I wanted to set up a RAID0 (striped array) on two HDDs to servce as cache for Duplicity backups. And I wanted to use GPT and only command line tools: sgdisk(8) for partitioning, and mdadm(8) for creating the software RAID. (I have usually just used Gparted, a GUI partitioning tool.)
All of this was done on Red Hat Enterprise Linux 6.5.
So, I have two (spinning disc) HDDs, each 931 GB, mounted as
First, zap any partitioning information they may have. (In all the examples below, "X" should be replaced by "a" or "b".)
# sgdisk -Z /dev/sdX
Next, partition them. The partitions have to be of type 0xFD00 "Linux RAID". You can do "sgdisk -L" to see a list of all available types. These type codes are not the same as the type codes used by fdisk(8).
The partitions will be 512 GB, leaving some for other uses.
# sgdisk -n 0:0:+512G -c 0:"cache" -t 0:0xFD00 /dev/sdX
# sgdisk -n 0:0:0 -c 0:"misc" /dev/sdX
The "0" first digit of the argument to "-n", "-c", and "-t" is shorthand for the first available partition number. In this case, the first line would be "1" and the second line would be "2".
In the second line, note that "-n 0:0:0" uses the default of starting at the first unallocated sector, and ending at the last allocateable sector on the drive, thereby using up the rest of the HDD for the "misc" partition 2. Leaving out the type specification, "-t", gives the default 0x8300 "Linux filesystem."
Print out the partition info to check:
# sgdisk -p /dev/sdX
Disk /dev/sdX: 1953525168 sectors, 931.5 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): xxxxxxxxxxxxxxx-xxx-xxxxxxxxxxxxxxxx
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 1953525134
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)
Number Start (sector) End (sector) Size Code Name
1 2048 1073743871 512.0 GiB FD00 cache
2 1073743872 1953525134 419.5 GiB 8300 misc
And we see that it has done what we expected.
Next, we create the RAID0 from /dev/sda1 and /dev/sdb1:
# mdadm -v -C /dev/md/mycomputer:cache -l stripe -n 2 /dev/sda1 /dev/sdb1
This creates a device /dev/md127 with a symbolic link for the readable name:
lrwxrwxrwx 1 root root 8 2017-10-18 18:11:31 -0400 /dev/md/mycomputer:cache -> ../md127
In mdadm v3.3.4 in RHEL6, I found that the name given to the "-C/--create" option would always end up being "mycomputer:something", where "something" was determined by what you actually give it. This name comes up after reboot.
The "-v" is for verbose output, "-l" is for the RAID level (which can be specified by integer, or string), "-n" is the number of devices, and the positional arguments are a list of the devices to be used.
Also, the integer N in /dev/mdN is determined by the system. It seems to start with 127.
For instance, doing
# mdadm -C /dev/md0
after rebooting gave this:
lrwxrwxrwx 1 root root 8 2017-10-18 18:11:31 -0400 /dev/md/mycomputer:0 -> ../md127
And doing
# mdadm -C /dev/md/cache
gave
lrwxrwxrwx 1 root root 8 2017-10-18 18:11:31 -0400 /dev/md/mycomputer:cache -> ../md127
I wised up on my third time through, and named it what it was going to pick, anyway.
The RAID needs to be "assembled" and activated at boot time. This is not done by default. To do this, a file /etc/mdadm.conf must be created. (Other distros may have a different location for this file.)
Assuming there is no such file, start by using mdadm(8) to output the array specification to the file:
# mdadm -Ds /dev/md/mycomputer:cache > /etc/mdadm.conf
# cat /etc/mdadm.conf
ARRAY /dev/md/mycomputer:cache metadata=1.2 name=mycomputer:cache UUID=xxxxxxxx
Very important: this UUID will not be the same as the UUID of the filesystem we will create later.
Add DEVICE, MAILADDR, and AUTO lines to /etc/mdadm.conf, resulting in:
DEVICE /dev/sda1 /dev/sdb1
MAILADDR [email protected]
AUTO +all
ARRAY /dev/md/mycomputer:cache metadata=1.2 name=mycomputer:cache UUID=xxxxxxxx
I did the next bit in single-user mode as I wanted this to be mounted as /var/cache, which is also used by several other things. Also, since it gets tiresome writing out the whole device name, I used the short name /dev/md127.
# telinit 1
# mkfs.ext4 /dev/md127
Next, I mounted the device in a temporary location to transfer the existing contents:
# mkdir /mnt/tmpmnt
# mount /dev/md127 /mnt/tmpmnt
# cd /var/cache
# tar cf - * | ( cd /mnt/tmpmnt ; tar xvf - )
Get the UUID of this new filesystem for use in /etc/fstab:
# blkid /dev/md127
yyyyyyyyy-yyyyyyyy-yyyyyyyyyy-yyyyyyyyy
And create an entry in /etc/fstab:
UUID=yyyyyyyyy-yyyyyyyy-yyyyyyyyyy-yyyyyyyyy /var/cache ext4 defaults 0 2
And reboot!
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