Databases Reference
In-Depth Information
Devices, filesystems, and operating systems vary in their support for the
TRIM
command,
which marks space as ready to reuse. Sometimes the device will
TRIM
when you delete
all of the files. If that happens between runs of the benchmark, the device will reset to
stage A, and you'll have to cycle it through stages A and B between runs. Another factor
is the differing performance when the device is more or less filled up. A repeatable
benchmark has to account for all of these factors.
As a result of the above complexities, vendor benchmarks and specifications are a
minefield for the unwary, even when they're reported faithfully and with good inten-
tions. You typically get four numbers from vendors. Here's an example of a device's
specifications:
1. The device can read up to 520 MB/s.
2. The device can write up to 480 MB/s.
3. The device can perform sustained writes up to 420 MB/s.
4. The device can perform 70,000 random 4 KB writes per second.
If you cross-check those numbers, you will notice that the peak IOPS (input/output
operations per second) of 70,000 random 4 KB writes per second is only about 274
MB/s, which is a lot less than the peak write bandwidths listed in points 2 and 3. This
is because the peak write bandwidth is achieved with large block sizes such as 64 KB
or 128 KB, and the peak IOPS is achieved with small block sizes.
Most applications don't write in such large blocks. InnoDB typically writes a combi-
nation of 16 KB blocks and 512-byte blocks. As a result, you should really expect only
274 MB/s of write bandwidth from this device—and that's in stage A, before the
garbage collector kicks in and the device reaches its steady-state long-term performance
levels!
You can find current benchmarks of MySQL and raw file I/O workloads on solid-state
devices at our blogs,
http://www.ssdperformanceblog.com
and
http://www.mysqlperfor
Solid-State Drives (SSDs)
SSDs emulate SATA hard drives. This is a compatibility feature: a replacement for a
SATA drive doesn't require any special drivers or interconnects.
Intel X-25E drives are probably the most common SSDs we see used in servers today,
but there are lots of other options. The X-25E is sold for the “enterprise” market, but
there is also the X-25M, which has MLC storage and is intended for the mass market
of laptop users and so forth. Intel also sells the 320 series, which a lot of people are
using as well. Again, this is just one vendor—there are many, and by the time this topic
goes to print, some of what we've written about SSDs will likely already be outdated.
