Just out of curiosity, we have a RHEL server running software RAID. There's a cronjob which kicks off a raid data check every so often to check the consistency of the array. From my understanding, the data check will utilize idle IO bandwidth so as to not interfere with the server's normal operation.
As can be seen from the iostat output below, the server is virtually idle. Why is the raid data check only using 6M/sec of IO bandwidth? During the initial RAID build, I have seen these machines reach 320M/sec+. Why is the data check not utilizing 200MB/sec as indicated as the max bandwidth in the kernel log entry?
Below is the log output from when the raid data check was last started:
Apr 28 01:00:01 xxxx kernel: md: data-check of RAID array md1
Apr 28 01:00:01 xxxx kernel: md: minimum _guaranteed_ speed: 1000 KB/sec/disk.
Apr 28 01:00:01 xxxx kernel: md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for data-check.
Apr 28 01:00:01 xxxx kernel: md: using 128k window, over a total of 3824596992k.
Apr 28 01:00:07 xxxx kernel: md: delaying data-check of md0 until md1 has finished (they share one or more physical units)
Here's some output of /proc/mdstat relating to md1:
md1 : active raid10 sdb2[1] sda2[0] sdd2[3] sdg2[6] sdh2[7] sde2[4] sdc2[2] sdf2[5]
3824596992 blocks super 1.1 512K chunks 2 near-copies [8/8] [UUUUUUUU]
[==========>..........] check = 53.4% (2045997824/3824596992) finish=5255.6min speed=5639K/sec
bitmap: 10/29 pages [40KB], 65536KB chunk
Output of iostat -m:
[root@xxxx ~]# iostat -m
Linux 2.6.32-358.2.1.el6.x86_64 (xxxx) 05/01/2013 _x86_64_ (32 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
2.92 0.07 0.83 1.26 0.00 94.92
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sde 35.25 1.26 0.38 1520624 456526
sdf 33.15 1.21 0.38 1461256 456526
sdb 35.49 1.24 0.38 1485991 460191
sda 36.80 1.28 0.38 1542868 460191
sdc 31.64 1.26 0.36 1519241 429095
sdd 30.00 1.21 0.36 1460629 429095
sdh 30.51 1.21 0.35 1460819 424707
sdg 31.90 1.26 0.35 1519924 424707
md0 2.72 0.01 0.01 8465 12014
md1 177.21 0.38 1.34 453189 1607605