I have a USB caddy that takes 4 drives, on USB3. I have four 2TB SATA disks in it and have just set up a RAID5 array across the four disks, formatted to ext4.
cat /proc/mdstat currently reports: md1 : active raid5 sdi1[4] sdh1[2] sdg1[1] sdf1[0] 5860145664 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_] [>....................] recovery = 1.2% (24142492/1953381888) finish=1134.2min speed=28348K/sec
That's about 20hrs to complete the build, running at max speed (/proc/sys/dev/raid/speed_limit_max is 500000 so I'm not hitting the upper limit there).
To be honest, rebuilding an array comprising 4x2TB disks over USB in 20hrs seems pretty good to me, so I'm not overly concerned. But should I be? Is running this configuration likely to cause me grief in future?
The purpose of the array is to store backups, so read/write speed is relatively unimportant. At this point the array is empty so wiping and starting again isn't a big deal if someone has better ideas.
(As an aside, does the above look right for USB3 speeds? It's my first experience of USB3 and I'm using a cheap PCI-E USB3 card so I have no idea how well it's performing.)
Mark