I was doing some bulk reads on an ext4 file system on
a simple mirrored device (3.13 x86_64 kernel):
md2 : active raid1 sdd3[0] sde3[1]
1932514496 blocks [2/2] [UU]
bitmap: 0/15 pages [0KB], 65536KB chunk
And I noticed that the reads were all hitting the first disk.
The second was basically idle. Here's "dstat -d -D md2,sdd,sde":
--dsk/md2-----dsk/sdd-----dsk/sde--
read writ: read writ: read writ
510k 55k: 484k 58k: 27k 58k
129M 0 : 129M 0 : 48k 0
108M 48k: 108M 60k: 576k 60k
123M 0 : 123M 8192B: 0 8192B
133M 0 : 133M 0 : 360k 0
132M 0 : 132M 0 : 20k 0
138M 0 : 138M 0 : 304k 0
128M 0 : 128M 0 : 896k 0
129M 0 : 129M 4096B: 64k 4096B
135M 0 : 135M 0 : 36k 0
116M 12k: 116M 24k: 36k 28k
117M 0 : 116M 4096B: 632k 0
127M 0 : 127M 0 : 288k 0
130M 0 : 130M 0 : 336k 0
133M 0 : 133M 0 : 212k 0
134M 0 : 134M 0 : 304k 0
130M 0 : 129M 4096B: 100k 4096B
128M 0 : 127M 0 : 280k 0
106M 12k: 106M 28k: 372k 28k
129M 0 : 129M 0 : 344k 0
134M 0 : 134M 0 : 196k 0
134M 0 : 134M 0 : 384k 0
129M 0 : 129M 0 : 304k 0
I thought (drivers/md/raid1.c:read_balance()) the driver was
supposed to do some striping on large reads.
While 125M/s is nice, more than that would be nicer.
The drives are identical, but are plugged in to different controllers.
sdd is on an AMD SB600 controller, while sde is on a PDC42819.
Is there some knob I need to adjust to make read balancing happen?
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