to Raid or not to Raid?

Discussion in 'Technical' started by ghys, Oct 27, 2006.

  1. ghys

    ghys New Member

    Hello everybody. My 1st post here but not my 1st visit. Amazing stuff in here and a big thanks to all contributors and the community.

    I got a simple yet complicated question for you all:

    We just bought a brand new HP server with 3x250Gb SATA hot swap drives.

    I will install Suse Linux Enterprise Server 10.1 on it, with all standard server services(DNS, DHCP, Web, security....) and i was wondering what configuration for the HD's would be the best for security of data, backup , restore in case of disaster, and easy of manageability.
    I had two choices in head:

    2 drives as mirrors and the 3rd one as standard SATA that would receive the data created by the backup software , then this data would be transfered also to an external backup site via secure ftp or VPN. This way if one drives goes down, we still have one working exactly as the other one.

    The other was simple RAID5 , for faster access to data and rebuilding in case of dead drive. But i heard that if the 1st drive goes down, we can not rebuild it with the other 2 drives. Is that true? I can't get more info on raid 5 for this.

    There you have it! Do you have some more suggesitons or comments on the setup , they would be very helpfull since i will start installation very soon.

    Thanks all for your time and help
  2. terabite

    terabite New Member

    RAID-1 is is the only way to nirvana

    Sounds like 250G is plenty of disk for this work regime!

    A good design but better is possible!
    No - that is not true - a RAID-5 array can be operated in a degraded mode after losing _any_ 1 of the 3 disks. When the replacement disk is introduced its sectors can be reverse engineered from the other 2 disks.

    But this is a less desirable configuration unless space it your prime goal and performance is secondary.

    But the best configuration is to establish drive 0 and 1 as a RAID-1 pair and use drive 2 as a hot spare. The downside is you only get 1/3rd of the storage but the upside is lots of sleep at night! When either drive 0 or 1 fails drive 2 will be automatically deployed and you can replace the faulty disk without ever having a non redundant system.

    If you _really_ need more storage consider getting a pair of other disks to operate in a RAID-1 set.

    Cheers Frank.
  3. djtremors

    djtremors ISPConfig Developer ISPConfig Developer

    I don't know who told you that rubbish unless I misunderstood what you meant.
    RAID 5 is your best option for data with 3 drives. The data is stripped across all 3 drives and should any one of them fail, the other 2 can continue as the parity is spread across all 3 drives. it doesn't matter which of the three drives fail they will always keep the system running. ;) it does degrade in performance when it dies but not too much, better than system being down and uninaccessible.

    RAID1 is slow on writes but can be fast on reads but could be a pain to increase the drive space later whereas a RAID 5 you can add more drives to the array.
    If you have created a large LVM partition on the RAID5 and you are running out of space, add another drive to the array and then resize the LVM as needed.

    For redundancy and speed, a RAID1/0 is good but requires 4 drives, hence I prefer 5 for growth.

    oh btw, I usually setup the system on a raid 1 and the data on a raid 5 ( you can then take down the data and rebuild it while the system is running. increase the raid size etc while remote ).

    my 2 cents.
    (ex Adaptec engineer ;) )
    Last edited: Oct 28, 2006
  4. ghys

    ghys New Member

    ok wow ... thanks for the great suggestions.
    Quick question, how do you setup a system with Raid 1 and Data as 5 when you only have 3 disks or am i missing something here.

    So in conclusion, the Raid 5 solution would be the best choice? The boss here wanted to grab a backup copy of data and burn on DVD and send to external site. Would not be a problem with Raid 5 right?

    thanks again for the suggestions.
  5. terabite

    terabite New Member

    Get RAID-1 son, forget the rest ;-)

    Short answer is ... you don't. The choice is either RAID-1 _or_ RAID-5 - not both!

    Longer answer - personally I avoid RAID-5 like the plague - not because it doesn't work - it does - but because I want the most reliable and least impact to performance (and rebuilding) if I lose a disc. Some will claim RAID-1 is slower on writes because it has to write 2x the data...this is a bit of a furphy unless you have dedicated RAID-5 hardware to compute the checksums faster than the CPU can issue disk writes. Where RAID-1 kills is on reads - it is possible to get more than 100% IO (of a single disc) buy issuing reads simultaneously to the pair of discs - and even more by position the heads to different cylinders and optimising those IO's to reduce seek and rotational latency.

    Nope - RAID-1 with 2 disks devoted as mirrors and the 3rd as a hot standby is my best practice. As I said before this presumes 250G will meet all of the servers storage requirements. If not then I prefer to get further storage capability than to butcher the config to use RAID-5.

    Never a problem with any RAID (or non RAID) config.

    Cheers, Frank.
  6. 3cwired_com

    3cwired_com New Member

    Like said above, I'd go with the mirror setup and another drive as spare.
  7. ghys

    ghys New Member

    well again thanks for the great sugegstions and comments,

    now my other concern , after reading and reading and reinstalling and tech support from SUSE and HP, they can't give me a straight answer for the Adaptec RAID card in my server. This does not look like a BIOS raid, since i have to enter CTRL+A to enter the setup utility so my guess.... this is an actual hardware raid setup. Adaptec 1420SA 4xSATA HostRaid controller.

    I did my RAID config inside this utility and everything works fine, except i have to work around some setup thing in LINUX at boot: broken_modules=sata_MV
    and aar81xx driver from Adaptec.
    The bug i got was that if i do a security update from SUSE ( patches and Kernel ) the aar81xx driver and RAID drivers are replaced or lost and i don<t have a boot device available. Got to revert to old installation.

    I wanted, after that, to go in 100% software RAID with my SUSE server, but i was a bit lost in the right way to partition the drives and get the right config and partition map and exactly how and what to raid.

    So for now i am working with a Adaptec RAID config that i can't get any upgrades or patches on.

    Suggestions anyone?

    Again many many thanks for your kind help and time
  8. 3cwired_com

    3cwired_com New Member

    Personally I am not sure of your options, but I would go with a hardware raid setup, I have a 3ware escalade card and I am totally happy with it. Some may recommend software raids, but they do have their limitations, and I think hardware raids' limitations are mainly price, and some compatibility. I am running centos 4.4 btw, and it works great.

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