Set up a secondary RAID-1 device

rmcconnell · 04-07-2017, 08:43 PM

I recently set up a RAID-1 device on a new computer using a pair of WD Blue SATA drives. The official documentation describes how to set up a bootable drive, so there are a few details missing for a secondary drive. Here is how I did it. Hopefully, this will help others avoid some of the mistakes I made. This mostly follows the RAID How-To for a while, but diverges where necessary.
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First, set up and copy the partition table. If your drives are larger than 2.3TB, you will have to use the gpt label type in cfdisk. DOS just won't handle it. These drives are all one partition, which is not bootable. The partition type is Linux Raid (86).

# cfdisk /dev/sdb
# fdisk -l /dev/sdb
# sfdisk -d /dev/sdb | sfdisk --force /dev/sdc
# fdisk -l

Next check to make sure there are no other multi-disk drives configured, then go ahead and configure yours. Make sure you get the correct device names. Since I am not using these as a boot device, I allowed it to use the newest edition of metadata.

# cat /proc/mdstat
# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
# cat /proc/mdstat
# mdadm --detail --scan > /etc/mdadm.conf

Now you need to create a mount point, add a line to fstab and format the drive, not necessarily in that order.

# mkfs.ext4 /dev/md0
# mkdir /raid0
# cd /etc
# joe fstab

Add this line to fstab:

/dev/md0 /raid0 ext4 defaults 1 2

Now you can mount your new RAID-1 drive.

# mount /dev/md0 /raid0

Finally, check the device status.

# cat /proc/mdstat

I was working with all new hardware, so I wasn't worried about losing any data. That made it a lot easier to try a few different options, and recover from a couple of mistakes before I allowed any data to be stored on it.

lazardo · 04-13-2017, 01:26 AM

next time

Code:

# mdadm --create /dev/md0 \
--chunk-size=128k --bitmap=internal --level=10 --layout=f2 \
--raid-devices=2 /dev/sdb1 /dev/sdc1

for about 2x streaming read bump wo write penalty.

Notes:
- do some homework on mdadam chunk size, but 128k performance is much better than the default.
- bitmaps significantly shorten reconstruction time if there is an issue.
- RAID10 can be used on a 2-disk array, 'f2' is the performance magic
- compare between 2-disk RAID1 (boot partition) and 2-disk RAID10-f2 (SSDs on SATA-6):

Code:

$ cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] 
md1 : active raid1 sdb1[1] sda1[0]
      102336 blocks [2/2] [UU]
      
md5 : active raid10 sdb5[1] sda5[0]
      207616768 blocks 128K chunks 2 far-copies [2/2] [UU]
      bitmap: 0/2 pages [0KB], 65536KB chunk

unused devices: <none>

Code:

$ sudo hdparm -tT /dev/md{1,5}

/dev/md1:
 Timing cached reads:   22882 MB in  2.00 seconds = 11453.19 MB/sec
 Timing buffered disk reads:  98 MB in  0.21 seconds = 460.36 MB/sec

/dev/md5:
 Timing cached reads:   22688 MB in  2.00 seconds = 11356.08 MB/sec
 Timing buffered disk reads: 2900 MB in  3.00 seconds = 966.04 MB/sec

Cheers,