Mark Knecht wrote:
Once all of that is in place then possibly more cores will help, but I
suspect even then it probably hard to use 4 billion CPU cycles/second
doing nothing but disk I/O. SATA controllers are all doing DMA so CPU
overhead is relatively *very* low.
There is the RAID5/6 parity calculations to be considered on writes and
this appears to be single threaded. There is an experimental multicore
kernel option I believe, but recent discussion indicates there may be
some problems with it.
A very quick test on a box here on a Xeon E5440 (4 x 2.8GHz) and a SAS
attached 16 x 750GB SATA md RAID6. The array is 72% full and probably
quite fragmented and currently the system is idle.
dd if=/dev/zero of=/mnt/storage/dump bs=1M count=20000
20000+0 records in
20000+0 records out
20971520000 bytes (21 GB) copied, 87.2374 s, 240 MB/s
Looking at the outputs of vmstat 5 and mpstat -P ALL 5 during this, one
core (probably doing parity generation) was around 7.56% idle and the
other 3 were around 88.5, 67.5 and 51.8% idle.
The same test run when the system was commissioned and the array was
empty, acheived 565MB/s writes.
Regards,
Richard
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