I wanted to create a RAID 5 array with four 1tb drives. I only have three, but I have a 750gb and 320gb drive lying around. I figured there was probably a way to combine them into a 1tb drive that I could use with the others.
Using Linux's LVM, I can create a logical partition from the two smaller drives as big as the 1tb drive.
$ pvdisplay
Lists all physical volumes managed by LVM. First we have to create the physical volumes for LVM. I prefer to create the volumes out of partitions, although you can do it from raw drives too.
First let's use fdisk to create "Linux LVM" partitions on the two drives.
$ fdisk /dev/sdb
Press "n" to create a new partition
Press "t" to set the partition type, and enter "8e" for Linux LVM.
When creating the partition, accepting the defaults will make it use the whole drive. I want to use the entire 750 drive and only part of the 320 drive, so that in total it has the same number of blocks as the 1tb drives. So I first created the "Linux RAID" partitions on the 1tb drives so I could see how many cylinders it listed, which ended up being 121601. So I created a partition the full size of the 750 drive (91201 cylinders), then created a 121601-91201 cylinder partition on the 320 drive.
$ fdisk -l /dev/sdb
Disk /dev/sdb: 320.0 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Device Boot Start End Blocks Id System
/dev/sdb1 1 30400 244187968+ 8e Linux LVM
/dev/sdb2 30401 38913 68380672+ 83 Linux
$ fdisk -l /dev/sdj
Disk /dev/sdj: 750.1 GB, 750156374016 bytes
255 heads, 63 sectors/track, 91201 cylinders
Device Boot Start End Blocks Id System
/dev/sdj1 1 91201 732572001 8e Linux LVM
Now that I have the two partitions ready, I moved on to LVM setup.
$ pvcreate /dev/sdb1
$ pvcreate /dev/sdj1
This sets up the two partitions as physical volumes for LVM.
Next is creating a logical volume group:
$ vgcreate vg_tb /dev/sdb1 /dev/sdj1
This creates a volume group called "vg_tb" using the two physical volumes sdb1 and sdj1.
Let's take a look at what we have so far:
$ pvdisplay
--- Physical volume ---
PV Name /dev/sdb1
VG Name vg_tb
PV Size 232.88 GB / not usable 832.50 KB
Allocatable yes
PE Size (KByte) 4096
Total PE 59616
Free PE 59616
Allocated PE 0
PV UUID 70hPKX-n11U-RcB6-0Kyt-1SOP-ni7E-2Y9hcE
--- Physical volume ---
PV Name /dev/sdj1
VG Name vg_tb
PV Size 698.64 GB / not usable 2.34 MB
Allocatable yes
PE Size (KByte) 4096
Total PE 178850
Free PE 178850
Allocated PE 0
PV UUID PzFb9b-lapG-KdT3-78nh-Gq75-F0Lo-I3xCrl
--- Physical volume ---
PV Name /dev/sda2
VG Name VolGroup00
PV Size 111.60 GB / not usable 2.86 MB
Allocatable yes
PE Size (KByte) 32768
Total PE 3571
Free PE 1
Allocated PE 3570
PV UUID 1wM65Z-3QGd-vDiq-mq1R-YhEE-Ackp-hh3g13
$ vgdisplay
--- Volume group ---
VG Name vg_tb
System ID
Format lvm2
Metadata Areas 2
Metadata Sequence No 1
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 0
Open LV 0
Max PV 0
Cur PV 2
Act PV 2
VG Size 931.51 GB
PE Size 4.00 MB
Total PE 238466
Alloc PE / Size 0 / 0
Free PE / Size 238466 / 931.51 GB
VG UUID olg9GP-x1sC-sFAD-TgWY-KIIx-YWNt-kL763n
--- Volume group ---
VG Name VolGroup00
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 3
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 2
Open LV 2
Max PV 0
Cur PV 1
Act PV 1
VG Size 111.59 GB
PE Size 32.00 MB
Total PE 3571
Alloc PE / Size 3570 / 111.56 GB
Free PE / Size 1 / 32.00 MB
VG UUID IZ25LV-oMOG-DwKK-QuFN-bqqp-ClUe-d71k5l
So far so good. The next step is to create a logical volume in the new volume group:
$ lvcreate vg_tb -n onetb -l 100%VG
Logical volume "onetb" created
This creates a new logical volume called "onetb" in the "vg_tb" group using 100% of the group's available space. Now let's take a look at the list of logical volumes:
$ lvdisplay
--- Logical volume ---
LV Name /dev/vg_tb/onetb
VG Name vg_tb
LV UUID sQKaq9-D6Mv-p8it-vWGW-O7DX-FGmC-cl5FSh
LV Write Access read/write
LV Status available
# open 0
LV Size 931.51 GB
Current LE 238466
Segments 2
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:2
--- Logical volume ---
LV Name /dev/VolGroup00/LogVol00
VG Name VolGroup00
LV UUID G5hlbb-tA3S-qhTS-03us-f9dl-1Vxy-9vDSU5
LV Write Access read/write
LV Status available
# open 1
LV Size 109.62 GB
Current LE 3508
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:0
--- Logical volume ---
LV Name /dev/VolGroup00/LogVol01
VG Name VolGroup00
LV UUID 7NZrY9-1wSJ-4fRp-VPnM-V07u-9rj8-vmtxTx
LV Write Access read/write
LV Status available
# open 1
LV Size 1.94 GB
Current LE 62
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:1
You can ignore all of the VolGroup00 things, those are the auto-created volumes from when I installed Fedora.
At this point, I have a new device at /dev/vg_tb/onetb which is the same size as my 1tb drives, and I can use it exactly as I would use the 1tb partition at /dev/sdf1.
Now it's time to create the RAID 5 array from these four volumes.
$ mdadm -v --create /dev/md1 --chunk=128 --level=5 --raid-devices=4 /dev/sdf1 /dev/sdh1 /dev/sdi1 /dev/vg_tb/onetb
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: size set to 976756608K
mdadm: array /dev/md1 started.
The array will begin syncing, and you can watch it by running:
$ watch -n 1 "cat /proc/mdstat"
Personalities : [raid6] [raid5] [raid4]
md1 : active raid5 dm-2[4] sdi1[2] sdh1[1] sdf1[0]
2930269824 blocks level 5, 128k chunk, algorithm 2 [4/3] [UUU_]
[=>...................] recovery = 5.5% (54017536/976756608) finish=593.1min speed=25925K/sec
md0 : active raid5 sdg1[0] sdc1[3] sde1[2] sdd1[1]
2197715712 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]
unused devices: <none>
While this is syncing, we can create the ext3 filesystem.
$ mke2fs -j -b 4096 -m 0 -E stride=32,stripe-width=96 /dev/md1
This creates an ext2 filesystem with journaling (ext3), the block size is 4kb, and 0% of the blocks are reserved for the superuser. The stride is calculated as the raid block size / ext2 block size (128k / 4k = 32). The stripe width is calculated as the stride value times the number of data disks in the array. In a 4-disk RAID 5 array, there are three data disks, one being for the parity data.
This will take some time, and will significantly slow down the sync process. Mine dropped from 25mb/s to around 1mb/s. I figure I'll let it create the filesystem so I can start copying data to it right away, and let it finish its sync on its own time.
While you're at it, you should set up munin to monitor the SMART data from all the drives as well as the status of the array using my munin-raid-monitor plugin.
Additional Reading:
- http://linux-raid.osdl.org/index.php/RAID_setup
- http://bfish.xaedalus.net/2006/11/software-raid-5-in-ubuntu-with-mdadm/
- http://www.linuxquestions.org/questions/linux-hardware-18/how-can-i-override-the-5.00-reserved-for-the-super-user-mkfs.ext3-creates-616546/
- http://www.linuxconfig.org/Linux_lvm_-_Logical_Volume_Manager
- http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/LV_create.html
