Support and Q&A for Solid-State Drives

     There’s a lot of excitement around the potential for the widespread adoption of solid-state drives (SSD) for primary storage, particularly on laptops and also among many folks in the server world.  As with any new technology, as it is introduced we often need to revisit the assumptions baked into the overall system (OS, device support, applications) as a result of the performance characteristics of the technologies in use.  This post looks at the way we have tuned Windows 7 to the current generation of SSDs.  This is a rapidly moving area and we expect that there will continue to be ways we will tune Windows and we also expect the technology to continue to evolve, perhaps introducing new tradeoffs or challenging other underlying assumptions.  Michael Fortin authored this post with help from many folks across the storage and fundamentals teams.  --Steven

Many of today’s Solid State Drives (SSDs) offer the promise of improved performance, more consistent responsiveness, increased battery life, superior ruggedness, quicker startup times, and noise and vibration reductions. With prices dropping precipitously, most analysts expect more and more PCs to be sold with SSDs in place of traditional rotating hard disk drives (HDDs).

In Windows 7, we’ve focused a number of our engineering efforts with SSD operating characteristics in mind. As a result, Windows 7’s default behavior is to operate efficiently on SSDs without requiring any customer intervention. Before delving into how Windows 7’s behavior is automatically tuned to work efficiently on SSDs, a brief overview of SSD operating characteristics is warranted.

Random Reads: A very good story for SSDs

SSDs tend to be very fast for random reads. Most SSDs thoroughly trounce traditionally HDDs because the mechanical work required to position a rotating disk head isn’t required. As a result, the better SSDs can perform 4 KB random reads almost 100 times faster than the typical HDD (about 1/10th of a millisecond per read vs. roughly 10 milliseconds).

Sequential Reads and Writes: Also Good

Sequential read and write operations range between quite good to superb. Because flash chips can be configured in parallel and data spread across the chips, today’s better SSDs can read sequentially at rates greater than 200 MB/s, which is close to double the rate many 7200 RPM drives can deliver. For sequential writes, we see some devices greatly exceeding the rates of typical HDDs, and most SSDs doing fairly well in comparison. In today’s market, there are still considerable differences in sequential write rates between SSDs. Some greatly outperform the typical HDD, others lag by a bit, and a few are poor in comparison.

Random Writes & Flushes: Your mileage will vary greatly

The differences in sequential write rates are interesting to note, but for most users they won’t make for as notable a difference in overall performance as random writes.

What’s a long time for a random write? Well, an average HDD can typically move 4 KB random writes to its spinning media in 7 to 15 milliseconds, which has proven to be largely unacceptable. As a result, most HDDs come with 4, 8 or more megabytes of internal memory and attempt to cache small random writes rather than wait the full 7 to 15 milliseconds. When they do cache a write, they return success to the OS even though the bytes haven’t been moved to the spinning media. We typically see these cached writes completing in a few hundred microseconds (so 10X, 20X or faster than actually writing to spinning media). In looking at millions of disk writes from thousands of telemetry traces, we observe 92% of 4 KB or smaller IOs taking less than 1 millisecond, 80% taking less than 600 microseconds, and an impressive 48% taking less than 200 microseconds. Caching works!

On occasion, we’ll see HDDs struggle with bursts of random writes and flushes. Drives that cache too much for too long and then get caught with too much of a backlog of work to complete when a flush comes along, have proven to be problematic. These flushes and surrounding IOs can have considerably lengthened response times. We’ve seen some devices take a half second to a full second to complete individual IOs and take 10’s of seconds to return to a more consistently responsive state. For the user, this can be awful to endure as responsiveness drops to painful levels. Think of it, the response time for a single I/O can range from 200 microseconds up to a whopping 1,000,000 microseconds (1 second).

When presented with realistic workloads, we see the worst of the SSDs producing very long IO times as well, as much as one half to one full second to complete individual random write and flush requests. This is abysmal for many workloads and can make the entire system feel choppy, unresponsive and sluggish.

Random Writes & Flushes: Why is this so hard?

Read more: Engineering Windows 7 

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