RAID vs Mirroring

Disk drives
Conceptually, I understand the difference between RAID and mirroring. But many times, things are not always as simple as they seem. That said, I'm configuring a new i5 520 with 35gb drives RAID protected versus 35gb drives mirrored and with IBM's recent offerings, the mirrored drives are less than the RAID drives. Is this as much of a no brainer as it seems?

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HI Teamjots,

I may be wrong here so forgive me if I misunderstood you… Mirroring is RAID… the common name for RAID-1, which is one of several RAID schemes.

Do you mean replication?

RAID has a number of schemes that offer various levels of protection in different configurations. The number of schemes has increased over time as each offers different pros and cons.

RAID-1, or mirroring, doesnt suffer from the write penalty that RAID-5 does (in calculating parity data), and RAID-1 can tolerate a single physical disk failure without service degradation. However, RAID-1 requires that you have double the usable capacity you require, as each physical disk requires another physical disk to mirror data to. So, if you need 35GB of usable capacity, you will need to have 70GB of physcial disk, also known as RAW capacity.

Other RAID schemes such as RAID-5 can reduce the amount of RAW disk required. RAID-5 uses an extra disk to store parity data that is used to ‘rebuild’ the data should a failure occur. So, if you set up a 4+1 RAID-5 scheme (4 data disks + 1 parity disk) you only lose 20% of RAW capacity to the protection scheme (parity data). e.g. you could get 400GB usable out of 500GB RAW.

Hope that helps – let me know if u need anymore info.


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  • Poppaman2
    The answer to your question depends entirely on what you are trying to do with the system being configured: it takes more processing power (on the RAID card or subsystem) to process RAID 5 information (parity needs to be calculated, using an algorithm such as XOR) than it does to process RAID 1 (write 2x from cache before purging, or send the information down two "pipes" instead of one). Benefits of RAID 1 (mirroring): virtually instantaneous recovery from a disk failure. Drawbacks: cost/loss of raw disk capacity. Benefits of RAID 5 (striping): Somewhat faster data READS (more spindles); less loss capacity than disk mirror (RAID 1) or disk duplexing (duplication of entire I/O subsystem); cheaper to implement than RAID 5. Drawbacks: slower WRITES (unless a fairly large cache is used); longer recovery times from failure; space constraints (how many slots do you have in your enclosure?). I have seen and used both RAID 1 and RAID 5 in a single system: RAID 1 for the OS and RAID 5 for data. There is also (for the truly paranoid among us) RAID 1,5 (or 5,1) in which a logical drive is striped and then mirrored (5,1) or mirrored and then striped (1,5). The benefits and drawbacks should be obvious....
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  • Ariens72
    I should caution you I'm not that familiar with the IBM i5 520 product line but here's a tidbit of information which may or may not help. Review this for starters Bottom line is RAID 5 will give you more disk space for the given number of disk drives. Why IBM provides the mirrored drives at lower cost would make me think they are suppling differnet drives. Lower rpm to balance the better throughput of mirroring versus RAID 5 or they are burying the cost of the RAID disk controller across the drives. Need a lot more info on type of applications being used, transactions to be processed, and uptime requirements to answer more specifically.
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