204.1. Configuring RAID

204.1 Configuring RAID

Weight: 3

Description: Candidates should be able to configure and implement software RAID. This objective includes using and configuring RAID 0, 1 and 5.

Key Knowledge Areas:

  • Software raid configuration files and utilities

Terms and Utilities:

  • mdadm.conf

  • mdadm

  • /proc/mdstat

  • partition type 0xFD

What is RAID?

RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a way of storing the same data in different places on multiple hard disks to protect data in the case of a drive failure. However, not all RAID levels provide redundancy.

How RAID Works?

RAID works by placing data on multiple disks and allowing input/output (I/O) operations to overlap in a balanced way, improving performance. Because the use of multiple disks increases the mean time between failures (MTBF), storing data redundantly also increases fault tolerance.

RAID arrays appear to the operating system (OS) as a single logical hard disk. RAID employs the techniques of disk mirroring or disk striping.

  • Mirroring copies identical data onto more than one drive.

  • Striping partitions each drive's storage space into units ranging from a sector (512 bytes) up to several megabytes(designed based on needs of a system). The stripes of all the disks are interleaved and addressed in order.

Parity is a concept, which is also streamlined into RAID technology as another way of storage method. It involves saving of information across the disk arrays, so that, the same information can be used to recreate or reconstruct the affected data, which is otherwise filled with errors or data loss, when disk drive fails.

Disk mirroring and disk striping can be combined on a RAID array.

RAID Controller

A RAID controller can be used as a level of abstraction between the OS and the physical disks, presenting groups of disks as logical units. Using a RAID controller can improve performance and help protect data in case of a crash.

A RAID controller can be used in both hardware- and software-based RAID arrays.

  • In a hardware-based RAID product, a physical controller manages the array. When in the form of a Peripheral Component Interconnect or PCI Express card, the controller can be designed to support drive formats such as SATA and SCSI. A physical RAID controller can also be part of the motherboard.

  • With software-based RAID, the controller uses the resources of the hardware system. While it performs the same functions as a hardware-based RAID controller, software-based RAID controllers may not enable as much of a performance boost.

RAID Levels

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance. The different data distribution layouts, are named by the word "RAID" followed by a number, for example RAID 0 or RAID 1. Each RAID level, provides a different balance among reliability, availability, performance, and capacity.

  • When RAID was introduced first in 1988 six different RAID Levels were introduced, known as "standrad RAID levels".

  • As time passes "Nested RAID Levels" are created based on a combination of "Standard RAID Levels"

  • and then there are some " Nonstandard RAID levels". Like RAID7 that are not popular and we don't talk about here.

RAID levels comparison chart:

RAID LEVELS

RAID0

RAID1

RAID5

RAID6

RAID10

Min # of Disks

2

2

3

4

4

Fault Tolerance

None

1 disk

1 disk

2 disks

Up to one disk failure in each sub-array

Disk Space Over-Head

None

50%

1 disk

2 disks

50%

Read Speed

fast

fast

slow

slow

fast

Write Speed

fast

fair

slow

slow

fair

Cost

cheap

high(disks)

high

very high

high(disks)

Usage

High End Workstations, data logging, real-time rendering, very transitory data

Operating System, transaction databases

Data warehousing, web serving, archiving

Data archive, backup to disk, high availability solutions, servers with large capacity requirements

Fast databases, application servers

For lpic2-202 examp we just expected to know about RAID Levels 0,1 and 5. Obviously we do not talk about any hardware-based RAID controller and we talk about software-raid in linux.

/proc/mdstat

/proc/mdstat provides a way to check the state of the md driver. It also shows information about software RAID Array if exists.

root@server1:~# cat /proc/mdstat
Personalities :
unused devices: <none>

Lets start creating a RAID Array, consist of two disk Drives with RAID level0:

root@server1:~# ls /dev/sd*
/dev/sda /dev/sda1 /dev/sda2 /dev/sda5 /dev/sdb /dev/sdc

partition type 0xFD

Each Disk Drive need to be formated with the special partition type 0xFD before play a role in RAID Array. This separating has some benefits, first if these disk drives moved to another system, operating system read this format and realize that its dealing with a RAID Array.

root@server1:~# fdisk /dev/sdb
Welcome to fdisk (util-linux 2.27.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-20971519, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-20971519, default 20971519):
Created a new partition 1 of type 'Linux' and of size 10 GiB.
Command (m for help): t
Selected partition 1
Partition type (type L to list all types): l
0 Empty 24 NEC DOS 81 Minix / old Lin bf Solaris
1 FAT12 27 Hidden NTFS Win 82 Linux swap / So c1 DRDOS/sec (FAT-
2 XENIX root 39 Plan 9 83 Linux c4 DRDOS/sec (FAT-
3 XENIX usr 3c PartitionMagic 84 OS/2 hidden or c6 DRDOS/sec (FAT-
4 FAT16 <32M 40 Venix 80286 85 Linux extended c7 Syrinx
5 Extended 41 PPC PReP Boot 86 NTFS volume set da Non-FS data
6 FAT16 42 SFS 87 NTFS volume set db CP/M / CTOS / .
7 HPFS/NTFS/exFAT 4d QNX4.x 88 Linux plaintext de Dell Utility
8 AIX 4e QNX4.x 2nd part 8e Linux LVM df BootIt
9 AIX bootable 4f QNX4.x 3rd part 93 Amoeba e1 DOS access
a OS/2 Boot Manag 50 OnTrack DM 94 Amoeba BBT e3 DOS R/O
b W95 FAT32 51 OnTrack DM6 Aux 9f BSD/OS e4 SpeedStor
c W95 FAT32 (LBA) 52 CP/M a0 IBM Thinkpad hi ea Rufus alignment
e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a5 FreeBSD eb BeOS fs
f W95 Ext'd (LBA) 54 OnTrackDM6 a6 OpenBSD ee GPT
10 OPUS 55 EZ-Drive a7 NeXTSTEP ef EFI (FAT-12/16/
11 Hidden FAT12 56 Golden Bow a8 Darwin UFS f0 Linux/PA-RISC b
12 Compaq diagnost 5c Priam Edisk a9 NetBSD f1 SpeedStor
14 Hidden FAT16 <3 61 SpeedStor ab Darwin boot f4 SpeedStor
16 Hidden FAT16 63 GNU HURD or Sys af HFS / HFS+ f2 DOS secondary
17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs fb VMware VMFS
18 AST SmartSleep 65 Novell Netware b8 BSDI swap fc VMware VMKCORE
1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid fd Linux raid auto
1c Hidden W95 FAT3 75 PC/IX bc Acronis FAT32 L fe LANstep
1e Hidden W95 FAT1 80 Old Minix be Solaris boot ff BBT
Partition type (type L to list all types): fd
Changed type of partition 'Linux' to 'Linux raid autodetect'.
Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

and lets do it for another Disk Drive:

root@server1:~# fdisk /dev/sdc
Welcome to fdisk (util-linux 2.27.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0xbc1e0832.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-20971519, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-20971519, default 20971519):
Created a new partition 1 of type 'Linux' and of size 10 GiB.
Command (m for help): t
Selected partition 1
Partition type (type L to list all types): fd
Changed type of partition 'Linux' to 'Linux raid autodetect'.
Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.
root@server1:~# ls /dev/sd* -l
brw-rw---- 1 root disk 8, 0 Jan 6 21:26 /dev/sda
brw-rw---- 1 root disk 8, 1 Jan 6 21:26 /dev/sda1
brw-rw---- 1 root disk 8, 2 Jan 6 21:26 /dev/sda2
brw-rw---- 1 root disk 8, 5 Jan 6 21:26 /dev/sda5
brw-rw---- 1 root disk 8, 16 Jan 6 22:14 /dev/sdb
brw-rw---- 1 root disk 8, 17 Jan 6 22:14 /dev/sdb1
brw-rw---- 1 root disk 8, 32 Jan 6 22:15 /dev/sdc
brw-rw---- 1 root disk 8, 33 Jan 6 22:15 /dev/sdc1

okey time to use Multiple Devices(Disk) administration tool to create RAID Array.

mdadm

Linux Software RAID devices are implemented through the md (Multiple Devices) device driver. mdadm is a tool for creating, managing, and monitoring RAID devices using the md driver in Linux. based on your distro you might need to install mdadm:

root@server1:~# apt install mdadm
root@server1:~# mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sdb1 /dev/sdc1
mdadm: chunk size defaults to 512K
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

Now /dev/md0 is created, consist of two 10 Gigabytes Disk Drives with RAID level 0.

root@server1:~# ls /dev/md*
/dev/md0
root@server1:~# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdb 8:16 0 10G 0 disk
└─sdb1 8:17 0 10G 0 part
└─md0 9:0 0 20G 0 raid0
sr0 11:0 1 1024M 0 rom
sdc 8:32 0 10G 0 disk
└─sdc1 8:33 0 10G 0 part
└─md0 9:0 0 20G 0 raid0
sda 8:0 0 50G 0 disk
├─sda2 8:2 0 1K 0 part
├─sda5 8:5 0 1021M 0 part [SWAP]
└─sda1 8:1 0 49G 0 part /
root@server1:~# ls /dev/md*
/dev/md0
root@server1:~# fdisk /dev/md0
Welcome to fdisk (util-linux 2.27.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x5b547cee.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-41906175, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-41906175, default 41906175):
Created a new partition 1 of type 'Linux' and of size 20 GiB.
Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

lets take a look at mdstat:

root@server1:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid0 sdc1[1] sdb1[0]
20953088 blocks super 1.2 512k chunks
unused devices: <none>

some usefull switches of mdadm

mdadm switch

Description

-v

--verbose

-C /dev/md01

--create /dev/md01

-l2

--level=2

-n2

--raid-devices=2

-x 3 /dev/sdX1 /dev/sdY1 /dev/sdZ1

--spare-device=3 /dev/sdX1 /dev/sdY1 /dev/sdZ1

mdadm.conf

Normally Linux system doesn’t automatically remember all the components that are part of the RAID set. This information has to be added it on mdadm.conf file under /etc directory. It helps to start, rebuild,re-activate the raid etc.., by default, the file will not be available, it has to be created manually.

Following command scan the availbe RAID levels on the system:

root@server1:~# mdadm --detail --scan
ARRAY /dev/md0 metadata=1.2 name=server1:0 UUID=f34215ab:8f1bf031:5597e3ff:02303fc7

and for more details about specific RAID Array:

root@server1:~# mdadm --detail /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Wed Jan 10 03:41:35 2018
Raid Level : raid0
Array Size : 20953088 (19.98 GiB 21.46 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Wed Jan 10 03:41:35 2018
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Chunk Size : 512K
Name : server1:0 (local to host server1)
UUID : f34215ab:8f1bf031:5597e3ff:02303fc7
Events : 0
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 33 1 active sync /dev/sdc1

and finally lets add configuration to mdadm.conf:

root@server1:~# mdadm --detail --scan >> /etc/mdadm/mdadm.conf
root@server1:~# cat /etc/mdadm/mdadm.conf
# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#
# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
#DEVICE partitions containers
# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes
# automatically tag new arrays as belonging to the local system
HOMEHOST <system>
# instruct the monitoring daemon where to send mail alerts
MAILADDR root
# definitions of existing MD arrays
# This file was auto-generated on Sat, 06 Jan 2018 22:23:08 -0800
# by mkconf $Id$
ARRAY /dev/md0 metadata=1.2 name=server1:0 UUID=f34215ab:8f1bf031:5597e3ff:02303fc7

and thats all our seetings have become persistent. Some other usefull mdadm commands:

Remove a Disk From Array

We can’t remove a disk directly from the array, unless it is failed, so we first have to fail it( if its not Failed normally):

mdadm --fail /dev/md0 /dev/sdb1
mdadm --remove /dev/md0 /dev/sdb1

Add a Disk to an Existing Array

mdadm --add /dev/md0 /dev/sdb1

Stop and Delete a Raid Array

mdadm --stop /dev/md0
mdadm --remove /dev/md0

and finally we can even delete the superblock from each drive

mdadm --zero-superblock /dev/sdb
mdadm --zero-superblock /dev/sdc

that all people.