Everytime I have to rebuild a Dell Server, it's already been decided which RAID configuration to use. Well I just want to know why they chose that RAID configuration and what is it for?
RAID is Redundant Array of Independent Disks and the main purpose for any chosen configuration is to 1) increase data reliability and 2) increase input/output performance. I'll put in a third reason 3) to make a hot swappable drive in case one of them fails.
Here is an example of an old beat up Dell 2850 with six perfectly salvagable drives I had to swap out and rebuild on another unit. Counting from the left, the first two drives are 73GB top and bottom. The remainder 300GB drives are in the in remainder four slots. Order matters. Considering labeling them in a numbered sequence. The first two drives are mirrored logical drives in a RAID 1 configuration; the last four drives are the physical drives in a RAID 5 configuration.
At the BIOS setup, you just choose the RAID for each drive in the bay array, but what does it all mean?
Settings available:
RAID level: 0, 1, 5, 10, or 50
RAID status of the virtual disk: failed, degraded, or optimal
Size of the virtual disk
Stripe element size
Operation currently in progress
RAID 0
* Striped copy
* data split evenly across disks
* no data redundancy
* makes a large logical disk out of a few smaller physical disks, say two 100GB striped together to make one 200GB
* it's bad when drives fail, file system cannot recover because data is split across the disks
* useful for large read-only NFS server where mounting many disks is clunky and redundancy is irrelevant.
RAID 1
* exact copy (or mirror) of data across two or more disks.
* read performance more important than data storage capacity
* the is array only as big as the smallest member disk.
* It's possible to "split the mirror," declare one disk as inactive, do a backup of that disk, then "rebuild" the mirror.
RAID 2
* obsolete
* stripes data at the bit (rather than block) level by Hamming code for error correction.
RAID 3
* less useful, now obsolete
* uses byte-level striping with a dedicated parity disk.
* cannot process multiple requests simultaneously
* requires synchronized spinning spindles in lockstep
RAID 4
* uses block-level striping with a dedicated parity (copy) disk, possible bottleneck.
* allows each member of the set to act independently when only a single block is requested.
* run by a disk controller
* can service multiple read requests simultaneously.
* does not need synchronized spinning spindles
* well implemented at the enterprise level by one company, NetApp using a Write Anywhere File Layout (WAFL) which stores system metadata (like inodes, block maps, and inode maps) in the same way application data is stored
* allows write to file system metadata blocks anywhere on the disk
RAID 5
* replaces RAID 3 and 4
* block-level striping with parity data distributed across all member disks
Example:
* four 1 TB drives can be made into two separate 1 TB redundant arrays under RAID 1
* or 2 TB under RAID 1+0
* the same four drives can be used to build a 3 TB array under RAID 5.
parity, an error checking bit
RAID 6
* extends RAID 5 by adding an additional parity block
* uses block-level striping with two parity blocks distributed across all member disks
* fast
* uses firmware and specialized ASICs for intensive parity calculations
* provides fault tolerance of two drive failures; the array continues to operate with up to two failed drives
Hot sparing
Hardware and software RAIDs with redundancy may support a hot spare drive; when a physical drive in the array is inactive until an active drive fails, then the system automatically replaces the failed drive with the spare, rebuilding the array with the spare drive.
Not Hot swappable
On another unit, I was trying to figure out why the drives from different manufacturers were not swappable. Turns out hardware RAID controllers use their own data layout formats, so it is not possible to span controllers from different manufacturers.
References: http://support.dell.com/support/edocs/storage/RAID/PERC5/en/UG/HTML/chapterh.htm
Wikipedia RAID
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