Databases Reference
In-Depth Information
latency for random writes was in the double-digit milliseconds, and in the worst case,
it was over a second.
6
(You should expect sub-millisecond writes.)
This specific comparison was for a customer who wanted to see whether Micron SSDs
would be better than 64 GB Intel SSDs, which they already used in the same
configuration. When we benchmarked the Intel drives, we found the same performance
characteristics. So we tried some other configurations of drives, with and without a SAS
expander, to see what would happen.
Table 9-1
shows the results.
Table 9-1. Benchmarks with SSDs on an Adaptec RAID controller
Drives
Brand
Size
SAS expander
Random read
Random write
34
Intel
64 GB
Yes
310 MB/s
130 MB/s
14
Intel
64 GB
Yes
305 MB/s
145 MB/s
24
Micron
50 GB
No
350 MB/s
120 MB/s
34
Intel
50 GB
No
350 MB/s
180 MB/s
None of these results approached what we should expect from so many drives. In gen-
eral, the RAID controller was giving us the performance we'd expect from six or eight
drives, not dozens. The RAID controller was simply saturated. The point of this story
is that you should benchmark carefully before investing heavily in hardware—the
results might be quite different from your expectations.
PCIe Storage Devices
In contrast to SATA SSDs, PCIe devices don't try to emulate hard drives. This is a good
thing: the interface between the server and the hard drives isn't capable of handling the
full performance of flash. The SAS/SATA interconnect has lower bandwidth than PCIe,
so PCIe is a better choice for high performance. PCIe devices also have much lower
latency, because they are physically closer to the CPUs.
Nothing matches the performance you can get from PCIe devices. The downside is that
they're relatively expensive.
All of the models we're familiar with require a special driver to create a block device
that the operating system sees as a hard drive. They use a mixture of strategies for their
wear leveling and other logic; some of them use the host system's CPU and memory,
and some have onboard logic controllers and RAM. In many cases the host system has
plentiful CPU and RAM resources, so using them is actually a more cost-effective strat-
egy than buying a card that has its own.
We don't recommend RAID with PCIe devices. They're too expensive to use with
RAID, and most devices have their own onboard RAID anyway. We don't really know
6. But that's not all. We checked the drives after the benchmark and found two dead SSDs and one with
inconsistencies.
