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Thread: Which raid?

  1. #11
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    Quote Originally Posted by epoch1970 View Post
    Interesting.
    The drives were mounted by the OS, or simply on-line in the electrical sense ?
    (Generally I won't keep a backup volume mounted once the copy is done; too many times I erased the backup and the original...)
    I said "mounted filesystems," so they were mounted. In fact, the backup partition on the second drive was corrupted while doing a backup. So unmounting it after the backup would not have done any good.

    I think that mistakenly deleting files is one of the dangers of RAID being used for at least temporary backups. I have a script that removes write permissions from all directories and files, which gets run after things are added to the RAID volume. Kind of a pain, as backup scripts have to first restore write permission and then remove again afterward. However, keeps me from accidentally deleting something important (which I did once!).

  2. #12
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    Quote Originally Posted by ncarver View Post
    I think that mistakenly deleting files is one of the dangers of RAID being used for at least temporary backups.
    It bears repeating - RAID is not backup. Not even "temporary" backup. Backup means having another set of data independent of the first. You don't get this from RAID. I don't mean to sound patronizing - It's clear to me that you understand this, but I worry that others reading might not.

    Mistaken deletions are the main reason I don't like RAID for my situation. Loss of data is far more likely to come from fat fingering than it is from hardware failure. My nightly rsync to another internal HD gives me at least a few hours to detect and undo any mistakes before they propagate to the second drive. With RAID I get no such grace period.

  3. #13
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    Which raid?

    jsprag wrote:
    > Mistaken deletions are the main reason I don't like RAID for my
    > situation. Loss of data is far more likely to come from fat fingering
    > than it is from hardware failure.


    Right, and RAID mirroring isn't generally trying to minimise data loss,
    it's trying to maximise uptime.

    Your rsync gives you something that RAID mirroring doesn't: a window to
    detect user error.

    RAID mirroring gives something that your rsync doesn't: the ability to
    survive disk failure without any loss of availability.

    Different goals.

    A nice combination might be provided by using the ZFS filesystem, if
    you're on Solaris/OpenSolaris (or MacOS). Mirror the disks for
    availability, and use ZFS's snapshot facility for protection against
    fat-fingers, and even better without having to dedicate twice the disk
    space to do it (like rsync does).

    That's how I'll be running SC on my new system...

    cheers,
    calum.

  4. #14
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    Quote Originally Posted by cdmackay View Post
    Your rsync gives you something that RAID mirroring doesn't: a window to
    detect user error.

    RAID mirroring gives something that your rsync doesn't: the ability to
    survive disk failure without any loss of availability.

    Different goals.
    Good summation. The two aren't mutually exclusive, of course. If a user wanted they could have RAID mirroring, another internal drive for nightly (or any other interval) backups, and an external backup. Starts to add up to a lot of drives though...

    Quote Originally Posted by cdmackay View Post
    A nice combination might be provided by using the ZFS filesystem, if
    you're on Solaris/OpenSolaris (or MacOS). Mirror the disks for
    availability, and use ZFS's snapshot facility for protection against
    fat-fingers, and even better without having to dedicate twice the disk
    space to do it (like rsync does).

    That's how I'll be running SC on my new system...

    cheers,
    calum.
    Interesting thought. Not that familiar with Solaris or ZFS but might be worth looking into.

  5. #15
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    Which raid?

    jsprag wrote:
    > Good summation. The two aren't mutually exclusive, of course. If a
    > user wanted they could have RAID mirroring, another internal drive for
    > nightly (or any other interval) backups, and an external backup.
    > Starts to add up to a lot of drives though...


    quite.

    This is why I like ZFS.

    [I ought to declare an interest, as I work for Sun; then again, it's all
    open-source, so... ]

    > Interesting thought. Not that familiar with Solaris or ZFS but might
    > be worth looking into.


    I recommend having a look; try downloading the latest OpenSolaris live
    cd and having a play.

    ZFS is a copy-on-write filesystem, so snapshots are implemented very
    naturally. When you take a snapshot, all that happens is that future
    writes to a particular block of a file are redirected, and the original
    block is kept for the snapshot, the new block for the "real" filesystem.
    It's possible to have thousands of snapshots. Very space efficient.

    The GNOME file-manager has an extension to browse the data in snapshots
    - which can also be accessed directly - via the "time slider", in
    practice very much like MacOS Time Machine, but integrated into the
    filesystem.

    ZFS also provides various levels of RAID, including striping, mirroring,
    and two levels of efficient versions of RAID5 (RAIDZ, RAIDZ2), without
    any need for a separate volume manager.


    There are SC build instructions for OpenSolaris on the wiki, and I'm
    looking towards making an unofficial nightly build regularly available,
    in the OpenSolaris IPS pkg format.

    cheers,
    calum.

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