Databases Reference
In-Depth Information
The good thing about SSDs is that they are readily available in lots of brands and mod-
els, they're relatively cheap, and they're a lot faster than hard drives. The biggest down-
side is that they're not always as reliable as hard drives, depending on the brand and
model. Until recently, most devices didn't have an onboard battery, but most devices
do have a write cache to buffer writes. This write cache isn't durable without a battery
to back it, but it can't be disabled without greatly increasing the write load on the
underlying flash storage. So, if you disable your drive's cache to get really durable stor-
age, you will wear the device out faster, and in some cases this will void the warranty.
Some manufacturers don't exactly rush to inform people about this characteristic of
the SSDs they sell, and they guard details such as the internal architecture of the devices
pretty jealously. Whether there is a battery or capacitor to keep the write cache's data
safe in case of a power failure is usually an open question. In some cases the drive will
accept a command to disable the cache, but ignore it. So you really won't know whether
your drive is durable unless you do crash testing. We crash-tested some drives and
found varying results. These days some drives ship with a capacitor to protect the cache,
making it durable, but in general,
if your drive doesn't brag that it has a battery or
capacitor, then it doesn't
. This means it isn't durable in case of power outages, so you'll
get data corruption, possibly without knowing it. A capacitor or battery is a feature you
should definitely look for in SSDs.
You generally get what you pay for with SSDs. The challenges of the underlying tech-
nology aren't easy to solve. Lots of manufacturers make drives that fail shockingly
quickly under load, or don't provide consistent performance. Some low-end manufac-
turers have a habit of releasing a new generation of drives every time you turn around,
and claiming that they've solved all the problems of the older generation. This tends
to be untrue, of course. The “enterprise-grade” devices are usually worth the price if
you care about reliability and consistently high performance.
Using RAID with SSDs
We recommend that you use RAID (Redundant Array of Inexpensive Disks) with SATA
SSDs. They are simply not reliable enough to trust a single drive with your data.
Many older RAID controllers weren't SSD-ready. They assumed that they were man-
aging spindle-based hard drives, and they did things like buffering and reordering
writes, assuming that it would be more efficient. This was just wasted work and added
latency, because the logical locations that the SSD exposes are mapped to arbitrary
locations in the underlying flash memory. The situation is a bit better today. Some
RAID controllers have a letter at the end of their model numbers, indicating that they
are SSD-ready. For example, the Adaptec controllers use a Z for this purpose.
Even flash-ready controllers are not
really
flash-ready, however. For example, Vadim
benchmarked an Adaptec 5805Z controller with a variety of drives in RAID 10, using
a 500 GB file and a concurrency of 16. The results were terrible: the 95th percentile
