[ ... ]
> abyss ~ # dd bs=1M count=2024 if=/dev/zero of=/dev/sdd conv=fdatasync
> 2024+0 records in
> 2024+0 records out
> 2122317824 bytes (2,1 GB, 2,0 GiB) copied, 14,1238 s, 150 MB/s
> abyss ~ # dd bs=1M count=2024 if=/dev/zero of=/dev/sdc conv=fdatasync
> 2024+0 records in
> 2024+0 records out
> 2122317824 bytes (2,1 GB, 2,0 GiB) copied, 14,7046 s, 144 MB/s
[ ... ]
> mdadm: automatically enabling write-intent bitmap on large array
[ ... ]
> abyss ~ # dd bs=1M count=2024 if=/dev/zero of=/dev/md/md6to conv=fdatasync
> 2024+0 records in
> 2024+0 records out
> 2122317824 bytes (2,1 GB, 2,0 GiB) copied, 21,1528 s, 100 MB/s
[ ... ]
This is not about "performance" but raw speed in a very
particular situation. The difference is not large and could be
meaningless for a variety of reasons:
* Had the initial 6TB sync finished (would take 15-17 hours)?
* Internal bitmaps can steal a fair bit of speed.
* For whatever weirdness is related to the appalling
implementation of the Linux page cache, IO reading (but less
so writing) an MD block device is often slower than to a file
on a filetree on that block device, plus writing via the page
cache costs a lot of CPU time.
Replication of the experiment on similar smaller disks:
soft# mdadm --create --verbose /dev/md/md6to --level=1 --raid-devices=2 /dev/sde3 /dev/sdf3
mdadm: size set to 243269440K
mdadm: array /dev/md/md6to started.
soft# grep -A2 'sd[ef]3' /proc/mdstat
md127 : active raid1 sdf3[1] sde3[0]
243269440 blocks super 1.0 [2/2] [UU]
[====>................] resync = 20.5% (49954880/243269440) finish=19.5min speed=164445K/sec
soft# grep -A2 'sd[ef]3' /proc/mdstat; dd bs=1M count=2024 if=/dev/zero of=/dev
/md/md6to conv=fdatasync
md127 : active raid1 sdf3[1] sde3[0]
243269440 blocks super 1.0 [2/2] [UU]
2024+0 records in
2024+0 records out
2122317824 bytes (2.1 GB) copied, 13.8561 s, 153 MB/s
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