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Stacknerd
Asked: 2015-01-05 15:25:37 +0800 CST

3disk in raid 1 using half/half

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Im using a mdadm software raid system on my new server which runs 3x120GB ssd drives.

Currently, this is the preinstalled configuration that came with the server 

cat /proc/mdstat
Personalities : [raid1] 
md2 : active raid1 sdb2[1] sda2[0] sdc2[2]
      96211904 blocks [3/3] [UUU]

md1 : active raid1 sda1[0] sdc1[2] sdb1[1]
      20478912 blocks [3/3] [UUU]

unused devices: <none>

I would like to have more space available than 120gb, because right now its using only 1 drive for the data, 1 in raid + the other 1 in spare backup.

My question goes as is : Is it possible to have 3 drives running a raid 1 ?

Example : 3x120GB = 360GB. Have 180GB available and 180GB in raid

Any help would be really appreciated.

For more information on my current setup to show the 120GB only 

df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs         16G  208K   16G   1% /dev
tmpfs            16G     0   16G   0% /dev/shm
/dev/md1         20G  1.3G   17G   7% /
/dev/md2         91G  200M   86G   1% /vz
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/etc/named
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/var/named
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/etc/named.conf
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/etc/named.rfc1912.zones
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/etc/rndc.key
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/usr/lib64/bind
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/etc/named.iscdlv.key
/dev/md1         20G  1.3G   17G   7% /var/named/chroot/etc/named.root.key
linux

2 Answers

  1. Best Answer

    It sounds like you want a RAID10 with two mirrors on your three disks. Perfectly normal setup with Linux software RAID.

    http://en.wikipedia.org/wiki/Linux_MD_RAID_10#Linux_MD_RAID_10

    It does exactly what you described. You decide how many copies (mirrors) of each data block you want, and it will stripe the data to fill all the disks.

    For example, if I have three devices (/dev/loop0, /dev/loop1, /dev/loop2), which are 100M each, I can accomplish what you suggested this way:

    > sudo mdadm --create --level raid10 -n 3 -p f2 /dev/md/test /dev/loop{0..2}
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md/test started.

    The -p f2 indicates that you want a "far" layout with "two" copies of each block device. And as you can see, the new RAID device is about 1.5 times the size of each disk:

    > sudo blockdev --getsize64 /dev/md/test /dev/loop{0..2} 
157286400
105906176
105906176
105906176
    • 1

  2. No, raid 1 requires an even number of disks. http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1

    • -1