Still, sometimes even streaming across multiple I/O channels to multiple NAND chips isn't enough to keep up with the data coming in across the bus that the computer expects the SSD to accept, so quite a few consumer SSDs contain some amount of DDR2 or DDR3 SDRAM, usually between 128 and 512 MB. Having a chunk of cache sitting there lets the SSD quickly receive data that it needs to write, even if it's too busy to actually write it at the moment; the data sits in the SDRAM cache until the controller is able to find time to send it down and actually commit it to NAND. All this happens transparently to the computer and you, the end user—regardless of whether or not the data has actually been written, the SSD controller reports back to the operating system that the write was completed successfully.
This greatly decreases the effective latency and increases the throughput of the SSD, but there's an obvious problem. The SDRAM in an SSD's cache is the same kind of SDRAM used for main memory—the kind that erases itself if it loses power. If the computer were to suffer a power loss while the SSD has data in cache that hasn't yet been committed to NAND, then that data would be completely gone, with consequences that could range from annoying to catastrophic. The most common consequence of a loss of uncommitted write cache would be file system corruption, which might or might not be repairable with a chkdsk or fsck; depending on what was being held in cache, the entire file system on the SSD could be unrecoverable.
This is obviously a bad thing, and most SSD manufacturers who bolster their drives' performance with a large SDRAM cache also include some mechanism to supply power to the SSD long enough to dump its cache contents out to NAND, usually in the form of a large capacitor. Whatever the mechanism, it only needs to provide the drive with power for a short amount of time, since it doesn't take terribly long to write out even a full 512MB of data to an SSD. However, not every SDRAM-cached SSD has a set of cache-powering capacitors—some have nothing at all, so be aware of the specs when picking one out. Most have something, though, and the speed benefits of stuffing some RAM into an SSD more than balance out the risks, which are quite manageable.
