I did some tests, starting with the default values of 256 for
stripe_cache_size and 3072 for read_ahead_kb, and doubling them both
until performance stopped improving. Here are the best results that I
saw:
# echo 2048 > /sys/block/md0/md/stripe_cache_size
# echo 24576 > /sys/block/md0/queue/read_ahead_kb
# iozone -a -y64K -q16M -s4G -e -f iotest -i0 -i1 -i2
random random
KB reclen write rewrite read reread read write
4194304 64 241087 259892 243478 248102 7745 16161
4194304 128 259503 261886 244612 247157 13417 26812
4194304 256 260438 268077 240211 238916 21884 37527
4194304 512 243511 250004 252507 252276 34694 48868
4194304 1024 244744 253905 258920 250495 52351 76356
4194304 2048 240910 250500 253800 265361 79848 100131
4194304 4096 244283 253516 271940 272117 101737 137386
4194304 8192 239110 246370 262118 269687 103437 164715
4194304 16384 240698 249182 239378 253896 119437 198276
250 MB/s reads and writes is quite nice for a 5 drive RAID-6.
But I still do not understand why it is necessary to increase the
stripe_cache_size to 16 full stripes in order to optimize sequential
write speed.
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