SSD Tiering versus Caching: Part 2

Posted on August 14, 2011 by tekinerd

A while back I wrote about some of the differences between caching and tiering when using solid state disk (SSD) drives in a PC or server.

Having just returned from the 2011 Flash Memory Summit in Santa Clara, I feel compelled to add some additional color around the topic given the level of confusion clearly evident at the show. Also I’d like to blatantly plug an upcoming evolution in tiering, called MicroTiering from our own company, Enmotus which emerged from stealth at the show.

The simplest high level clarification that emerged from the show I’m glad to say matched what we described in our earlier blog (SSD Caching versus Tiering): caching makes a copy of frequently accessed data from a hard drive and places it in the SSD for future reads, whereas tiering moves the data permanently to the SSD and it’s no longer stored on the hard drive. Caching speeds up reads only at this point with a modified caching algorithm to account for SSD behavior versus RAM based schemes, whereas tiering simply maps the host reads and writes to appropriate storage tier with no additional processing overhead. So in teiring, you get the write advantage and of lesser benefit, the incremental capacity of the SSD which becomes available to the host as usable storage (minus some minor overheads to keep track of the mapping tables).

Why the confusion? One RAID vendor in particular, along with several caching companies, are calling their direct attached storage (or DAS) caching solution “tiering”, even though they are only caching the data to speed up reads and data isn’t moved. Sure write based caching is coming, but it’s still fundamentally a copy of the data that is on the hard drive not a move and SSD caching algorithms apply.

Where Caching is Deployed

SSD caching has a strong and viable place in the world of storage and computing at many levels so it’s not a case of tiering versus caching, but more when to use either or both. Also, caching is relatively inexpensive and will most likely end up bundling for free in PC desktop applications with the SSD you are purchasing for Windows applications for example, simply because this is how all caching ends up i.e. “free” with some piece of hardware, an SSD in this case. Case in point is Intel and Matrix RAID, which has now been enhanced with it’s own caching scheme called Smart Response Technology (SRT) currently available for Z68 flavor motherboards and systems.

In the broader sense, we are now seeing SSD caching deployed in a number of environments:

  • Desktops (eventually notebooks with both SSD and hard drives) bundled with SSDs or as standalone software e.g. Intel SRT and Nvelo (typically Windows only)
  • Server host software based caching e.g. FusionIO, IOturbine, Velobit (Windows and VMware)
  • Hardware PCIe adapter based server RAID SSD caching e.g. LSI’s CacheCade (most operating systems)
  • SAN based SSD caching software, appliances or modules within disk arrays e.g. Oracle’s ZFS caching schemes (disk arrays) or specialist appliances that transparently cache data into SSDs in the SAN network.

Where Data Tiering is Deployed

Tiering is still fundamentally a shared SAN based storage technology used in large data sets. In its current form, it’s really an automated way to move data to and from slow, inexpensive bulk storage (e.g. SATA drives, possibly even tape drives) to fast, expensive storage based on its frequency of access or “demand”. Why? So data managers can keep expensive storage costs to a minimum by taking advantage of the fact that typically less than 20% of data is being accessed over any specific period of time. Youtube is a perfect example. You don’t want to store a newly uploaded video and keep it stored on a large SSD disk array just in case it becomes highly popular versus the other numerous uploads. Tiering automatically identifies that the file (or more correctly a file’s assocatied low level storage ‘blocks’) is starting to increase in popularity, and moves it up to the fast storage for you automatically. Once on the higher performance storage, it can handle a significantly higher level of hits without causing excessive end user delays and the infamous video box ‘spinning wheel’. Once it dies down, it moves it back making way for other content that may be on the popularity rise.

Tiering Operates Like A Human Brain

The thing I like about teiring is that it’s more like how we think as humans i.e. pattern recognition over a large data set, with an almost automated and instant response to a trend rather than looking at independent and much smaller slices of data as with caching. A tiering algorithm observes data access patterns on the fly and determines how often and more importantly, what type of access is going on and adapts accordingly. For example, it can determine if an access pattern is random or sequential and allocate storage to the right type of storage media based on it’s characteristics. A great “big iron” example solution is EMC’s FAST, or the now defunct Atrato.

Tiering can also scale better to multiple levels of storage types. Whereas caching is limited to either RAM, single SSDs or tied to a RAID adapter, tiering can operating on multiple tiers of storage from a much broader set up to and including cloud storage (i.e. a very slow tier) for example.

MicroTeiring

At the show, I introduced the term MicroTiering, one of the solutions our company Enmotus will be providing in the near future. MicroTiering is essentially a direct attach storage version of its SAN cousin but applied on the much smaller subset of storage that is inside the server itself. It’s essentially a hardware accelerated approach to teiring at DAS level that doesn’t tax the host CPU and facilitates a much broader set of operating systems and hypervisor support versus the narrow host SSD caching only offerings we see today that are confined to just a few environments.

Tiering and Caching Together

The two technologies are not mutually exclusive. In fact, it is more than likely that tiering and caching involving SSDs will be deployed together as they both provide different benefits. For example, caching tends to favor the less expensive MLC SSDs as the data is only copied and handles the highly read only transient or none critical data, so loss of the SSD cache itself is none critical. It’s also the easiest way to add a very fast, direct attached SSD cache to your sever provided your operating system or VM environment can handle it.

On the other hand, as tiering relocates the data to the SSD, SLC is preferable for it’s higher performance on reads and writes, higher resilience and data retention characteristics. In the case of DAS based tiering solutions like MicroTiering, it is expected that tiering may also be better suited to virtual machine environments and databases due to it’s inherent and simpler write advantage, low to zero host software layers and VMware’s tendencies to shift the read-write balance more toward 50/50.

What’s for sure, lots of innovation and exciting things still going on this space with lots more to come.

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Snapshot and Full Image Backup for Windows Home Server

Posted on June 5, 2011 by tekinerd

One of the short comings of the original 32 bit Windows Home Server (WHS) for me was the lack of any built in tools to backup the primary WHS boot drive. While using a RAID 1 boot drive would protect me from a disk drive crash, it didn’t offer the capability to “rewind” back to a former backup copy to fix a system drive corruption issue. To compound the problem, my favorite Windows disk image backup utility doesn’t support server based operating systems, presumably because they have a higher cost enterprise class version they sell into the classic server markets.

So for most of us, the problem remains on how to easily backup and restore the primary boot volume of the WHS server or upgrading the entire server hardware without losing the current WHS configuration. The good news is that backup and restore for a primary boot drive becomes significantly easier when you are running as a virtual machine. Better still, these come for free if implementing WHS on VMware’s ESXi hypervisor as illustrated in At Home with ESXi posted earlier.

Additional Backup Options Provided by a WHS ESXi Setup

With VMware ESXi, you automatically get two ways to create backups of your primary home server boot drive via the vSphere utility run from your regular PC:

  • Snapshot – takes a point in time copy of the complete WHS virtual machine on the same physical drive as the primary WHS boot image. Total time around 3 mins and take as many as you have room for on the disk.
  • Full Image Copy – the complete WHS virtual machine is copied via the network to your local PC or a network drive. Total time will be several hours depending on your network speed (70GBytes take a while to copy across home network)

Read more at System Drive Backup Options for Windows Home Server on VMware ESXi.

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Loving Personal Cloud Storage with SugarSync

Posted on January 26, 2011 by tekinerd

I love simple-to-use technology, being a great advocate of the KISS principle. I could writesugarsync_logo a thesis on how many times I’ve seen engineers and marketers create funky steps in products to do the most basic things, usually because not enough attention was paid pre-design to figuring out ahead of time the multiple ways different end users may use a product out of the box. 

So I was delighted when within minutes, I was using my first real cloud storage application and sync’ing up files on my laptop to a virtual storage drive at cloud storage provider sugarsync popupSugarSync. The first 5G is free so there’s plenty of room for an average user’s photos and also music files as a starter. My first step was selecting my non-personal files such as downloaded iTunes music file directory (warnings provided that this was for back purposes only) and photos. The easy to setup configuration utility started sync’ing my photo and music files almost instantly after I identified which directories to use, which took a few hours to get fully copied over via my cable modem connection. The SugarSync folks provide a nice little task bar popup menu and applet to access the online files and see where you’re sync is up to if you get curious. Once complete, it only copies over the changes, adds or deletions helping keeping the on-going traffic to an absolute minimum.

I then downloaded the iPhone SugarSync app. That’s when I got reallysugarsync_iphone excited. I could clearly see my computer’s files and more importantly, it was very simple to get to them and display the photos directly on my iPhone. What I liked even better was that I could select a file and send it to someone while out roaming about. For all intents and purposes, it was as though I was emailing it directly from my laptop. Very handy if you have specific documents that you may want to send someone instantly while travelling. Adding another PC was also easy. From the SugarSync web site, after logging in you can download the sync application to connect the new PC into the cloud storage based drive I’d created for myself earlier, and add new share areas from that PC also.

Viewing which files were up on the cloud drive was just a matter of clicking on the task bar applet installed (pop up menu shown above) and selecting the View Files in SugarSync File Manager option which opens up their file manager like application. From there, it was pretty simple to see what you had up on the drive as well as select files to share or send to others if required.

sugarsync screen

Alternatively, you can get to a similar view by visiting the web site and logging in from any PC. Nice if you need to use someone else’s computer to get to your data.

So far, I really do recommend the SugarSync approach for those of you looking for a simple file backup and sharing mechanism for photos and music. The free 5G certainly helps draw you in, especially as they don’t ask you for your credit card details for that “just in case you go over” situation – a nice change. I’m not quite there on storing more personal or sensitive data as I’m still in the wait-and-see-mode with respect to trusting that my data is stored securely. We just don’t have enough history to know how secure this type of approach is yet but hopefully time will demonstrate this.

That said, nice job SugarSync on a great and easy-to-use product!

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Will Traditional Disk Array Vendors Survive?

Posted on December 10, 2010 by tekinerd

The more you look at how SAN disk array systems are evolving, the more you start to wonder how traditional disk array systems vendors can survive long term without a significant change in their business model. What’s been a business primarily focused on building solid high-availability RAID arrays is now facing a serious commoditization challenge as standard Intel compatible server hardware is adapted to storage device applications. Furthermore, the latest or pending acquisitions of 3PAR, Isilon and now Compellent, puts significant pressure on the last few independent disk array vendors to start beefing up their virtualization or unified storage play.

There are a few industry trends that should raise concerns for traditional players:

  • There are fewer margin dollars available for incremental R&D for pure RAID disk arrays, not helped by the continuing drop in disk drive prices
  • Standard Intel compatible off the shelf CPUs, memory, chipsets and RAID components developed for higher volume servers and workstations are now being utilized in storage platforms running standard operating systems, or open storage software Linux or ZFS based  (e.g. OpenFiler)
  • We continue to see new hybrid storage-server hardware product offerings from white box equipment builders, including dual controller SBB solutions, that facilitate easier disk array and storage virtualization integration
  • SSD device performance increases are leading to newer approaches to storage systems that may not lend themselves to traditional disk array controllers and slow speed SANs
  • We are seeing many storage management features and thin provisioning making their way into server hypervisors, in many cases as “free” standard components
  • Large scale data intensive environments, such as those used by search engines, are favoring non-RAID, non-SAN architectures (e.g. Hadoop) utilizing low cost networked PC technology and Linux (JBOS – just a bunch of servers)

Does this spell the end for traditional RAID array guys? If you read many of the press around RAID today, then the outlook does not look good. However, in the short term there is still business to be had. The good news is that the storage equipment market, along with infrastructure upgrades in traditional corporate markets, takes many years to transition to new technologies. You just cannot switch out storage when something new comes along. It’s like trying to upgrade the train carriages while the train is moving; not practical, dangerous and will pretty much upset customers if you try to do it. You have to migrate non-disruptively to any new storage system to avoid significant service or application disruption. But this will not last forever.

They literally have to starting thinking outside the box. What is clear is that these traditional disk array vendors really need to start taking note of the highly successful Dell EqualLogic effect i.e. storage that is scalable at network system level, simple and cost effective from an operating cost standpoint as opposed to the device centric model they currently use.  The market has certainly rewarded this virtualized, scalable networked approach in spades given Dell’s reported revenue successes with this product family over the past few years, even though  it is more costly on a dollar per Gigabyte basis.

Some pure play disk array vendors or divisions such as LSI, Dot Hill and Xyratex (who OEM their products to larger companies such as Dell, HP and IBM) are already migrating upward by acquiring or building storage virtualization appliance companies, but it will take time for them to adjust to the new world as it takes a significant shift in the types of engineers to develop network level, virtualized storage. Then there is the issue of doubling investment for a while as you sustain one and develop another approach. Selling styles also change as the system level knowledge needs to target a different type of buyer in many cases. In their favor, they have the robustness, service and know-how advantage i.e. they know how to make, sell and support storage extremely well.

It wouldn’t be the first time an industry had to reinvent itself in order to survive and it will be interesting to see how it pans out over the next several years, and most importantly, who survives until the next round of commoditization.

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SSD Caching versus Tiering

Posted on November 8, 2010 by tekinerd

In some recent discussions, I sensed there is some confusion around solid state device (SSD) storage used as a storage tier vs, a cache. While there are some similarities and both are intended to achieve the same end result i.e. acceleration of data accesses from slower storage, there are some definite differences which I thought I’d try to clarify here. This is from my working viewpoint here, so please do post comments if  you feel differently.

Firstly, SSD caching is temporary storage of data in an SSD cache whereas true data tiering is classed as a semi-permanent movement of data to or from an SSD storage tier. Both are based on algorithms or policies that ultimately result in data being copied to, or removed from, SSDs. To clarify further, if you were to unplug or remove your SSDs, for the caching case, the user data is still stored in the primary storage behind the SSD cache and is still from the original source (albeit slower) whereas in a data tier environment, the user data (and capacity) is no longer available if the SSD tier were removed as the data was physically moved to the SSDs and most likely removed from the original storage tier.

Another subtle difference between caching and teiring is if the SSD capacity is visible or not. In a cached mode, the SSD capacity is totally invisible i.e. the end application simply sees the data accessed much faster if it has been previously accessed and is still in cache store (i.e. a cache hit). So if a 100G SSD cache exists in a system with say 4TB of hard disk drive (HDD) storage, the total capacity is still only 4TB i.e. that of the hard disk array, with 100% of the data always existing on the 4TB with copies only of the data in the SSD cache based on the caching algorithm used. In a true data tiering setup using SSDs, the total storage is 4.1TB and though this may be presented to a host computer as one large virtual storage device, part of the data exists on the SSD and the remainder on the hard disk storage. Typically, such small amounts of SSD would not be implemented as a dedicated tier, but you get the idea if say 1TB of SSD storage was being used in a storage area network system of 400TB of hard drive based storage creating 401TB of usable capacity.

So how does data make it into a cache versus a tier? Cache and block level automated data tiering controllers monitor and operate on statistics gathered from the stream of storage commands and in particular the addresses of the storage blocks being accessed.

SSD Caching Simplified

Caching models typically employ a lookup table method based on the block level address (or range of blocks) to establish if the data the host is requesting has been accessed before and potentially exists in the SSD cache. Data is typically moved more quickly into an SSD cache versus say tiering where more analysis of the longer term trend is typically employed which can span hours if not days in some cases. Unlike DRAM based caches however where it is possible to cache all reads, a little more care and time is taken with SSDs to ensure that excessive writing to the cache is avoided given the finite number of writes an SSD can tolerate. Most engines use some form of “hot-spot” detection algorithm to identify frequently accessed regions of storage and move data into the cache area once it has been established there is a definite frequent access trend.

Traditional caching involves one of several classic caching algorithms which result in either read-only or read and write caching. Cache algorithms and approaches vary by vendor and dictate how a read from the HDD storage results in a copy of the original data entering the cache table and how long it ”lives” in the cache itself. Subsequent reads to that same data who’s original location was on the hard drive can now be sent from the SSD cache instead of the slower HDD i.e. a cache hit (determined using a address lookup in the cache tables). If this is the first time data is being accessed from a specific location on the hard drive(s), then the data must first be accessed from the slower drives and a copy made in the SSD cache if the hot spot checking algorithms deems necessary (triggered by the cache miss).

Caching algorithms often try to use more sophisticated models to pre-fetch data based on a trend and store it in the cache if it thinks there a high probability it may be accessed soon e.g. in sequential video streaming or VMware virtual machine migrations where it is beneficial to cache data from the next sequential addresses and pull them into the cache at the same time as the initial access. After some period of time or when new data needs to displace older or stale data in the cache, a cache flush cleans out the old data. This may also be triggered by the hot spot detection logic determining that the data is now “cold”.

The measure of a good cache is how many hits it gets versus misses. If data is very random and scattered over the entire addressable range of storage with infrequent accesses back to the same locations, then the effectiveness is significantly lower and sometimes detrimental to overall performance as there is an overhead in attempting to locate data in the cache on every data access.

SSD Auto Tiering Basics

An automated data tiering controller treats the SSD and HDDs as two separate physical islands of storage, even if presented to the host application (and hence the user) as one large contiguous storage pool (a virtual disk). A statistics gathering or scanning engine collects data over time and looks for data access patterns and trends that match a pre-defined set of policies or conditions. These engines use a mix of algorithms and rules that indicate how and when a particular block (or group of blocks) of storage is to be migrated or moved.

The simplest “caching like” approach used by a data tiering controller is based on frequency of access. For example, it may monitor data blocks being accessed from the hard drives and if it passes a pre-defined number of accesses per hour “N” for a period of time “T”, then a rule may be employed that says when N>1000 AND T>60 minutes, move the data up to the next logical tier. So if data being accessed a lot from the hard drive and there are only two tiered defined, SSD being the faster of the two, the data will be copied to the SSD tier (i.e. promoted) and the virtual address map that converts real time host addresses to the physical updated to point data to the new location in SSD storage. All of this of course happens behind a virtual interface to the host itself who has no idea the storage just moved to a new physical location. Depending on the tiering algorithm and vendor, the data may be discarded on the old tier to free up capacity. The converse is also true. If data is infrequently accessed and lives on the SSD tier, it may be demoted to the HDD tier based on similar algorithms.

More sophisticated tiering models exist of course, some that work at file layers and look at the specific data or file metadata to make more intelligent decisions about what to do with data.

Where is SSD Caching or Tiering Applied?

Typically, SSD caching is implemented as a single SATA or PCIe flash storage device along with an operating system driver layer software in a direct attached storage (DAS) environment to speed up Windows or other operating system accesses. In much larger data center storage area networks (SAN) and cloud server-storage environments, there are an increasing number of dedicated rackmount SSD storage units that can act as a transparent cache at LUN level where the caching is all done in the storage area network layer, again invisible to the host computer. The benefit of cache based systems are that they can be added transparently and often non-disruptively (other than the initial install). Unlike with tiering, there is no need to setup dedicated pools or tiers of storage i.e. they can be overlaid on top of an existing storage setup.

Tiering is more often found in larger storage area network based environments with several disk array and storage appliance vendors offering the capability to tier between different disk arrays based on their media type or configuration. Larger tiered systems often also use other backup storage media such as tape or virtual tape systems. Automated tiering can substantially reduce the management overhead associated with backup and archival of large amounts of data by fully automating the movement process, or helping meet data accessibility requirements of government regulations. In many cases, it is possible to tier data transparently between different media types within the same physical disk array e.g. a few SSD drives in RAID 1 or 10, 4-6 SAS drives in a RAID 10 and 6-12 SATA drives in a RAID  i.e. 3 distinct tiers of storage. Distributed or virtualizaed storage environments also offer either manual or automated tiering mechanisms that work within their proprietary environments. At the other end of the spectrum, file volume manager and storage virtualization solutions running on the host or in a dedicated appliance can allow IT managers to organize existing disk array devices of different types and vendors and sort them into tiers. This is typically a process that requires a reasonable amount of planning and often disruption, but can yield tremendous benefits once deployed.

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Cloudy Server Growth

Posted on October 6, 2010 by tekinerd

Infoworld recently posted an interesting article (The numbers don’t lie — cloud computing boosts server sales) about how server market revenues have increased by around 11 percent in the second quarter of 2010, with quarterly revenues coming in around $10.9 billion for the quarter. In fact, if you look at IDC’s 2Q10 press release, unit shipments were up 23.8% year over year which is impressive, reportedly the fastest quarter over quarter growth in 5 years. Much of this can certainly be attributed to recovery of the server market segment, but as David Linthicum discusses, shouldn’t overall server growth start declining in the face of server virtualization and cloud computing? Hm… good point.

In many ways, growth in the face of a technology shift caused by more efficient cloud and utility based computing models should not be that surprising given we’ve seen a similar phenomenon before. We lived through it in the early broadband communications days where the concept of an overlay network was used to introduce new services non-disruptively i.e. rather than replace the old network, a new one was built along side hence the term “overlay”. So it is not an unlikely scenario in the cloud age where both corporate and public computer-storage networks are effectively overlaying new systems on top of their existing systems to minimize disruptions and in many cases, testing this stuff out incrementally before they turn it all over to cloud and virtualization – be it internal or externally hosted.

This definitely seemed to be the case when I recently called my hosting provider to ask why FrontPage extensions could no longer be enabled for a website I was putting together as a temporary placeholder for another project. Turned out that all new customers were being steered to the new “grid computer system” which doesn’t support FrontPage extensions anymore, so I had to be moved back to the “legacy” systems. Bingo…. two networks, one for old and one for new. Reminded me of a tour of a central office in Austin, Texas we were given when developing the early broadband network technologies. We were there to review how they planned to roll out DSL when it eventually arrived. I was surprised then to see a lone Siemens digital switch in the corner providing ISDN services purely for data as an overlay service instead of using the already huge installed base of AT&T switches they’d been using for mainline customers and installing ISDN line cards. The technology and switch could handle it, but for service and disruption reasons it was not the preferred way to roll out an infant service.

Bottom line. Traditional models always stick around much longer than us technologists think for usually non-technical reasons, even when there are many advantages to ditching the old stuff for the new. Another reminder of the hype curve age we live in.

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Windows Home Server to the Rescue

Posted on October 6, 2010 by tekinerd

I have now been running WHS since Dec 2007 on a small self-built VIA based ITX system, chosen because it was small and ran on much lower power and generated less heat and noise than a conventional PC or low cost server allowing me to leave it on all the time. It’s worked like a charm since it was turned on in 2007 and not crashed once! Since then I’ve added a second WHS using the VIA Artigo shoebox PC which I use for projects like the Earthquake monitoring project referred to on this site a few months back.

We have a pretty active PC home. I have gamer kids (I’m one of them) in the house with 3 dedicated gaming PCs mixed in with 3 laptops (2 college, 1 work), a digital audio workstation for audio and midi recording, plus a home built media center PC in the family room. Operating systems are a mix of Windows XP, Vista 32 bit, Vista 64 bit and Windows 7. All of the PCs have the WHS Connector software, though only 2 of them (my home desktop and home laptop) wake up automatically to do backup all the time as I found that the laptop would wake up in my hotel room looking for my WHS when travelling! The rest, I tend to run manual backups or turn on the automated backup only when I know the work laptop is going to be stationary for a while or I have a lot of new content I’m creating.

Having lost data from my pre-WHS writing days, I’d already developed a healthy habit of making sure I had multiple copies of important data (e.g. photos, audio/midi projects) in multiple locations so I’ve managed to avoid catastrophic loss of personal data with a little careful management so far. WHS really helped automate and simplify this previously manual process. On occasion however, there are situations where a recovery of a complete PC or file is still necessary beyond the normal “copy off your backup USB drive” scenario.

(Read the full article on MSWHS.COM here.)

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Intel Patsburg and Software RAID

Posted on September 16, 2010 by tekinerd

I just got done reading the “Intel eats crow on software RAID” writeup from the The UK Register. On one side I’m really happy to see that server based software RAID (or Virtual RAID Adapters, VRAs, as we called them at Ciprico and DotHill) coming into the spotlight again. Performance, especially now with SSD usage on the rise, is definitely one of the strengths of a software RAID solution which has the ability to scale to much faster rates than a hardware RAID adapter in terms of raw IOPs or MB/s. After all, it’s using the power of a 2-3GHz multi-core Intel or AMD CPU coupled to a very fast memory and I/O bus, versus some fixed function, 800M-1.2GHz embedded RAID CPU hanging off the PCIe bus.

On the other hand, asking if software RAID is faster than or can replace hardware RAID is not really the right question to be asking here. Sure, software RAID with persistent storage like SSDs is changing the landscape as far as making a pure host based software RAID viable, but for traditional hard disk drives not much has changed. There’s a lot of volatile elements (i.e. gone if the lights go out) type storage stages used all the way from the application that wrote the data, through the storage IO device (be it a hardware accelerated RAID adapter or simple IO device), through the 32+MBytes of cache on the drive if you left it enabled, until you actually arrive at the persistent media storage platter. Oh, and then there is VMware ESX which can’t support a conventional software RAID stack yet.

So let’s get some perspective here.

First, as any good RAID vendor will tell you, it’s not so much about software vs hardware RAID, it’s about who is providing your RAID stack, how many “RAID bytes served so far”, how good service and support is and essentially how much you trust the vendor offering the software RAID stack. This is where a RAID stack’s “age” and pedigree is important regardless of its implementation. Being a good software RAID provider goes well beyond making it fast. It’s how robust your solution is and also how great your support is when things don’t work right and you need help fixing it. Hard disks (and SSDs are no exception) throw all sorts of curve balls at you and only the robustness of your RAID vendor’s test and compatibility labs can really filter a lot of this out. It often takes a knowledgeable RAID systems engineer to figure out that it either was, or was not, the fault of the RAID stack in the first place. My deepest respect is for those folks that have to spend their Sundays way into the wee hours of the morning figuring these sort of things out when the fault defies conventional logic.

Second, on the technology side, RAID is always implemented in software in IT application regardless if host or hardware based. It either runs on the host CPU (software, chipset or host RAID) or on a dedicated CPU on the RAID adapter (hardware RAID), sometimes in host software with some assistance from the hardware (e.g. XOR calculations). Granted, one runs in an unpredictable OS environment and the other in a more closed and predictable embedded one, but they end up doing the same thing in software on different CPUs. While there are cases where software RAID may be sufficient and more affordable as it eliminates much of the hardware cost, there are probably just as many cases where it just doesn’t work well at all. Case in point being VMware ESX (see earlier post on this topic here) where there are no commercially available, bootable software RAID solutions available, plus there are less general CPU cycles available anyhow. So hardware RAID tends to win out here. Also, software RAID doesn’t protect your data fully from a system power loss unless you are protecting the whole server with a dedicated UPS which can do an orderly shutdown of the system in the event of a plant power loss. Then there are the video editing crowd that maybe use their host CPUs for video compression, another case where software RAID often fails due to lack of enough available CPU cycles.

So, the key questions to be asking about software RAID in my mind are not how fast it can go, but:

  • How robust is the RAID stack in question i.e. how many “bytes were served” before you got to it, who else is using it a mission critical environment?
  • How would my business be impacted by a server power loss running software RAID? Can I live with a UPS to protect the whole server as I have a fast means of getting back to a fully operational level?
  • Who’s going to support it when it goes wrong and how good is this support when it comes to knowing both the RAID stack strengths and limitations?
  • Are you comfortable buying a RAID solution a chip vendor or storage vendor, the latter who makes their livelihood from creating highly robust disk array systems? You may be perfectly ok with the former.

All of these will depend on just how important your data is and more importantly, how quickly you can restore the system to full operation in the event of a hardware failure.

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