Will Hybrid drives conquer enterprise storage?

Toyota Hybrid Synergy Drive Decal: RAC Future Car Challenge by Dominic's pics (cc) (from Flickr)
Toyota Hybrid Synergy Drive Decal: RAC Future Car Challenge by Dominic's pics (cc) (from Flickr)

I saw where Seagate announced the next generation of their Momentus XT Hybrid (SSD & Disk) drive this week.  We haven’t discussed Hybrid drives much on this blog but it has become a viable product family.

I am not planning on describing the new drive specs here as there was an excellent review by Greg Schulz at StorageIOblog.

However, the question some in the storage industry have had is can Hybrid drives supplant data center storage.  I believe the answer to that is no and I will tell you why.

Hybrid drive secrets

The secret to Seagate’s Hybrid drive lies in its FAST technology.  It provides a sort of automated disk caching that moves frequently accessed OS or boot data to NAND/SSD providing quicker access times.

Storage subsystem caching logic has been around in storage subsystems for decade’s now, ever since the IBM 3880 Mod 11&13 storage control systems came out last century.  However, these algorithms have gotten much more sophisticated over time and today can make a significant difference in storage system performance.  This can be easily witnessed by the wide variance in storage system performance on a per disk drive basis (e.g., see my post on Latest SPC-2 results – chart of the month).

Enterprise storage use of Hybrid drives?

The problem with using Hybrid drives in enterprise storage is that caching algorithms are based on some predictability of access/reference patterns.  When you have a Hybrid drive directly connected to a server or a PC it can view a significant portion of server IO (at least to the boot/OS volume) but more importantly, that boot/OS data is statically allocated, i.e., doesn’t move around all that much.   This means that one PC session looks pretty much like the next PC session and as such, the hybrid drive can learn an awful lot about the next IO session just by remembering the last one.

However, enterprise storage IO changes significantly from one storage session (day?) to another.  Not only are the end-user generated database transactions moving around the data, but the data itself is much more dynamically allocated, i.e., moves around a lot.

Backend data movement is especially true for automated storage tiering used in subsystems that contain both SSDs and disk drives. But it’s also true in systems that map data placement using log structured file systems.  NetApp Write Anywhere File Layout (WAFL) being a prominent user of this approach but other storage systems do this as well.

In addition, any fixed, permanent mapping of a user data block to a physical disk location is becoming less useful over time as advanced storage features make dynamic or virtualized mapping a necessity.  Just consider snapshots based on copy-on-write technology, all it takes is a write to have a snapshot block be moved to a different location.

Nonetheless, the main problem is that all the smarts about what is happening to data on backend storage primarily lies at the controller level not at the drive level.  This not only applies to data mapping but also end-user/application data access, as cache hits are never even seen by a drive.  As such, Hybrid drives alone don’t make much sense in enterprise storage.

Maybe, if they were intricately tied to the subsystem

I guess one way this could all work better is if the Hybrid drive caching logic were somehow controlled by the storage subsystem.  In this way, the controller could provide hints as to which disk blocks to move into NAND.  Perhaps this is a way to distribute storage tiering activity to the backend devices, without the subsystem having to do any of the heavy lifting, i.e., the hybrid drives would do all the data movement under the guidance of the controller.

I don’t think this likely because it would take industry standardization to define any new “hint” commands and they would be specific to Hybrid drives.  Barring standards, it’s an interface between one storage vendor and one drive vendor.  Probably ok if you made both storage subsystem and hybrid drives but there aren’t any vendor’s left that does both drives and the storage controllers.

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So, given the state of enterprise storage today and its continuing proclivity to move data around accross its backend storage,  I believe Hybrid drives won’t be used in enterprise storage anytime soon.

Comments?

 

SSD news roundup

The NexGen n5 Storage System (c) 2011 NexGen Storage, All Rights Reserved
The NexGen n5 Storage System (c) 2011 NexGen Storage, All Rights Reserved

NexGen comes out of stealth

NexGen Storage a local storage company came out of stealth today and is also generally available.  Their storage system has been in beta since April 2011 and is in use by a number of customers today.

Their product uses DRAM caching, PCIe NAND flash, and nearline SAS drives to provide guaranteed QoS for LUN I/O.  The system can provision IOP rate, bandwidth and (possibly) latency over a set of configured LUNs.    Such provisioning can change using policy management on a time basis to support time-based tiering. Also, one can prioritize how important the QoS is for a LUN so that it could be guaranteed or could be sacrificed to support performance for other storage system LUNs.

The NexGen storage provides a multi-tiered hybrid storage system that supports 10GBE iSCSI, and uses MLC NAND PCIe card  to boost performance for SAS nearline drives.  NexGen also supports data deduplication which is done during off-peak times to reduce data footprint.

DRAM replacing Disk!?

In a report by ARS Technica, a research group out of Stanford is attempting to gang together server DRAM to create a networked storage system.  There have been a number of attempts to use DRAM as a storage system in the past but the Stanford group is going after it in a different way by aggregating together DRAM across a gaggle of servers.  They are using standard disks or SSDs for backup purposes because DRAM is, of course, a volatile storage device but the intent is to keep all in memory to speed up performance.

I was at SNW USA a couple of weeks ago talking to a Taiwanese company that was offering a DRAM storage accelerator device which also used DRAM as a storage service. Of course, Texas Memory Systems and others have had DRAM based storage for a while now. The cost for such devices was always pretty high but the performance was commensurate.

In contrast, the Stanford group is trying to use commodity hardware (servers) with copious amounts of DRAM, to create a storage system.  The article seems to imply that the system could take advantage of unused DRAM, sitting around your server farm. But, I find it hard to believe that.  Most virtualized server environments today are running lean on memory and there shouldn’t be a lot of excess DRAM capacity hanging around.

The other achilles heel of the Stanford DRAM storage is that it is highly dependent on low latency networking.  Although Infiniband probably qualifies as low latency, it’s not low latency enough to support this systems IO workloads. As such, they believe they need even lower latency networking than Infiniband to make it work well.

OCZ ups the IOP rate on their RevoDrive3 Max series PCIe NAND storage

Speaking of PCIe NAND flash, OCZ just announced speedier storage, upping the random read IO rates up to 245K from the 230K IOPS offered in their previous PCIe NAND storage.  Unclear what they did to boost this but, it’s entirely possible that they have optimized their NAND controller to support more random reads.

OCZ announces they will ship TLC SSD storage in 2012

OCZ’s been busy.  Now that the enterprise is moving to adopt MLC and eMLC SSD storage, it seems time to introduce TLC (3-bits/cell) SSDs.  With TLC, the price should come down a bit more (see chart in article), but the endurance should also suffer significantly.  I suppose with the capacities available with TLC and enough over provisioning OCZ can make a storage device that would be reliable enough for certain applications at a more reasonable cost.

I never thought I would see MLC in enterprise storage so, I suppose at some point even TLC makes sense, but I would be even more hesitant to jump on this bandwagon for awhile yet.

Solid Fire obtains more funding

Early last week Solid Fire, another local SSD startup obtained $25M in additional funding.  Solid Fire, an all SSD storage system company,  is still technically in beta but expect general availability near the end of the year.   We haven’t talked about them before in RayOnStorage but they are focusing on cloud service providers with an all SSD solution which includes deduplication.  I promise to talk about them some more when they reach GA.

LaCIE introduces a Little Big Disk, a Thunderbolt SSD

Finally, in the highend consumer space, LaCie just released a new SSD which attaches to servers/desktops using the new Apple-Intel Thunderbolt IO interface.  Given the expense (~$900) for 128GB SSD, it seems a bit much but if you absolutely have to have the performance this may be the only way to go.

 

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Well that’s about all I could find on SSD and DRAM storage announcements. However, I am sure I missed a couple so if you know one I should have mentioned please comment.

OCZ’s new Octane SATA SSD pushes latency limits below 100μsec

(c) 2011 OCZ (from their website)OCZ just announced that their new Octane 1TB SSD can perform reads and writes under a 100 μsec. (specifically “Read: 0.06ms; Write: 0.09ms”).  Such fast access times boggle the imagination and even with SATA 3 seems almost unobtainable.

Speed matters, especially with SSDs

Why would any device try to reach a 90μsec write access time and a 60μsec read access time? With the advent of high-speed stock trading where even distance matters, a lot, latency is becoming a hot topic once again.

Although from my perspective it never really went away (see my Storage throughput vs. IO response time and why it matters post).  So access times measured in 10’s of μsec. is just fine by me.

How SSD access time translates into storage system latency or response time is another matter.  But one can see some seriously fast storage system latencies (or LRT) in TMS’s latest RAMSAN SPC-1 benchmark results, under ~90μsec measured at the host level! (See my May dispatch on latest SPC performance).  On the other hand, how they measure 90μsec host level latencies without a logic analyzer attached is beyond me.

How are they doing this?

How can a OCZ’s SATA SSD deliver such fast access times? NAND is too slow to provide this access time for writes so there must be some magic.  For instance, NAND writes (programing) can take on the order of a couple of 100μsecs and that doesn’t include the erase time of more like 1/2msec.  So the only way to support a 90μsec write or storage system access time with NAND chips is by buffering write data into an “ondevice” DRAM cache.

NAND reads are quite a bit faster on the order of 25μsec for the first byte and 25nsec for each byte after that.  As such, SSD read data could conceivably be coming directly from NAND.  However you have to set aside some device latency/access time to perform IO command processing, chip addressing, channel setup, etc.  Thus, it wouldn’t surprise me to see them using the DRAM cache for read data as well.

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I never thought I would see sub-1msec storage system response times but that was broken a couple of years ago with IBM’s Turbo 8300.   With the advent of DRAM caching for NAND SSDs and the new,  purpose built all-SSD storage systems, it seems we are already in the age of sub-100μsec response times.

I fear to get much below this we may need something like the next generation SATA or SAS to come out and even faster processing/memory speeds. But from where I sit sub-10μsec response times don’t seem that far away.  By then, distance will matter even more.

Comments?

Graphene Flash Memory

Model of graphene structure by CORE-Materials (cc) (from Flickr)
Model of graphene structure by CORE-Materials (cc) (from Flickr)

I have been thinking about writing a post on “Is Flash Dead?” for a while now.  Well at least since talking with IBM research a couple of weeks ago on their new memory technologies that they have been working on.

But then this new Technology Review article came out  discussing recent research on Graphene Flash Memory.

Problems with NAND Flash

As we have discussed before, NAND flash memory has some serious limitations as it’s shrunk below 11nm or so. For instance, write endurance plummets, memory retention times are reduced and cell-to-cell interactions increase significantly.

These issues are not that much of a problem with today’s flash at 20nm or so. But to continue to follow Moore’s law and drop the price of NAND flash on a $/Gb basis, it will need to shrink below 16nm.  At that point or soon thereafter, current NAND flash technology will no longer be viable.

Other non-NAND based non-volatile memories

That’s why IBM and others are working on different types of non-volatile storage such as PCM (phase change memory), MRAM (magnetic RAM) , FeRAM (Ferroelectric RAM) and others.  All these have the potential to improve general reliability characteristics beyond where NAND Flash is today and where it will be tomorrow as chip geometries shrink even more.

IBM seems to be betting on MRAM or racetrack memory technology because it has near DRAM performance, extremely low power and can store far more data in the same amount of space. It sort of reminds me of delay line memory where bits were stored on a wire line and read out as they passed across a read/write circuit. Only in the case of racetrack memory, the delay line is etched in a silicon circuit indentation with the read/write head implemented at the bottom of the cleft.

Graphene as the solution

Then along comes Graphene based Flash Memory.  Graphene can apparently be used as a substitute for the storage layer in a flash memory cell.  According to the report, the graphene stores data using less power and with better stability over time.  Both crucial problems with NAND flash memory as it’s shrunk below today’s geometries.  The research is being done at UCLA and is supported by Samsung, a significant manufacturer of NAND flash memory today.

Current demonstration chips are much larger than would be useful.  However, given graphene’s material characteristics, the researchers believe there should be no problem scaling it down below where NAND Flash would start exhibiting problems.  The next iteration of research will be to see if their scaling assumptions can hold when device geometry is shrunk.

The other problem is getting graphene, a new material, into current chip production.  Current materials used in chip manufacturing lines are very tightly controlled and  building hybrid graphene devices to the same level of manufacturing tolerances and control will take some effort.

So don’t look for Graphene Flash Memory to show up anytime soon. But given that 16nm chip geometries are only a couple of years out and 11nm, a couple of years beyond that, it wouldn’t surprise me to see Graphene based Flash Memory introduced in about 4 years or so.  Then again, I am no materials expert, so don’t hold me to this timeline.

 

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Comments?

Pure Storage surfaces

1 controller X 1 storage shelf (c) 2011 Pure Storage (from their website)
1 controller X 1 storage shelf (c) 2011 Pure Storage (from their website)

We were talking with Pure Storage last week, another SSD startup which just emerged out of stealth mode today.  Somewhat like SolidFire which we discussed a month or so ago, Pure Storage uses only SSDs to provide primary storage.  In this case, they are supporting a FC front end, with an all SSDs backend, and implementing internal data deduplication and compression, to try to address the needs of enterprise tier 1 storage.

Pure Storage is in final beta testing with their product and plan to GA sometime around the end of the year.

Pure Storage hardware

Their system is built around MLC SSDs which are available from many vendors but with a strategic investment from Samsung, currently use that vendor’s storage.  As we know, MLC has write endurance limitations but Pure Storage was built from the ground up knowing they were going to use this technology and have built their IP to counteract these issues.

The system is available in one or two controller configurations, with an Infiniband interconnect between the controllers, 6Gbps SAS backend, 48GB of DRAM per controller for caching purposes, and NV-RAM for power outages.  Each controller has 12-cores supplied by 2-Intel Xeon processor chips.

With the first release they are limiting the controllers to one or two (HA option) but their storage system is capable of clustering together many more, maybe even up to 8-controllers using the Infiniband back end.

Each storage shelf provides 5.5TB of raw storage using 2.5″ 256GB MLC SSDs.  It looks like each controller can handle up to 2-storage shelfs with the HA (dual controller option) supporting 4 drive shelfs for up to 22TB of raw storage.

Pure Storage Performance

Although these numbers are not independently verified, the company says a single controller (with 1-storage shelf) they can do 200K sustained 4K random read IOPS, 2GB/sec bandwidth, 140K sustained write IOPS, or 500MB/s of write bandwidth.  A dual controller system (with 2-storage shelfs) can achieve 300K random read IOPS, 3GB/sec bandwidth, 180K write IOPS or 1GB/sec of write bandwidth.  They also claim that they can do all this IO with an under 1 msec. latency.

One of the things they pride themselves on is consistent performance.  They have built their storage such that they can deliver this consistent performance even under load conditions.

Given the amount of SSDs in their system this isn’t screaming performance but is certainly up there with many enterprise class systems sporting over 1000 disks.  The random write performance is not bad considering this is MLC.  On the other hand the sequential write bandwidth is probably their weakest spec and reflects their use of MLC flash.

Purity software

One key to Pure Storage (and SolidFire for that matter) is their use of inline data compression and deduplication. By using these techniques and basing their system storage on MLC, Pure Storage believes they can close the price gap between disk and SSD storage systems.

The problems with data reduction technologies is that not all environments can benefit from them and they both require lots of CPU power to perform well.  Pure Storage believes they have the horsepower (with 12 cores per controller) to support these services and are focusing their sales activities on those (VMware, Oracle, and SQL server) environments which have historically proven to be good candidates for data reduction.

In addition, they perform a lot of optimizations in their backend data layout to prolong the life of MLC storage. Specifically, they use a write chunk size that matches the underlying MLC SSDs page width so as not to waste endurance with partial data writes.  Also they migrate old data to new locations occasionally to maintain “data freshness” which can be a problem with MLC storage if the data is not touched often enough.  Probably other stuff as well, but essentially they are tuning their backend use to optimize endurance and performance of their SSD storage.

Furthermore, they have created a new RAID 3D scheme which provides an adaptive parity scheme based on the number of available drives that protects against any dual SSD failure.  They provide triple parity, dual parity for drive failures and another parity for unrecoverable bit errors within a data payload.  In most cases, a failed drive will not induce an immediate rebuild but rather a reconfiguration of data and parity to accommodate the failing drive and rebuild it onto new drives over time.

At the moment, they don’t have snapshots or data replication but they said these capabilities are on their roadmap for future delivery.

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In the mean time, all SSD storage systems seem to be coming out of the wood work. We mentioned SolidFire, but WhipTail is another one and I am sure there are plenty more in stealth waiting for the right moment to emerge.

I was at a conference about two months ago where I predicted that all SSD systems would be coming out with little of the engineering development of storage systems of yore. Based on the performance available from a single SSD, one wouldn’t need 100s of SSDs to generate 100K IOPS or more.  Pure Storage is doing this level of IO with only 22 MLC SSDs and a high-end, but essentially off-the-shelf controller.

Just imagine what one could do if you threw some custom hardware at it…

Comments?

SATA Express combines PCIe and SATA

SATA Express plug configuration (c) SATA-IO (from SATA-IO.org website)SATA-IO recently announced a new specification for an PCIe and SATA-IO specification (better described in the presentation) that will provide a SATA device interface directly connected to a server’s PCIe bus.

The new working specification offers either 8Gbps or 16Gbps depending on the number of PCIe lanes being used and provides a new PCIe/SATA-IO plug configuration.

While this may be a boon to normal SATA-IO disk drives I see the real advantage lies with an easier interface for PCIe based NAND storage cards or Hybrid disk drives.

New generation of PCIe SSDs based on SATA Express

For example, previously if you wanted to produce a PCIe NAND storage card, you either had to surround this with IO drivers to provide storage/cache interfaces (such as FusionIO) or provide enough smarts on the card to emulate an IO controller along with the backend storage device (see my post on OCZ’s new bootable PCIe Z-drive).  With the new SATA Express interface, one no longer needs to provide any additional smarts with the PCIe card as long as you can support SATA Express.

It would seem that SATA Express would be the best of all worlds.

  • If you wanted a directly accessed SATA SSD you could plug it in to your SATA-IO controller
  • If you wanted networked SATA SSDs you could plug it into your storage array.
  • If you wanted even better performance than either of those two alternatives you could plug the SATA SSD directly into the PCIe bus with the PCIe/SATA-IO interface.

Of course supporting SATA Express will take additional smarts on the part of any SATA-IO device but with all new SATA devices supporting the new interface, additional cost differentials should shrink substantially.

SATA-IO 3.2

The PCIe/SATA-IO plug design is just a concept now but SATA expects to have the specification finalized by year end with product availability near the end of 2012.  The SATA-IO organization have designated the SATA Express standard to be part of SATA 3.2.

One other new capability is being introduced with SATA 3.2, specifically a µSATA specification designed to provide storage for embedded system applications.

The prior generation SATA 3.1, coming out in products soon, includes the mSATA interface specification for mobile device storage and the USM SATA interface specification for consumer electronics storage.   And as most should recall, SATA 3.0 provided 6Gbps data transfer rates for SATA storage devices.

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Can “SAS Express” be far behind?

Comments?

OCZ’s latest Z-Drive R4 series PCIe SSD

OCZ_Z-Drive_RSeries (from http://www.ocztechnology.com/ocz-z-drive-r4-r-series-pci-express-ssd.html)
OCZ_Z-Drive_RSeries (from http://www.ocztechnology.com/ocz-z-drive-r4-r-series-pci-express-ssd.html)

OCZ just released a new version of their enterprise class Z-drive SSD storage with pretty impressive performance numbers (up to 500K IOPS [probably read] with 2.8GB/sec read data transfer).

Bootability

These new drives are bootable SCSI devices and connect directly to a server’s PCIe bus. They come in half height and full height card form factors and support 800GB to 3.2TB (full height) or 300GB to 1.2TB (half height) raw storage capacities.

OCZ also offers their Velo PCIe SSD series which are not bootable and as such, require an IO driver for each operating system. However, the Z-drive has more intelligence which provides a SCSI device and as such, can be used anywhere.

Naturally this comes at the price of additional hardware and overhead.   All of which could impact performance but given their specified IO rates, it doesn’t seem to be a problem.

Unclear how many other PCIe SSDs exist today that offer bootability but it certainly puts these drives in a different class than previous generation PCIe SSD such as available from FusionIO and other vendors that require IO drivers.

MLC NAND

One concern with new Z-drives might be their use of MLC NAND technology.  Although OCZ’s press release said the new drives would be available in either SLC or MLC configurations, current Z-drive spec sheets only indicate MLC availability.

As  discussed previously (see eMLC & eSLC and STEC’s MLC posts), MLC supports less write endurance (program-erase and write cycles) than SLC NAND cells.  Normally the difference is on the order of 10X less before NAND cell erase/write failure.

I also noticed there was no write endurance specification on their spec sheet for the new Z-drives.  Possibly,  at these capacities it may not matter but, in our view, a write endurance specification should be supplied for any SSD drive, and especially for enterprise class ones.

Z-drive series

OCZ offers two versions of their Z-drive the R and C series, both of which offer the same capacities and high performance but as far as I could tell the R series appears to be have more enterprise class availability and functionality. Specifically, this drive has power fail protection for the writes (capacitance power backup) as well as better SMART support (with “enterprise attributes”). These both seem to be missing from their C Series drives.

We hope the enterprise attribute SMART provides write endurance monitoring and reporting.  But there is no apparent definition of these attributes that were easily findable.

Also the R series power backup, called DataWrite Assurance Technology would be a necessary component for any enterprise disk device.  This essentially saves data written to the device but not to the NAND just yet from disappearing during a power outage/failure.

Given the above, we would certainly opt for the R series drive in any enterprise configuration.

Storage system using Z-drives

Just consider what one can do with a gaggle of Z-drives in a standard storage system.  For example, with 5 Z-drives in a server, it could potentially support 2.5M IOPs/sec and 14GB/sec of data transfer with some resulting loss of performance due to front-end emulation.  Moreover, at 3.2TB per drive, even in a RAID5 4+1 configuration the storage system would support 12.8TB of user capacity. One could conceivably do away with any DRAM cache in such a system and still provide excellent performance.

What the cost for such a system would be is another question. But with MLC NAND it shouldn’t be too obscene.

On the other hand serviceability might be a concern as it would be difficult to swap out a failed drive (bad SSD/PCIe card) while continuing IO operations. This could be done with some special hardware but it’s typically not present in standard, off the shelf servers.

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All in all a very interesting announcement from OCZ.  The likelihood that a single server will need this sort of IO performance from a lone drive is not that high (except maybe for massive website front ends) but putting a bunch of these in a storage box is another matter.  Such a configuration would make one screaming storage system with minimal hardware changes and only a modest amount of software development.

Comments?

Intel’s 320 SSD “8MB problem”

Intel SSD 320_001 by TAKA@P.P.R.S (cc) (from Flickr)
Intel SSD 320_001 by TAKA@P.P.R.S (cc) (from Flickr)

Read a recent news item on Intel being able to replicate their 320 SSD 8MB problem that some customers have been complaining about.

Apparently the problem occurs when power is suddenly removed from the device.  The end result is that the SSD’s capacity is restricted to 8MB from 40GB or more.

I have seen these sorts of problems before.  It probably has something to do with table updating activity associated with SSD wear leveling.

Wear leveling

NAND wear leveling looks very similar to virtual memory addressing and maps storage block addresses to physical NAND pages. Essentially something similar to a dynamic memory page table is maintained that shows where the current block is located in the physical NAND space, if present.  Typically, there are multiple tables involved, one for spare pages, another for mapping current block addresses to NAND page location and offset, one for used pages, etc.  All these tables have to be in some sort of non-volatile storage so they persist after power is removed.

Updating such tables and maintaining their integrity is a difficult endeovor.  More than likely some sort of table update is not occurring in an ACID fashion.

Intel’s fix

Intel has replicated the problem and promises a firmware fix. In my experience this is entirely possible.  Most probably customer data has not been lost (although this is not a certainty), it’s just not accessible at the moment. And Intel has reminded everyone that as with any storage device everyone should be taking periodic backups to other devices, SSDs are no exception.

I am certain that Intel and others are enhancing their verification and validation (V&V) activities to better probe and test the logic behind wear leveling fault tolerance, at least with respect to power loss. Of course, redesigning the table update algorithm to be more robust, reliable, and fault tolerant is a long range solution to these sorts of problems but may take longer than a just a bug fix.

The curse of complexity

But all this begs a critical question, as one puts more and more complexity outboard into the drive are we inducing more risk?

It’s a perennial problem in the IT industry. Software bugs are highly correlated to complexity and thereby, are ubiquitous, difficult to eliminate entirely, and often escape any and all efforts to eradicate them before customer shipments.  However, we can all get better at reducing bugs, i.e., we can make them less frequent, less impactful, and less visible.

What about disks?

All that being said, rotating media is not immune to the complexity problem. Disk drives have different sorts of complexity, e.g., here block addressing is mostly static and mapping updates occur much less frequently (for defect skipping) rather than constantly as with NAND, whenever data is written.  As such, problems with power loss impacting table updates are less frequent and less severe with disks.  On the other hand, stiction, vibration, and HDI are all very serious problems with rotating media but SSDs have a natural immunity to these issues.

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Any new technology brings both advantages and disadvantages with it.  NAND based SSD advantages include high speed, low power, and increased ruggedness but the disadvantages involve cost and complexity.  We can sometimes tradeoff cost against complexity but we cannot eliminate it entirely.

Moreover, while we cannot eliminate the complexity of NAND wear leveling today, we can always test it better.  That’s probably the most significant message coming out of today’s issue.  Any product SSD testing has to take into account the device’s intrinsic complexity and exercise that well, under adverse conditions. Power failure is just one example, I can think of dozens more.

Comments?