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RAID levels continued
RAID 1:
This level is also called mirroring. This level provides redundancy by writing same data to all the member disks. So we get mirror copy on every disk. This level works with two or more disks; those have parallel access of data at high transfer rates. When RAID 1 is used for two separate hard disks, in that case we can use either of the disks. If one disk fails, the other disk is ready to work. The disadvantage of RAID 1 is that it takes long to write data to disk. This version of RAID is secure but expensive too. This comes at a high cost because we write the same data to all the disks in array, which is wastage of disk space. Suppose, we have setup (/) root partition with RAID level 1. If it was of 50 GB in one disk, now with RAID 1 it will be consuming 100 GB. It means we are able to use only 50% of the disk space.
RAID 4:
This RAID level uses parity bit to protect the data. All the parity bits are stored on separate disk. This option is not allowed in Red Hat Enterprise installations, as it takes the same amount of space as in RAID 5 ( discussed ahead ). Yet RAID 5 has more advantages and that is why RAID 4 is not supported in Enterprise Linux.
Parity is generated on the basis of contents (i.e. the data ) of other disks in array. This parity information is used to reconstruct the data if any one of the disks in array is failed.
The RAID 4 is generally used with accompanying technologies like write back caching. It is better suited with transaction I/O rather than large data transfers. Capacity in RAID 4 is equal to the capacity of member disks ( or partitions ) minus capacity of one disk (or partition ).
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