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    RAID 1: two or three discs?


    Hello,

    I want to build a server where all my files on one disk are backed up on another disk. I thought of trying (never did this before) to use RAID 1 seems it seem to fit the purpose.

    My question is this;

    Should I use two disks; with the OS + Application Programs installed on both disks?

    Or

    Should I use three disks; with the OS + Application Programs installed on one disk, and the files I want to backed up on the other two disks (with RAID 1 mapping the files from one disk to the other).
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    Two disks. If you already have three disks, use RAID 5 instead.

    RAID is not a backup solution, it's a redundancy solution. A backup solution is used to recover from loss of data. A redundancy solution is used to avoid service interruption during a hardware failure.

    That is the reason you should use 2 disks instead of 1+2. From the standpoint of avoiding service disruption, your OS is just as important as your data.

    If your data is important, make sure you have an actual backup solution in place as well.
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    Originally Posted by E-Oreo
    If your data is important, make sure you have an actual backup solution in place as well.
    So RAID 1 is not enough as a backup? I understand what you mean by redundancy, however with RAID 1, if one of the drives goes bollocks, shouldn't I still be able to recover the files from the second drive? and then I replace the bad-drive and I am safe again? Or it dose not work like that?
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    RAID protects you by providing zero down time through the failure of a single physical hard disk. If the second disk fails before you've replaced the bad disk, and the system has resynchronised, then you're screwed (data is lost for good).

    Fire, theft(of the physical machine), flood, malice(human or malware), incompetence can also cause permanent loss of your data as well. To protect against these, you need a backup solution as well.
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    Originally Posted by salem
    Fire, theft(of the physical machine), flood, malice(human or malware), incompetence can also cause permanent loss of your data as well. To protect against these, you need a backup solution as well.
    So to be extra sure I should have a third disk for backup which is not installed in the same machine. Like that I'll be double sure that data is not lost!!
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    A good backup system takes two things into account: location and time. The quality and cost of your backup system are directly related to those.

    The backup should physically be located away from the primary source. At least a separate disk. Better, a separate server. Better, a separate building. Best, a separate city.

    It all depends on how much you can spend and how important your data is. Is it acceptable to lose your data in the event of a computer virus or admin incompetance? Then a separate disk is OK. Is it acceptable to lose your data in the event of a fire? Then a separate server is OK. Is it acceptable to lose your data in the event of a nature disaster? Then a separate building is OK. Is it unacceptable to lose your data? Then you need to have it in a separate city.

    The second element is time: how frequently you make backups and how long you store each backup. This is determined by how much data you can afford to lose and how long you think it will take for you to recognize a problem.

    Can you afford to lose one day worth of data? Then backup daily. Can you afford to lose one hour worth of data? Then backup hourly. etc.

    Are you sure you can detect a problem within one week? Then store one week worth of backups. Can you detect a problem within one month? Then store one month worth of backups. etc.

    Most people use a system where they do something like:
    * backup daily
    * store one week worth of daily backups
    * store one month worth of weekly backups
    * store one year worth of monthly backups

    Comments on this post

    • sim085 agrees : Thank You!
    PHP FAQ

    Originally Posted by Spad
    Ah USB, the only rectangular connector where you have to make 3 attempts before you get it the right way around

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