Definitive RAID Thread.

Discussion in 'Mac Help/Tips' started by mischief, Jul 2, 2002.

  1. mischief macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #1
    Okay, A lot of people have asked what RAID is. RAID is: Rapid Array of Inexpensive Disks.

    The first 2 kinds are:

    RAID 0 (Zero). RAID 0 is where 2 or more drives are mapped into a single large volume. Good for massive storage, bad for redundancy.

    RAID 1. RAID 1 is mapping multiple drives identically. This is done to create redundancy for the purpose of Data protection.

    RAID 1+0 (I'm assuming) is a pair of RAID 0 volumes mapped by software as RAID1. That is: A RAID 1 made up of a group of RAID 0 volumes.

    Could someone explain the other types so we can have them for the record?:)
     
  2. AlphaTech macrumors 601

    AlphaTech

    Joined:
    Oct 4, 2001
    Location:
    Natick, MA
    #2
    RAID 0+1 is an array that is both stripped and mirrored. Essentially, one RAID 0 array is mirrored with another. This means, that of the four drives used, you only have the size of two of them.

    Something else to be aware of. All RAID systems use the size of the smallest drive on the array. If you have three 80GB drives, and one 60GB drives, it only uses 60GB of the 80GB drives (each), yielding 240GB instead of 300GB. For that reason, along with others, you want to use identical hard drives. It is also recommended to use the exact same model drives, and people [typically] purchase them all at the same time to ensure this.

    BTW, RAID 1 is called mirroring, and writes the data to both drives. You only see one of the drives used (if you use a pair). It is used for data integrity/redundance reasons so that if one drive fails, you don't loose everything.

    This might assist some...

    Striping (RAID 0)
    _
    Reads and writes sectors of data interleaved between multiple drives. When any disk member fails, it affects the entire array. Performance is better than a single drive since the workload is balanced between the array members. This array type is for high performance systems. Identical drives are recommended for performance as well as data storage efficiency. The disk array data capacity is equal to the number of drive members times the smallest member capacity. For example, one 1GB and three 1.2GB drives will form a 4GB (4 x 1GB) disk array.

    Mirroring (RAID 1)
    _ Writes duplicate data on to a pair of drives while reads are performed in parallel. ATA RAID 1 is fault tolerant because each drive of a mirrored pair is installed on separate IDE channels. If one of the mirrored drives suffers a mechanical failure (e.g. spindle failure) or does not respond, the remaining drive will continue to function. This is called Fault Tolerance. If one drive has a physical sector error, the mirrored drive will continue to function.

    Striping/Mirroring (RAID 0+1)
    _ A combination of both above array types. It can increase performance by reading and writing data in parallel while protecting data with duplication. A minimum of four drives needs to be installed. With a four-drive disk array, two pairs of drives are striped. Each pair mirrors the data on the other pair of striped drives. The data capacity is similar to a standard Mirroring array with half of total capacity dedicated for redundancy.
     
  3. AlphaTech macrumors 601

    AlphaTech

    Joined:
    Oct 4, 2001
    Location:
    Natick, MA
  4. evildead macrumors 65816

    evildead

    Joined:
    Jun 18, 2001
    Location:
    WestCost, USA
    #4
    RAID

    You cant forget RAID 5 and the others.


    I use RAID 5 on all my arrays at work. Most of them are 9 drive Arrays (73GB Fibre Drives) RAID 5 alows for a hot spaire. No data loss and disaster recovery. I mirror 2 RAID 5 Arrays for ultra redundancy on prodution systems.


    -evildead
     
  5. mischief thread starter macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #5
    Quote from TOE in Hardware Rumors.

    Thanx Toe!

    "Level 0 is "striping," where all the drives are combined together into one big volume. The advantage is blistering speed. If the computer needs to write 100 MB to a 4-disk RAID, it only has to write 25MB to each disk, so it goes four times as fast. Add more disks and it gets faster. The killer benchmarks on the Xserve are based on a 4-disk Level 0 RAID. However, the BIG disadvantage of striping is that if any drive fails, all the data is lost. In the case of that 100 MB file, 25MB worth of it would be gone, and there's no way to recover it. Striping is most often used for video cvapture, where they ghave to suck down a ton of data as fast as possible. Once the data is captured, then it is moved to a more reliable drive.

    Level 1 is kinda the opposite. It is called mirroring. It only works with two drives, and the computer writes exactly the same information to each drive. There is no speed increase at all, but if one drive fails, NO data is lost at all. This provides extreme data security.

    Level 0+1 mixes the two methods, and only works with 4 drives, I think. It is a mirror of two stripes. Or a stripe of two mirrors. In any event, data gets striped over two mirrors, so with four drives, it goes twice as fast as one drive, but there is no chance of data being lost.

    Level 5 is the most popular one for big operations. It is more complex. It uses 3 or more drives (often up to 12, and even more). If n is the number of drives, it writes 1/(n-1) of the data to each drive except for one. So if there are 12 drives, it writes 1/11th of the data to each of 11 drives. The last drive is called a "parity drive," and it contains nothing but parity data. If one of those 11 drives fails, it can be rebuilt from the information on the other 11, plus the parity information. The L5 RAID can figure out what is missing by comparing the parity information to what information it still has. The advantages of L5 are extreme speed (with twelve drives, you supposedly go 11 times faster than one drive) and good protection against failure. The probems are the time it takes to rebuild a bad drive, and if two drives should happen to fail, all is lost. Also, the more drives you add the faster it is supposed to go, but the parity data gets more complex and there is more demand on the RAID controller, so it can start to slow down too.

    Level 10 is a lot like 0+1, but it has no limitation on the number of drives. It is a mirror of two big stripes. So with 12 drives, you would have two striped arrays of 6 drives, and those two stripes would mirror each other. The disadvantage is that it uses a lot of drives compared to the amount of storage space you get. But the advantage is that it is EXTREMELY difficult to ever lose any data with L10, and it is still very fast. Also, there is no time spent creating parity data. So with a 12-drive L10 RAID, you get 6-drives worth of data writing at a full 6 times faster than a normal drive, and there is very very little chance of losing any data. "
     
  6. AlphaTech macrumors 601

    AlphaTech

    Joined:
    Oct 4, 2001
    Location:
    Natick, MA
    #6
    Re: RAID

    Anything above RAID 0+1 is a bit silly for home use. RAID 5 is fine/good for businesses, but who has data that needs the protection of RAID 5 at home??
     
  7. mischief thread starter macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #7
    Re: Re: RAID

    OK. If this was an Apple Store I'd agree. The thing is: There's a lot of de-facto IT guys out there that have to do a lot of this stuff for their companies. Plus people ask a lot. ;)
     
  8. edesignuk Moderator emeritus

    edesignuk

    Joined:
    Mar 25, 2002
    Location:
    London, England
    #8
    I for one have found this very informative, thanks very much guys. :)
     
  9. AlphaTech macrumors 601

    AlphaTech

    Joined:
    Oct 4, 2001
    Location:
    Natick, MA
    #9
    Re: Re: Re: RAID

    True, but how many people set up RAID 5 at HOME??? I am also not talking about a business that you base out of your home, since that is not a home user environment, but a business. HmmmmmmmMMM???? :D :p ;)
     
  10. mischief thread starter macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #10
    Re: Re: Re: Re: RAID

    Ah yes but: Without a thread like this HOW WOULD THEY EVER KNOW which is appropriate? People are timid about asking questions.
     
  11. AlphaTech macrumors 601

    AlphaTech

    Joined:
    Oct 4, 2001
    Location:
    Natick, MA
    #11
    Re: Re: Re: Re: Re: RAID

    Then they are truely deserving of the title: Foolish Mortal... :rolleyes:

    If you don't know something, ask, if you can't get an answer, research one for yourself. All it takes is a brain to figure THAT part out... oh wait, there I go assuming that people have [or use their] brains. :p
     
  12. nixd2001 macrumors regular

    Joined:
    Aug 21, 2002
    Location:
    UK
    #12
    Given the name of the thread, this thread seems a reasonable place to ask my question:

    What (cheap) options are there for creating RAID setups using IDE drives and a PCI card for a (new) PM G4, with a particular focus on increasing read/write performance rather than data robustness/redundancy?

    Thanks in advance
     
  13. mischief thread starter macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #13
    I don't know what U define as "cheap" but:

    Assuming you keep the original drive to boot from you have 3 bays to play with.

    I'll assume you have no desire to put a HD in the bottom optical bay as it will likely require some slight mods to the cse to actually screw it down.

    Sonnet makes a very nice RAID 133 Host card for $200.00 capable of mapping 4 drives as 2 RAID volumes and provides Hardware-based RAID for both mirroring and striping.

    Maxtor (who bought Quantum) makes some fairly inexpensive drives in larger sizes. Think in terms of paying about $1.25 per GB below 120 GB and $2.00 Per GB there-after.
     
  14. beatle888 macrumors 68000

    beatle888

    Joined:
    Feb 3, 2002
    #14
    it is my understanding that if i plugged in a
    strip raid system on my tibook to do serious
    photoshop work (ti667) that this system
    would be WAY faster since im using the
    slow internal drive that came stock. actually
    even with a fast firewire drive i should see
    major speed improvements. am i correct?
     
  15. mischief thread starter macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #15
    Yes and no.

    you'd see a significant improvement but your system bus would still be a bottleneck. The nicest external RAID's I've seen are the LaCie rackable ones. U can also get Gigabit Ethernet compatable NAS RAID units that are even faster (Gigabit Ethernet speeds rather than 400Gb/sec.)That's Network Area Storage.
     
  16. beatle888 macrumors 68000

    beatle888

    Joined:
    Feb 3, 2002
    #16
    that's ok. i just need photoshops VM to speed
    up :) if this powerbook will at least push a
    500mb file around with 1gig of ram dedicated
    to photoshop and a giganet raid then i will be
    set till 2004 easily :D

    will you please tell me more about the ethernet
    raid system?

    i can find out about the lacie at their website.

    thank you for your advice.


    hmmm i just went to the lacie site and they
    had a firewire drive their with 35mb a second.

    i'll keep looking.
     
  17. mischief thread starter macrumors 68030

    mischief

    Joined:
    Aug 1, 2001
    Location:
    Santa Cruz Ca
    #17
    First of all FW is 400 Megabits per second, If Lacie is listing 35 MegaBytes per second, that's pretty good.

    Now: as to VM. VM should be assigned to internal disks, which on a powerbook are confined to ATA 66 at (I believe) 5400 RPM. That's not too bad considering that 5400 RPM is pretty fast on a drive that's using smaller platters.

    You can do a couple of Things: on OS X you can partition 1 GB for use as a swap partition, thus keeping most VM tasks separate from your storage space. On OS 9 you can actually assign a RAMDisk as the boot volume, which is pretty slick.:D

    Also: when using an external monitor: keep the bit-depth synchronous between the Powerbook's display and the monitor to keep GPU cycles to a minimum.
     
  18. beatle888 macrumors 68000

    beatle888

    Joined:
    Feb 3, 2002
    #18
    here't the specs on the lacie


    * Speed: 400Mbps - 50MB/s FireWire® Interface (IEEE1394 /iLink®)
    * Capacity: up to 160GB of additional storage
    * Audio-video: up to 1000 hours of MP3 music-files/100 hours of compressed video
    * Scalable: daisy-chain up to 63 hot-plug devices
    * The best technology: up to 7200rpm / ATA100 / 2MB cache inside / Oxford911
    * Plug & play for MacOS & Windows

    the reason why im considering firewire instead
    of sccsi is because i have a tibook.

    "Now: as to VM. VM should be assigned to internal disks"

    Oh, I'm talking about PhotoShop scratch disk not OSX VM.

    I thought if I got a nice fast external drive
    for a photoshop scratch disk I would see
    better photoshop performance?
     

Share This Page