Category: SSD Performance

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”

Not all SSDs are Created Equal

Results of the SNIA PTS on Seventeen SSDs and one HDDSSDs vary widely in performance.  This is something that becomes amazingly clear when a number of these devices are put through a battery of tests.

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 “Not all SSDs are Created Equal”

An HDD Cache for an SSD?

University of TorontoA colleague – Isilon’s Rob Peglar – pointed out an interesting paper written by researchers at the University of Toronto in collaboration with Microsoft.  The paper makes a case for using an HDD to cache writes to an SSD to improve storage system performance.

“Wait a minute!” you say.  “An HDD as a cache for an SSD?  This can’t be possible!” Continue reading “An HDD Cache for an SSD?”

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)?”