap they are courtesy of an obscure company with an unpronouncable name, and complacency on the part of manufacturers!

Storage - What IS non-disruptive?

This grows out of a discussion going on over here:

The two main systems up for discussion here, are the IBM SVC 2145 and the Hitachi Data Systems USP-V class. So, let's start with some background on the systems.
The IBM SVC 2145 is like most IBM products, an amalgamam of acronyms. The official name is the IBM System Storage SAN Volume Controller. The SVC is a Storage Virtualization Engine, presenting hosts with a unified front-end, SAN Admins with a single point of management, and utilizing any number of disk controllers behind it. It is also the current (and probably forever) record holder of just about every SPC benchmark and a large number of TPC benchmarks. The currently available (and not the announced) hardware offers from 8 to 32 4Gbit FC ports per cluster, as many disks as you can fit into 4,096 MDisks as of 4.2.1 - using RAID5 in 4+P that's 16,384 spindles! Maximum actual spanning is limited to 512 spindles for a single VDisk, which is still jaw-dropping.
The Hitachi USP-V is a beast. A lovely, lovely beast I want for my very own at home (except for the power bills.) It doesn't hold a lot of records because not enough people show it love, in my opinion. The latest generation offers up to 1,152 drives in 1-4 expansion frames with up to 128 flash drives, 512GB of cache, and up to 224 Fiber Channel ports. Everyone else, eat your hearts out. The USP-V also includes all of Hitachi's fancy software offerings, and the ability to assume control of and pseudo-virtualize external storage arrays like IBM DS4000's, HDS AMS-series, and so on.

If you just went "wait, these sound like very different products" that's because they are. You see, the IBM SVC is a Storage Virtualization Engine and the HDS USP-V has a Storage Virtualization Engine. But the SVC offers no storage disks of it's own whereas on the HDS USP-V being disk storage is the core functionality. In other words, we're comparing something with no disks to something that's built entirely around disks. Vendors are getting very good at blurring these lines very badly, or explaining their products very vaguely, resulting in some pretty bad customer confusion.

So which is which and what is what? Let's start with the IBM SVC. The IBM SVC is two to eight 1U systems in an appliance form, which you stick in front of supported disk controllers to provide extent-based virtualization (or not) and/or provide you with a single point of management and presentation for a variety of hosts. Connections to hosts and storage are via standard Fiber Channel, and cluster interconnect uses standard Gigabit Ethernet. The SVC also offers the advantage of using IBM's SDD multipath driver which is available for every major OS out there in driver or pluggable software module form.
This brings us to the HDS USP-V. The HDS USP-V is a high end enterprise storage system boasting some of the most impressive specs you can find. It's highly configurable and customizable. It offers gobs of high speed SAS disks as well as support for SATA disks using HDS' RAID1+, RAID5 and RAID6 algorithms. Like the IBM DS8000's, disks go in 4 at a time at the minimum. The internals are connected via Hitachi's proprietary (not in a bad way) Universal Star Network. External storage is attached via Fiber Channel.
(As a note; I'm not including iSCSI because it's only an announced feature on the SVC, and NAS is the realm of the USP, not the USP-V. Plus we're not comparing features, dangit!)

Now if you read the original post on Wikibon that started us down this road, you'll notice that it was about what constitutes non-disruptive block level virtualization. (Or extents, if you want. Pick your poison.) Some folks have said that only the USP-V does it. But, that's not true. You see, the SVC also does everything the USP-V does. So what's going on here? Well, there's two problems.
One, the USP-V is storage with virtualization while the SVC is just virtualization. Two, the definition of non-disruptive is.. ambiguous at best, tenuous at other times, and just crazy at still others.
Let's start with my definition of non-disruptive. My definition of non-disruptive is being able to perform hardware repairs, software upgrades and hardware upgrades without impacting the production environment beyond performance. Most folks will tell you that my definition is pretty darn reasonable.
HDS and IBM like to redefine "non-disruptive" on a per-product basis, to suit their needs. It's marketing, don't pretend to be surprised, okay? This is what they are paid to do.
So, if we go by my definition, why do both of these systems offer you the potential of non-disruptive maintenance and upgrades? Because they both do. And the SVC may even slightly edge out the USP-V in this regard because of it's appliance nature. (I'll explain, I promise!)

The caveat that both systems are subject to is the actual storage subsystem and attached hosts. Yes. The disks and the end consumers of LUNs. The USP-V can legitimately make that claim because without adding external storage, it's still a USP, offering non-disruptive firmware upgrades and hardware replacement within practical limits.
The SVC can also legitimately claim to be non-disruptive block-level virtualization too. Why? Because the SVC itself can do everything HDS claims to be do non-disruptively as well. Data migration between arrays, between nodes (IO Groups, in fact,) and even between clusters. All that it requires is that the storage subsystems behind it be able to do firmware and hardware maintenance non-disruptively.

This is also where their claims fall apart. For our example, we'll be using the venerable LSI designed-and-built IBM DS4800 storage controller and EXP810 shelves. For the record, I hate the DS4k/DS5k because it is absolute crap for enterprise storage. But it's cheap so it's everywhere. It's still crap.
Given the DS4800 as an External Storage Array on the USP-V and the SVC, both solutions fail the non-disruptive claims. Even the ones their own manufacturers make. Why? Because it's a DS4800, and they're both dependent on it. Firmware upgrades on the DS4800 are fraught with terror and most decidedly disruptive, requiring all IO be stopped to the DS4k. That means any arrays with data on the DS4k must have IO stopped at the virtualizing layer before maintenance can be performed. Which means stopping them at the USP-V or the SVC. Which means shutting down production environments using those LUNs. Was that a "whoops" I just heard?
But by the same token, if we take a USP-V with an AMS2500 behind it, and an SVC with an AMS2500 behind it, both pass because maintenance on the AMS2500 is non-disruptive. See how it works? Now you can dislike marketing crap just as much as I do!
However, within the isolated products themselves - that being the USP-V and only the SVC (with no storage,) all tasks are non-disruptive. As of 4.3.x you can take an SVC from 2145-4F2 hardware to 2145-8G4 hardware in the middle of the day with no impact to your production environment beyond performance, with proper planning.
So why does the SVC slightly edge out the USP-V in non-disruptive? Because the SVC is an appliance you put in a standard rack. If you put each IO Group (two nodes) in separate racks, which you should, the SVC can continue to operate normally through physical moves - excepting when managed storage is shut down for moves. The USP-V can't, because the controller is in a single dedicated frame. Yep. That's the entirety of it.

What about host side? HDS claims to be unique in their ability to migrate a disk non-disruptively between USP-V frames. Well, one, the SVC can do this with Metro mirroring (not to be confused with Global mirroring! Global is the one for long distances!) or within a cluster using VDisk mirroring or FlashCopy (which HDS has equivalents for for in-frame, predictably!) Two, apply brakes when the reality of hosts hits.
No, Virginia, there isn't any such thing as a free lunch and migration between frames or between clusters will always, always be disruptive at the host level. You're changing WWNNs and WWPNs on the controllers, even if the rest isn't changing. And you think a host will just smile and eat it? Boy, don't I wish - that would save me SO much trouble, both past and present! No, no. The hosts will get very, very upset with you. So what's the procedure?
Well, I can't speak to the USP-V's since I haven't done it. But I imagine it's somewhat like the SVC's with obvious key differences. The USP-V is doing a migration from Frame A to Frame B. The SVC is mirroring data between Cluster A and Cluster B. To complete the migration on SVC? You unmount the disks at the host. Reverse the mirror direction. Rediscover disks on the host. Mount the disks from Cluster B. Verify everything is happy, and break the mirror. 5-15 minutes of downtime, typically. My guess is that the USP-V is similar in needing to stop IO and unmount, start migration, rediscover disks to pick up the new frame's ownership, and remount while migration is in progress. In theory, this could be fixed in software, but it's a very difficult problem to fix.

So, what've we learned today? One, don't take the vendor's definition of non-disruptive at face value. Ever. Two, do your own homework and don't just settle for "magic quadrants" and glossies. Insist on tech demos that don't simply consist of the vendor demonstrating the feature on a cherry picked array. Insist on hands on time. Insist on talking to real customers.
This post is a great example of exactly why you should. I learned about things the USP-V can do inter-frame that I didn't know before. And hopefully people learned about things the SVC can do that IBM marketing didn't tell them. (Seriously, IBM marketing sucks.)

And I can hope everybody learned a bit about the practical limitations of any storage virtualization solution, be it HDS, IBM, EMC or Joe's Computer Shack.

Aaaaaand the disclaimer!
I don't work for IBM, Hitachi, any subsidiaries, VARs, or BFFs. Despite my lust for the combination of SVC + 2x AMS-2500 in my home, both IBM and HDS have failed to ship me either, or even so much as a cheap mug! I hope you guys are listening, 'cause I could use a new mug after my Sun Customer Appreciation mug broke. ;)