Seagate has just announced an upgrade to the company’s Momentus XT hybrid HDD family. Seagate announced in August that the company had already shipped over one million units of its first generation Momentus XT since its May 2010 introduction.
For those unaware of what a hybrid HDD is, the short answer is that it’s a way to get HDD capacities and SSD speeds at a price marginally higher than that of an HDD. Continue reading
The 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
One 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
An 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
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
The 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
One 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
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
Calypso Systems ran the SNIA SSD Performance Test Specification (PTS), outlined in an earlier post in this blog, on seventeen SSDs and a single HDD. The results appear, in miniature, in the graphic for this post. Continue reading