On 09/08/2017 11:06 AM, hw wrote:
Make a test and replace a software RAID5 with a hardware RAID5. Even
with
only 4 disks, you will see an overall performance gain. I´m guessing
that
the SATA controllers they put onto the mainboards are not designed to
handle
all the data --- which gets multiplied to all the disks --- and that the
PCI bus might get clogged. There´s also the CPU being burdened with the
calculations required for the RAID, and that may not be displayed by
tools
like top, so you can be fooled easily.
That sounds like a whole lot of guesswork, which I'd suggest should
inspire slightly less confidence than you are showing in it.
RAID parity calculations are accounted under a process named
md<number>_raid<level>. You will see time consumed by that code under
all of the normal process accounting tools, including total time under
"ps" and current time under "top". Typically, your CPU is vastly faster
than the cheap processors on hardware RAID controllers, and the
advantage will go to software RAID over hardware. If your system is CPU
bound, however, and you need that extra fraction of a percent of CPU
cycles that go to calculating parity, hardware might offer an advantage.
The last system I purchased had its storage controller on a PCIe 3.0 x16
port, so its throughput to the card should be around 16GB/s. Yours
might be different. I should be able to put roughly 20 disks on that
card before the PCIe bus is the bottleneck. If this were a RAID6
volume, a hardware RAID card would be able to support sustained writes
to 22 drives vs 20 for md RAID. I don't see that as a compelling
advantage, but it is potentially an advantage for a hypothetical
hardware RAID card.
When you are testing your 4 disk RAID5 array, microbenchmarks like
bonnie++ will show you a very significant advantage toward the hardware
RAID as very small writes are added to the battery-backed cache on the
card and the OS considers them complete. However, on many cards, if the
system writes data to the card faster than the card writes to disks, the
cache will fill up, and at that point, the system performance can
suddenly and unexpectedly plummet. I've fun a few workloads where that
happened, and we had to replace the system entirely, and use software
RAID instead. Software RAID's performance tends to be far more
predictable as the workload increases.
Outside of microbenchmarks like bonnie++, software RAID often offers
much better performance than hardware RAID controllers. Having tested
systems extensively for many years, my advice is this: there is no
simple answer to the question of whether software or hardware RAID is
better. You need to test your specific application on your specific
hardware to determine what configuration will work best. There are some
workloads where a hardware controller will offer better write
performance, since a battery backed write-cache can complete very small
random writes very quickly. If that is not the specific behavior of
your application, software RAID will very often offer you better
performance, as well as other advantages. On the other hand, software
RAID absolutely requires a monitored UPS and tested auto-shutdown in
order to be remotely reliable, just as a hardware RAID controller
requires a battery backed write-cache, and monitoring of the battery state.
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