Policy tab, and, after you read the warning label, select “Enable advanced performance.” This page is a bit confusing: it says you cannot modify the WCE (write cache enable) performance, but then it gives you two radio buttons that do so.

When I selected “Advanced write performance,” I expected to see great performance—but not so. I could never get good write performance. This could be a problem with the device driver or with the device. The article “A Design for High-Performance Flash Disks” ⁶ explains the problem and offers a solution. Clearly, a little intelligence in the disk controller could buffer the writes and give performance comparable to the 1,100 I/Os per second that we get with sequential writes—but that is not what I see with the current software-hardware configuration.

WHAT IF FLASH DISKS DELIVERED THOUSANDS OF IOPS AND WERE “BIG”?

My tests and those of several others suggest that flash disks can deliver about 3,000 random 8-KB reads per second, and with some reengineering, about 1,100 random 8-KB writes per second. Indeed, it appears that a single flash chip could deliver nearly that performance, and there are many chips inside the “box”—so the actual limit could be four times or more. Even the current performance, however, would be very attractive for many enterprise applications. For example, the TPC-C benchmark has approximately equal reads and writes. Using the graphs in figure 2, a weighted average of the four-deep, 8-KB random read rate ( 2,804 IOPS) and the four-deep, 8-KB sequential write rate ( 1,233 IOPS) results in a harmonic average of 1,713 IOPS (one-deep is 1,624 IOPS). TPC-C systems are configured with approximately 50 disks per CPU. For example, the most recent Dell TPC-C system has 90 15,000-RPM 36-GB SCSI disks costing $45,000 (with $10,000 extra for maintenance that gets “discounted”). Those disks are 68 percent of the system cost. They deliver about 18,000 IOPS. That is comparable to the requests per second of 10 flash disks. So we could replace those 90 disks with 10 NSSDs if the data would fit on 320 GB (it does not). That would save a lot of money and power ( 1.3Kw of power and

1.3Kw of cooling).

The current flash disks are built with 16-GB NAND flash. When, in 2012, they are

built with a 1-terabit part, the device will have 2 TB of capacity and will indeed be able to store the TPC-C database. We could replace a $44,000 disk array with a few (say, 10) $400 flash disks (maybe).

The system diagram of the Samsung NSSD suggests many opportunities for innovation: interesting RAID options for fault tolerance (combining the ideas from “A Design for High-Performance Flash Disks” ⁷ with a nonvolatile storage map and a block-buffer, and with writing RAID- 5 stripes of data across the chip array), adding a battery, adding logic for copy-on-write snapshots, and so on. These devices enable whole new approaches to file systems. They are potential gap fillers between disks and RAM, and they are interesting “hot data” storage devices in their own right.