>I'll just call it sync access pattern overhead then.
As another data point, I've been adding more and more
drives to a RAID-1 array. Yesterday I just added a fourth
disk which is still syncing.
mdadm --grow /dev/md0 -n4
mdadm --manage /dev/md0 --add /dev/sde
md0 : active raid1 sde1[4] sdc1[0] sdb1[2] sdd1[1]
488383936 blocks [4/3] [UUU_]
[=====>...............] recovery = 27.6% (134963968/488383936) finish=1739.7min speed=3384K/sec
In general I'm seeing somewhere between 2 to 4 MB/s on the currently
running sync. Which is fine, no problem. The array is live and
currently handling some traffic. But these are pretty fast disks, and
running simple things like
hdparm -tT /dev/sde
nice dd_rescue /dev/md0 /dev/null
all show pretty big numbers. This provides some intuition that there
may be room for improvement in sync speed. Kernel 2.6.14, no fancy
RAID bitmaps involved.
Jeff
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