In this article I will share the steps to configure software raid i.e. linear raid which is pretty much similar to RAID 0 with little difference which we will go through in the below chapters. I have written another article with comparison and difference between various RAID types using figures including pros and cons of individual RAID types so that you can make an informed decision before choosing a RAID type for your system.
RAID 0 stripping vs Linear RAID
If you run multiple disks in a non-redundant configuration, and you want to be able to recover something when any of the disks fails, then, at the very least, you need to ensure that your data is not striped over all devices. When using a striped layout, parts of most files will end up on all disks, which means that when you lose any disk, you've irrevokably lost parts of nearly every file, not to mention parts of the file-system structures themselves. This means, as you seem to already surmise, that you need to use linear allocation, which essentially appends the constituent devices' extents together, and gives you some chance of recovering files that exist entirely on the surviving device(s) after a device failure.
MD_SB_DISK, as defined by the kernel. By default, that maximum is 27, but since arrays can also act as member disks, this limit is avoidable. In addition, a maximum of 256 software RAID devices are available for use.
Linear (Append) Mode
Linear RAID requires a minimum of two disks, but does not require that member disks be the same size or type. Since the system writes to each disk until it is full, the speed and size of individual disks is largely irrelevant, in terms of aggregate RAID performance.
What is Chunk Size in Linear RAID?
chunk-size would normally define the number of kilobytes to write to each member disk for arrays that support disk striping. However, when working with linear raid arrays, the chunk-size defines the rounding factor. Each component disk is sized so that it is a multiple of the rounding factor. Because of the way the RAID superblock is placed on each array member, rounding factors of less than 64 KB are effectively equal to 64 KB.
Create Linear RAID Array
Currently I have added two virtual disks (/dev/sdb and /dev/sdc) to my virtual machine for the demonstration of this article. There are no partitions available on these disks which we will create in the upcoming chapters.
[root@node1 ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 30G 0 disk
├─sda1 8:1 0 512M 0 part /boot
└─sda2 8:2 0 27.5G 0 part
├─centos-root 253:0 0 25.5G 0 lvm /
└─centos-swap 253:1 0 2G 0 lvm [SWAP]
sdb 8:16 0 4G 0 disk
sdc 8:32 0 4G 0 disk
sr0 11:0 1 1024M 0 rom
Important Rules of partitioning
Here are some points to remember when you are partitioning.
- If you don’t require autodetection and plan to use whole disks as array members, you don’t need to partition member disks individually.
- Set partitions to type Linux Raid Auto (
0xFD) if you want the kernel to automatically start arrays at boot time. Otherwise, leave them as Linux (0x83). - RAID-0 and linear raid arrays can contain partitions that have varying sizes without losing any disk space. Remember that when the smaller disks that belong to a RAID-0 become full, only the remaining disks are striped. So you might see variable performance on a RAID-0 with member disks of differing sizes as the array fills up.
- Using matched drives is strongly recommended when working with any nonlinear raid array.
Partitioning with fdisk
You don’t need to partition disks before using them in an array, but partitioning does provide a couple of advantages.
- Partitioning is necessary if you want the kernel to automatically start arrays, because the md driver uses the partition type to identify member disks.
- md devices don’t support partitioning directly, but in some cases, having a filesystem that spans an entire array is undesirable. Using software RAID for system partitions means that smaller partitions are necessary. After all, you don’t want /var or
/bootto span a whole array.
/dev/sda, for example). This means that you won’t be able to autostart arrays, however, so you’ll have to include commands to start md devices in your system initialization scripts.Next we will start creating partitions on our disks /dev/sdb and /dev/sdc using fdisk
[root@node1 ~]# fdisk /dev/sdb Welcome to fdisk (util-linux 2.23.2). 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 Building a new DOS disklabel with disk identifier 0xbea244e4. Command (m for help): n Partition type: p primary (0 primary, 0 extended, 4 free) e extended Select (default p): p Partition number (1-4, default 1): 1 First sector (2048-8388607, default 2048): ⇐ We pressed Enter leaving it to the default value Using default value 2048 Last sector, +sectors or +size{K,M,G} (2048-8388607, default 8388607): ⇐ We pressed Enter leaving it to the default value Using default value 8388607 Partition 1 of type Linux and of size 4 GiB is set Command (m for help): t ⇐ Change the partition type Selected partition 1 Hex code (type L to list all codes): l ⇐ List the available partition types 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 C: 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 eb BeOS fs e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a5 FreeBSD ee GPT f W95 Ext'd (LBA) 54 OnTrackDM6 a6 OpenBSD ef EFI (FAT-12/16/ 10 OPUS 55 EZ-Drive a7 NeXTSTEP f0 Linux/PA-RISC b 11 Hidden FAT12 56 Golden Bow a8 Darwin UFS f1 SpeedStor 12 Compaq diagnost 5c Priam Edisk a9 NetBSD f4 SpeedStor 14 Hidden FAT16 <3 61 SpeedStor ab Darwin boot f2 DOS secondary 16 Hidden FAT16 63 GNU HURD or Sys af HFS / HFS+ fb VMware VMFS 17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs fc VMware VMKCORE 18 AST SmartSleep 65 Novell Netware b8 BSDI swap fd Linux raid auto 1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid fe LANstep 1c Hidden W95 FAT3 75 PC/IX be Solaris boot ff BBT 1e Hidden W95 FAT1 80 Old Minix Hex code (type L to list all codes): fd ⇐ Change the partition type of selected partition to fd Changed type of partition 'Linux' to 'Linux raid autodetect' Command (m for help): w ⇐ Save the changes The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks.
Next we will repeat the same steps for /dev/sdc
[root@node1 ~]# fdisk /dev/sdc Welcome to fdisk (util-linux 2.23.2). 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 Building a new DOS disklabel with disk identifier 0xe215a659. Command (m for help): n Partition type: p primary (0 primary, 0 extended, 4 free) e extended Select (default p): p Partition number (1-4, default 1): First sector (2048-8388607, default 2048): Using default value 2048 Last sector, +sectors or +size{K,M,G} (2048-8388607, default 8388607): Using default value 8388607 Partition 1 of type Linux and of size 4 GiB is set Command (m for help): t Selected partition 1 Hex code (type L to list all codes): fd Changed type of partition 'Linux' to 'Linux raid autodetect' Command (m for help): p Disk /dev/sdc: 4294 MB, 4294967296 bytes, 8388608 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk label type: dos Disk identifier: 0xe215a659 Device Boot Start End Blocks Id System /dev/sdc1 2048 8388607 4193280 fd Linux raid autodetect Command (m for help): w The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks.
Update the partition table in the kernel.
[root@node1 ~]# partprobe
Now list the available partitions on your node and verify the changes. So now we have two new partitions /dev/sdb1 and /dev/sdc1 for setting up linear mode software raid.
[root@node1 ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 30G 0 disk
├─sda1 8:1 0 512M 0 part /boot
└─sda2 8:2 0 27.5G 0 part
├─centos-root 253:0 0 25.5G 0 lvm /
└─centos-swap 253:1 0 2G 0 lvm [SWAP]
sdb 8:16 0 4G 0 disk
└─sdb1 8:17 0 4G 0 part
sdc 8:32 0 4G 0 disk
└─sdc1 8:33 0 4G 0 part
sr0 11:0 1 1024M 0 rom
Create Linear Software RAID
To create a software raid using linear mode execute below command
[root@node1 ~]# mdadm -Cv -llinear -n2 /dev/md0 /dev/sd{b,c}1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
OR you can also use the long version of this command to create linear raid
# mdadm --create --verbose --level=linear --raid-devices=2 /dev/md0 /dev/sdb1 /dev/sdc1
Here,
-C, --create Create a new array. -v, --verbose Be more verbose. -l, --raid-level Select the RAID level: linear, 0, 1, 4, or 5. -n, --raid-disks Set the number of member disks in the array.
mdadm automatically activate newly created linear raid arrays. Information about the array and its member disks is now available via the /proc/mdstat pseudo file.
[root@node1 ~]# cat /proc/mdstat
Personalities : [linear]
md0 : active linear sdc1[1] sdb1[0]
8380416 blocks super 1.2 0k rounding
unused devices: <none>
Create filesystem
Next, create a file-system on the new software raid array. We will create ext4 filesystem on our linear raid array
[root@node1 ~]# mkfs.ext4 /dev/md0
mke2fs 1.42.9 (28-Dec-2013)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
524288 inodes, 2095104 blocks
104755 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=2145386496
64 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
Create mount point
Create a mount point for accessing the software raid array
[root@node1 ~]# mkdir /linear_raid
Next mount the raid array on the created directory
[root@node1 ~]# mount /dev/md0 /linear_raid/
Check the mount status which also gives more details about mount point, available space etc details.
[root@node1 ~]# df -h /linear_raid/ Filesystem Size Used Avail Use% Mounted on /dev/md0 7.8G 36M 7.3G 1% /linear_raid
Next add an entry for the array to /etc/fstab file so it will be mounted automatically when the system restarts.
[root@node1 ~]# tail -n 1 /etc/fstab /dev/md0 /linear_raid ext4 defaults 0 0
/etc/fstab entry could not be properly checked and mounted. So if the kernel doesn’t automatically start your array, an entry in /etc/fstab might be preventing the system from booting successfully. It’s a good idea to place commands that will manually start arrays in your initialization scripts before filesystems are checked and mounted, even if you’re already successfully using autodetection. This will provide additional stability and, at worst, display some innocuous warnings on the console.
Verify the software raid changes
Post reboot verify the raid status
[root@node1 ~]# cat /proc/mdstat
Personalities : [linear]
md0 : active linear sdc1[1] sdb1[0]
8380416 blocks super 1.2 0k rounding
unused devices: <none>
Lastly I hope the steps from the article to configure Software RAID using Linear RAID on Linux was helpful. So, let me know your suggestions and feedback using the comment section.
References:
Managing RAID in Linux
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