-f, --force
Insist that mdadm accept the geometry and layout
specified without question. Normally mdadm will not
allow creation of an array with only one device, and
will try to create a raid5 array with one missing
drive (as this makes the initial resync work faster).
With --force, mdadm will not try to be so clever.
And indeed, when I created the array with 5 disks, it marked one as a spare:
# mdadm --detail /dev/md1
/dev/md1:
Version : 00.90.03
Creation Time : Sat Jan 27 13:30:36 2007
Raid Level : raid5
Array Size : 1953545984 (1863.05 GiB 2000.43 GB)
Device Size : 488386496 (465.76 GiB 500.11 GB)
Raid Devices : 5
Total Devices : 5
Preferred Minor : 1
Persistence : Superblock is persistent
Update Time : Sat Jan 27 13:52:08 2007
State : clean, degraded, recovering
Active Devices : 4
Working Devices : 5
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 64K
Rebuild Status : 27% complete
UUID : 5ad3ba82:30b256f3:c70f55c8:1f40abbd
Events : 0.194
Number Major Minor RaidDevice State
0 8 48 0 active sync /dev/sdd
1 8 64 1 active sync /dev/sde
2 8 80 2 active sync /dev/sdf
3 8 96 3 active sync /dev/sdg
5 8 112 4 spare rebuilding /dev/sdhAnd you can see that 4 disks are reading, and 1 is writing:
Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn ... sdd 198.00 45824.00 0.00 45824 0 sde 198.00 45824.00 0.00 45824 0 sdf 200.00 45824.00 0.00 45824 0 sdg 203.00 46080.00 0.00 46080 0 sdh 203.00 0.00 46080.00 0 46080 ...
Neat!
It makes sense why it's done this way: 4 disks doing sequential reads and 1 doing sequential writes is faster than 5 disks doing mixed reads and writes.
But really? 6 hours?
I'd prefer an option where all disks are zeroed, and then initial resync is skipped. Yes, array wouldn't be immediately usable like it is with the kernel doing the background sync for me, but I think I could zero a 460 GB disk quicker than 6 hours... based on 100 MB/s filesystem writes I saw, assuming I can do even more to a raw block device, should be about an hour) But I can't see how... --assume-clean may be what I'm looking for? Do I just zero all the devices myself first, then re-create the array?
I wouldn't normally mind, but I want to performance-test several configurations and 6 hour waits seriously kills my flow. :)
