I recently set up a simple 4-disk raid 10 array with mdadm under Ubuntu 16.04 for a simple home server.
One thing that keeps happening though, is that there’s constant write activity on the disks, even when no applications are running.
When running cat /proc/mdstat this is my output:
Personalities : [raid10] [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4]
md0 : active raid10 sdc[1] sde[3] sdb[0] sdd[2]
7813774336 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]
bitmap: 2/59 pages [8KB], 65536KB chunk
iostat is showing there’s a constant ~2.5MB/s write happening on the array:
Linux 4.10.0-33-generic (Arcade) 01/09/17 _x86_64_ (8 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
1.34 0.01 0.42 0.23 0.00 98.00
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 8.63 61.34 2106.75 1027046 35271981
sdb 7.76 0.76 1280.29 12734 21435121
sde 7.69 0.25 1279.97 4185 21429673
sdd 7.71 0.29 1279.97 4892 21429669
sdc 7.73 0.14 1280.29 2272 21435121
md0 4.40 2.22 2534.38 37251 42431464
and mdadm -D /dev/md0 shows the following:
/dev/md0:
Version : 1.2
Creation Time : Thu Aug 31 17:20:07 2017
Raid Level : raid10
Array Size : 7813774336 (7451.80 GiB 8001.30 GB)
Used Dev Size : 3906887168 (3725.90 GiB 4000.65 GB)
Raid Devices : 4
Total Devices : 4
Persistence : Superblock is persistent
Intent Bitmap : Internal
Update Time : Fri Sep 1 21:57:22 2017
State : active
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Layout : near=2
Chunk Size : 512K
Name : Arcade:0 (local to host Arcade)
UUID : 5f57f3bc:5eaae757:3eed62b9:fb5e4848
Events : 11890
Number Major Minor RaidDevice State
0 8 16 0 active sync set-A /dev/sdb
1 8 32 1 active sync set-B /dev/sdc
2 8 48 2 active sync set-A /dev/sdd
3 8 64 3 active sync set-B /dev/sde
Am I correct in assuming it’s still working on the bitmap pages or is something going on I should be investigating?