Flash Performance and Reliability

As we can see from the table, read latencies are quite good, taking just 10s of microseconds to complete. Program latency is higher and more variable, as low as 200 microseconds for SLC, but higher as you pack more bits into each cell. To get good write performance, you will have to make use of multiple flash chips in parallel. Finally, erases are quite expensive, taking a few milliseconds typically. Dealing with this cost is central to modern flash storage design.

Reliability

Let’s now consider the reliability of flash chips. Unlike mechanical disks, which can fail for a wide variety of reasons (including the gruesome and quite physical head crash, where the drive head actually makes contact with the recording surface), flash chips are pure silicon and in that sense have fewer reliability issues to worry about. The primary concern is wear out; when a flash block is erased and programmed, it slowly accrues a little bit of extra charge. Over time, as that extra charge builds up, it becomes increasingly difficult to differentiate between a 0 and a 1. At the point where it becomes impossible, the block becomes unusable.

The typical lifetime of a block is currently not well known. Manufacturers rate MLC-based blocks as having a 10,000 P/E (Program/Erase) cycle lifetime, that is, each block can be erased and programmed 10,000 times before failing. SLC-based chips, because they store only a single bit per transistor, are rated with a longer lifetime, usually 100,000 P/E cycles. However, recent research“Write Endurance in Flash Drives: Measurements and Analysis” by Simona Boboila, Peter Desnoyers. FAST ’10, San Jose, California, February 2010. A cool paper that reverse engineers flash-device lifetimes. Endurance sometimes far exceeds manufacturer predictions, by up to 100×. has shown that lifetimes are much longer than expected.

One other reliability problem within flash chips is known as a disturbance. When accessing a particular page within a flash, it is possible that some bits get flipped in neighboring pages. Such bit flips are known as read disturbs or program disturbs, depending on whether the page is being read or programmed, respectively.

TIP: THE IMPORTANCE OF BACKWARDS COMPATIBILITY

Backwards compatibility is always a concern in layered systems. By defining a stable interface between two systems, one enables innovation on each side of the interface while ensuring continued interoperability. Such an approach has been quite successful in many domains: operating systems have relatively stable APIs for applications, disks provide the same block-based interface to file systems, and each layer in the IP networking stack provides a fixed unchanging interface to the layer above.

Not surprisingly, there can be a downside to such rigidity, as interfaces defined in one generation may not be appropriate in the next. In some cases, it may be useful to think about redesigning the entire system entirely. An excellent example is found in the Sun ZFS file system“ZFS: The Last Word in File Systems” by Jeff Bonwick and Bill Moore. Available here: http://www.ostep.org/Citations/zfs_last.pdf. Was this the last word in file systems? No, but maybe it’s close.. By reconsidering the interaction of file systems and RAID, the creators of ZFS envisioned (and then realized) a more effective integrated whole.