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    SubjectRe: lack of raw disk devices
    > Okay. I've seen this dicussion many times before. I need to know
    > now. What the hell is the difference a RAW block device and a
    > (err.... NONRAW?) regular block device?

    Depends on whom you talk to :-) In a traditional (BSD style?) raw
    device, read/write-operations from/to a raw device do not use the
    disk cache. This has two effects:
    - repeated disk access to the same location is slower
    - if the system crashes, there are no unflushed buffers
    - memory consumption is lower
    (well, that's three effects). Linux supports only the second one.

    Database implementors are especially interested in the second
    property, as it is a pre-requisite for safe transactions: the DB needs
    to know that the data is really on the disk when the write(2) call
    returns, so that recovery can occur should the system fail later.

    Even though you cannot disable the read cache in Linux, you can have
    the second of these properties, by given O_SYNC to the open(2) call.
    So you can disable the write caching if you want. There seems some
    dissense whether applications want to disable the read cache as well.

    Hope this helps,

     \ /
      Last update: 2005-03-22 13:41    [W:0.018 / U:13.740 seconds]
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