by May 5, 2003 0 comments



Last month we talked about the latest release of SCOLinux 4 (Powered by United Linux), its key features and installation. We touched upon a core feature of the OS for implementing Large Volume Groups over RAID (Redundant Array of Inexpensive Disks). This month, we’ll look at it in more detail. While implementing it is pretty easy, it requires a thorough understanding of RAID concepts and large volumes. 

RAID 6?
Recently, non-official designations such as RAID 6 have been offered at the very high-end of the market. These are just RAID 5 implementations but with multiple parity writes, so that the array can withstand the failure of multiple drives simultaneously. Obviously, only very high-end systems need such redundancy. 

Large volumes
Generally, we assume that a disk partition size is static and we can add a fixed amount of data to it. LVM (Large Volume Manager), however, provides a method by which you can add new disk volumes to a pre-created partition. For instance, suppose you have a /local partition of 1 GB, which contains all data of your local users, and the number of users increases and you need to add another GB of space to accommodate their data. The traditional method will be to pick up a 2 GB hard drive and copy all data from the /local partition to it and finally mount it as /local. Instead, if you use LVM, the only thing you have to do is insert a new 1 GB hard drive in your machine and add its storage units to the /local partition. This helps you grow your volumes significantly. 

Expert Practitioner shows the list of partitions in your system

RAID
RAID is an array of disks used either to provide redundancy or enhance performance or do both, depending upon the level you implement. There are five RAID levels, and each varies in the features and functionality it provides. SCO Linux allows you to create Level 0 (Strip sets), 1 (Mirrors) and 5 (Striped Set(s) with Parity). The striping or level 0 is a performance enhancement mechanism, which increases the read and write speed of data proportionally to the number of disks (Minimum
2) by simultaneously writing an equally divided chunk of data in every disk. In mirroring or level 1, all data is rewritten in a twin disk so that if any disk gives an error, the data can be retrieved from the other disk. But this slows down the machine as the data needs to be written twice. The stripe set with parity or level 5 is a combination of striping (level 0) and parity. Here, parity is a piece of information calculated for each byte of data by mathematical property of XOR (Exclusive OR) and stored in a drive which can be used for generating the data back in case of any error. This combination reduces the overall read and write time of the disk array and also provides a secured data storage system. This feature makes it the most popular implemented RAID type. It needs at least three disks to work. 

By combining RAID with Large Volume feature, the OS is supposed to provide a really fail-safe, robust and flexible disk storage system. It will be fast, easily upgradeable and secure. We configured it using the YaST configuration tool in SCO Linux, and it worked well.

Volume group on RAID 5 
You’ll need at least three hard drives with enough unformatted space. First, create a RAID partition using YaST2. You can easily run it by typing “yast2” from the terminal in GNOME or KDE. Then, select System from the right-hand side of the form that pops up and click on Partitioner from the Modules List and press Enter. This pops up a warning message, which can be safely ignored and you can proceed. It will then launch Expert
Partitioner.

Next, click on the Create button and it will pop a list of hard drives present in your system. Select them one by one and press OK. It will then ask you whether you want to create a primary or extended partition. Select Primary partition and hit Enter. It then pops up another form. 

Check the ‘do not format’ option and select ‘0xFD Linux RAID’ for File system ID, and press OK. It will create the basic RAID partitions. In the Expert Partition screen, click on the RAID button. This will drop down a menu, from where you select Create RAID. It will then ask for the RAID type (0, 1 or 5). Select RAID 5 and click on the Next button and add all the partitions to the RAID device by first selecting the partition and then clicking on the Add button. Finally, click on the Next button. It will ask for the file system type and mount point of the RAID device. Select ‘do not format’, leave the other fields unchanged and click on Finish.

Configure large volumes
Now, close the Partitioner by applying the changes and go back to the YaST dialogue box. Then select LVM, and click Launch. It will then start up the LVM dialogue box. 

This will show two columns, one for physical and the other for logical volumes. You’ll find that the newly created RAID volume is present in the physical volume list. Add this volume to the default volume group (system) by clicking on Add Volume button. Here you can add as many Volumes as you have. These volumes can be normal physical unformatted partitions or RAID devices. Finally, click on the Add button below the logical volumes column and define a mount point and file system type of the volume and click OK. You will find a new logical volume is created for your RAID device.

To add a new physical volume to your newly created logical volume, add the volume to your volume group and click on Edit.

This will pop up a dialogue box. Click on the “max” button just below the partition size field. And the new volume’s size will be added to the logical volume without damaging your data at all.

The feature is quite useful and works very well. 

Anindya Roy

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