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Take a look at the disk throughput of this two disk software RAID 1 array in a Linux 2.6.26-2-686 Debian system :

Disk throughput per device

As you can see, one device reads at a speed apparently close to its top throughput while the other writes likewise. What is happening ?

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10:44 root@kivu /var/log# grep resync *
dmesg:[    6.950037] md: resync of RAID array md0
dmesg:[    6.950037] md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for resync.
kern.log:Aug  1 18:18:25 kivu kernel: [    6.950037] md: resync of RAID array md0
kern.log:Aug  1 18:18:25 kivu kernel: [    6.950037] md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for resync.
kern.log:Aug  2 01:02:47 kivu kernel: [28104.777816] md: md0: resync done.

There - that was a RAID resynchronization, triggered shortly after a reboot.

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