Peter Rabbitson wrote:
Michael Guntsche wrote:
Is it possible that my computer is just too slow to get good read
results?
unlikely
While reading is a little bit faster it's nowhere near the speed I
get on
md0 itself.
I would guess that you did not set the correct read-ahead values for
the LV. If you do not specify anything it will default to 128k (256
sectors), which is terribly small for sequential reads. On the
contrary the MD device will do some clever calculations and set its
read-ahead correctly depending on the raid level and the number of
disks. Do:
blockdev --setra 65536 <your lv device>
and run the tests again. You are almost certainly going to get the
results you are after.
I will just comment that really large readahead values may cause
significant memory usage and transfer of unused data. My observations
and some posts indicate that very large readahead and/or chunk size may
reduce random access performance. I believe you said you had 512MB RAM,
that may be a factor as well.
Also, blockdev will allow you to diddle readahead on the device,
/dev/sdX, the array /dev/mdX, and the lv /dev/mapper/NAME. The
interaction of these, and the performance results of having the same
exact amount of readhead memory used in different way is a fine topic
for a thesis, conference paper, magazine article, or nightmare.
Unless you are planning to use this machine mainly for running
benchmarks, I would tune it for your actual load and a bit of worst case
avoidance.
--
Bill Davidsen <davidsen@xxxxxxx>
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark
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