DRAM vs. SLC SSD – Which Makes More Sense?

DRAM and SLC NAND price per Gigabyte HistoryThe SSD guy usually argues that NAND flash (as an SSD) fits into the memory/storage hierarchy for two reasons:

  • SSDs are cheaper than DRAM but more expensive than an HDD
  • SSDs are slower than DRAM but faster than an HDD

An SSD is usually a very good alternative to adding more DRAM to a system, but this is not always the case.  Users need to look carefully at the SSD they plan to use to make sure they are getting the most for their system spend. Continue reading “DRAM vs. SLC SSD – Which Makes More Sense?”

SSDs and Caching

IBM: Effect of Data Placement on SSD EffectivenessOne of the SSD Guy’s favorite subjects is caching and SSDs.  This is because I wrote a book on processor caches in the early 1990s, and the advent of SSD caches in storage systems hearkens back to the technology detailed in that book.

Caching works well whenever there are two layers in the memory hierarchy since the fast expensive layer can replicate data in the slow inexpensive layer to accelerate the processor’s performance. Continue reading “SSDs and Caching”

SSD Fast Erase

All the NAND chips in an SSD can be erased simultaneouslyAn interesting feature that exists in many SSDs is the ability to quickly erase all the data on the device.  The military is especially interested in this feature because it helps prevent sensitive data from falling into the wrong hands.

For example, let’s say your helicopter crashed when on a mission to assassinate the leader of a major terrorist organization.  If the HDD or SSD inside the cockpit was recovered by that organization the data might be extracted to help undermine future missions. Continue reading “SSD Fast Erase”

What Happens when SSDs Fail?

What happens at the end of an SSD's life?There’s a lot of “Fear, Uncertainty, and Doubt” – FUD – circulating about SSDs and their penchant for failure.  NAND flash wears out after a set number of erase/write cycles, a specification known as the flash’s endurance.

While some caution is warranted, a good understanding of how SSDs really behave will help to allay a lot of this concern. Continue reading “What Happens when SSDs Fail?”

SSDs and RAID

RAID ConfigurationThe SSD Guy has been asked a number of questions lately about SSDs and RAID.  Most of these center around the difference in failure behaviors between SSDs and HDDs – HDDs fail randomly (if ever), while SSDs fail relatively predictably due to wear.

Oddly enough, SSD failures due to wear make them a little friendlier than HDDs.  The wear mechanism is managed by the controller in the SSD.  SSDs have spare blocks, and the controller manages those blocks, so the controller understands exactly how much wear the SSD has undergone and how much room is left before the SSD will start to have difficulties. Continue reading “SSDs and RAID”

SSD Garbage Collection

garbage cansOne of the thorniest issues in SSD design how to manage erasing blocks that are no longer in use.  That’s saying a lot, because NAND flash presents so very many difficult challenges like wear leveling, bad block management, error correction, and write amplification.

The difficulty stems from the fact that all of today’s software was written for HDDs which don’t behave like the flash in an SSD.  An HDD can over-write existing data with new data.  In a flash SSD, a block must be erased before being over-written and this can take a half a second – a huge amount of time in the world of computing.  Since the software doesn’t accommodate flash’s “erase-before-write” needs, the controller inside the SSD must take care of this bit of housekeeping.  Unused and unerased blocks are moved out of the way and erased in the background.  This is called the “garbage collection” process. Continue reading “SSD Garbage Collection”

Sometimes SSDs Don’t Improve System Speed

Slow, Slow, Slow!The SSD Guy attended TechTarget‘s Storage Decisions Conference last week in San Francisco.  Dennis Martin of Demartek gave a very good presentation called Making the Case for Solid-State Storage.

Demartek tests a lot of systems based on various forms of storage.

I really liked an expression that Mr. Martin shared to compare SSDs to HDDs.  He said that SSDs cost dollars per gigabyte and pennies per IOPS, while HDDs cost pennies per gigabyte and dollars per IOPS.  This is a really good way to think about the strengths and weaknesses of these two technologies.  There is every reason to use a mix of both. Continue reading “Sometimes SSDs Don’t Improve System Speed”

Are HDDs Obsolete?

This looks like more fun than reading The SSD GuyAn article in the Storage Newsletter caught The SSD Guy’s eye when it ran in July.  The article consisted of a press release followed by an editorial comment:

While hard drives still have the cost advantage, it appears it’s becoming akin to sticking with a horse-drawn buggy in an age of automobiles by arguing that the upfront cost for a car is so much more than the cost of a horse. At some point, it just doesn’t make sense to ride a horse. How soon until the IT world gets there?

Continue reading “Are HDDs Obsolete?”

What’s the SNIA SSD Performance Test Specification (PTS)?

Click Here for the SNIA SSSI PTSThe Storage Networking Industry Association – SNIA – determined a few years back that it should address SSDs since they were about to become an important part of most storage systems.  To this end SNIA created the Solid State Storage Initiative, or SSSI.

They didn’t name it after SSDs since there will clearly come a time when flash stops pretending it’s an HDD and abandons standard HDD mechanical and interface specifications. Continue reading “What’s the SNIA SSD Performance Test Specification (PTS)?”