Hard disk drive makers are adding flash storage to their conventional spinning-platter drives to improve performance and are targeting the performance PC market. Wait a second, haven’t we seen this before? As Rocky eventually said to Bullwinkle, “but that trick never works!”

Flash as a Cache

Using flash memory as a disk cache is a pretty good idea. Flash has awesome random read performance and fairly good write speed (compared to a hard disk drive, at least). That’s why more and more enterprise storage vendors are adding flash as a disk cache, not just a plain tier of storage.

EMC is the latest to make the move, announcing “FAST Cache” for their midrange Clariion and Celerra enterprise storage systems last week. They join NetApp, Sun, and others already offering similar capability. Fusion-IO has been the champion PCIe flash provider, but STEC is expected to join them soon.

See my post, Is Flash A Disk Or A Cache?

Flash-as-a-cache hasn’t been as easy to roll out as flash-as-a-disk, but it promises to be more effective. An array that completely integrates flash can take advantage of its positives (fast random read, fast-ish write, low power) without stumbling over its shortcomings (big write blocks, shorter lifespan).

Flash in a Disk

Although EMC is doing the right thing by adding FAST Cache, their implementation uses disk drive form factor flash rather than the PCI cards selected by others. It may prove more-difficult to optimize the system for the characteristics of flash when one is writing through a conventional disk drive interface like Fibre Channel or SAS. Would-be flash-and-platter drives face the same issue: How do you use flash effectively when it’s abstracted from the server and presented as a conventional disk?

The hybrid hard disk drive (H-HDD) method, rolled out back in 2007, added ATA commands allowing a compatible operating system to specify whether data sent to a hybrid drive should be written to flash or disk. These products were paired with Windows Vista’s ReadyBoost and SuperFetch to produce performance gains that never materialized in practice. The so-called “ReadyDrive” has become a footnote in history, along with Intel’s “Robeson” effort to add a flash cache to the motherboard.

It is unclear what the new generation of hybrid hard drives allegedly on the drawing boards at Toshiba and Seagate would look like. It is unlikely that they would use the H-HDD interface, but they will likely be aimed at the same performance laptop and desktop market. Servers have continued migrating towards advanced enterprise storage systems that pack their own cache, reducing the impact of bare hybrid drives.

Ingredients for Success

Rather than repeat the mistakes of the past, these companies could integrate real smarts into the disk controller, allowing it to autonomously move data to the flash cache to improve everyday performance without any special operating system support. This “tiered storage in a can” approach might deliver the goods that old-fashioned H-HDDs never could.

© sfoskett for Stephen Foskett, Pack Rat, 2010. | Hybrid SSD/Hard Disk Drives: This Time For Sure!
This post was categorized as Enterprise storage, Terabyte home, Virtual Storage. Each of my categories has its own feed if you'd like to filter out or focus on posts like this.

Fram approaching in front of iceberg upernavik, CC-by-SA copyright Kim Hansen

Solid state (NAND flash) storage is all the rage right now, but there are many lingering questions regarding its true performance, reliability, and cost. But no question is more important in determining its ultimate usefulness than that of location: Where should flash storage be placed to maximize return on investment?

Storage companies have argued that flash disks can be used most effectively in external storage devices, arguing that it’s simpler to just leverage existing storage technologies. Server companies have tended to prefer to place it inside the server, asking why, if flash disks are capable of massive random I/O performance and extremely low latency, one would put them at the other end of a Fibre Channel or iSCSI connection, which introduces latency and tends to combine I/O operations?

The Case For Servers

The argument for placing flash in (or very close to) servers boils down to two key contentions:

1. Distance = latency, so moving quick flash devices away from I/O-hungry CPUs erodes their effectiveness
2. Granularity (or lack thereof) is the core problem facing storage management, so moving flash (and other types of storage) closer to the omniscient application is likely to bring greater effectiveness

The server folks are relying on a technical argument – that placing high-speed cache where it could theoretically do the most good is the right decision. And they are right, in a perfect world: A flash-aware application talking to a low-latency flash device over PCI ought to really fly!

There is some disagreement in the server-side argument as well: Is flash a “fake disk” or a new level of caching between RAM and storage? It seems that the pitch leans toward the latter, even when the SSD appears as a disk drive. This is what Fusion-IO is pitching: They skip old-school disk connections like SATA and SAS altogether, placing their storage on PCI Express and asking hardware and software vendors to integrate it as best they can. Consider Sun’s flash integration for ZFS, for example. Note, by the way, that Intel’s Turbo Memory products also offer PCIe flash, despite what you might be hearing.

Of course, we can just use a flash drive in place of an internal hard drive. Just about everyone makes something like this now, and they work pretty well in some cases. Then there are hybrid drives, which have gone nowhere so far.

But will this work? We need operating systems and applications that can make use of this local flash, and that has been a problem. Intel’s flash-on-the-motherboard idea never caught on, even as Vista included ReadyBoost, because the truth is that operating systems, file systems, or applications must be re-engineered to really make use of flash in a server. That’s happening, but slowly.

The Case for Arrays

Then we turn to the other end of the storage pipe. EMC put flash in the DMX in January, and Compellent is doing it as we speak. IBM went wild with a bunch of Fusion-IO drives and an SVC over the summer, too. All this proves that flash works in storage arrays!

Why? Simply because modern storage arrays are already engineered to make good use of disk drive capabilities. This is a “what works” strategy – even though it doesn’t sound as nice in theory, the array doesn’t need lots of re-engineering to see some benefit from flash. And post-RAID virtualized systems like that Compellent can really make hay with a few super-speed flash drives, since they can move hot blocks to flash dynamically.

Sure, there is latency between the CPU and the flash drive, but storage arrays are really computers in their own right. So they can derive the same benefit from flash that a server could, and they can share that benefit to connected servers rather than leaving it locked up.

Why Not Everywhere?

How about we end the debate. Flash works great in the server, and it works great in the array. Why not just put it anywhere it makes sense in your particular environment? Have an operating system, application, or file system that can make use of server-side flash? Go buy a Fusion-IO card! Have a virtualized enterprise storage array? Get some SSD there, too. And remember that it’s not all about NAND flash – RAM-based solid state storage from companies like Texas Memory Systems, Gear6, and Violin are even faster!

But remember one thing: This stuff is still very very expensive, so you have to really need the performance to make a case for flash.

Image by Kim Hansen, GFDL or CC-BY-SA (source)

© sfoskett for Stephen Foskett, Pack Rat, 2008. | SSD: So Close and Yet So Far
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Now a handful of storage players are talking about SSD’s again, including some respectable names like LSI and Xiotech and some lesser-known outfits like Solid Data Systems and Texas Memory Systems. IDC is predicting mainstream uptake of the technology, too, but note that they’re mostly talking about the PC market, not enterprise storage.

I say that if SSD ever gains footing in the enterprise, it’ll most likely be in a virtualized hybrid system, acting like a mega-cache. Or maybe a non-RAM permacache, if you will! Of course, we’ve seen just how much uptake that feature has had, eh EMC? But hey, it could happen. Even though Apple is soaking up more than a quarter of the world’s NAND flash this year, maybe they’ll force prices down. Or maybe Samsung’s or Hynix’s PRAM will conquer the world!

Let’s get serious, folks. What good is SSD? It improves random access read performance over disk at the expense of longevity, throughput, and, well, expense! ReadyBoost looked like a nice application for flash memory, since the potential content was bounded and could fit on a flash drive, but it seems to have done precisely nothing for performance.

If enterprise applications could benefit from better random access performance, we’d be putting large amounts of memory in front of the disks already. Oh, wait, that’s right, we already do that! Modern enterprise arrays have gobs of cache, more than any SSD, and use it quite effectively. Remember the old RAID-5 penalty?

SSD just serves to remind me of one of those performance-tuning axioms I learned long ago: It’s always better to let an intelligently-designed system manage itself than try to second-guess it. This applies to all areas of system performance tuning, from filesystem and LUN layouts to cache tuning. And this is why permacache and things like it never caught on.

And it’s why SSD will continue to play just a bit part in the enterprise until it’s just as cheap as disk. Like that’ll happen anytime soon.

Edit: Looks like I got the name wrong – the EMC SSD was the Orion.  Introduced in 1989, this evolved into the Symmetrix in 1992.  I used one of these back at Texaco in the 1990′s.

© sfoskett for Stephen Foskett, Pack Rat, 2007. | Wherefore Art Thou, Solid State Disks?
This post was categorized as Apple, Computer History, Enterprise storage. Each of my categories has its own feed if you'd like to filter out or focus on posts like this.