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    SubjectRe: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)
    On Sun, Aug 30, 2009 at 06:44:04PM +0400, Michael Tokarev wrote:
    >> If you lose power with the write caches enabled on that same 5 drive
    >> RAID set, you could lose as much as 5 * 32MB of freshly written data on
    >> a power loss (16-32MB write caches are common on s-ata disks these
    >> days).
    >
    > This is fundamentally wrong. Many filesystems today use either barriers
    > or flushes (if barriers are not supported), and the times when disk drives
    > were lying to the OS that the cache got flushed are long gone.

    While most common filesystem do have barrier support it is:

    - not actually enabled for the two most common filesystems
    - the support for write barriers an cache flushing tends to be buggy
    all over our software stack,

    >> For MD5 (and MD6), you really must run with the write cache disabled
    >> until we get barriers to work for those configurations.
    >
    > I highly doubt barriers will ever be supported on anything but simple
    > raid1, because it's impossible to guarantee ordering across multiple
    > drives. Well, it *is* possible to have write barriers with journalled
    > (and/or with battery-backed-cache) raid[456].
    >
    > Note that even if raid[456] does not support barriers, write cache
    > flushes still works.

    All currently working barrier implementations on Linux are built upon
    queue drains and cache flushes, plus sometimes setting the FUA bit.



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