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    SubjectRe: Disk write cache (Was: Hyper-Threading Vulnerability)
    On Mon, 16 May 2005, Jeff Garzik wrote:

    > Matthias Andree wrote:
    >> On Sun, 15 May 2005, Mikulas Patocka wrote:
    >>> Note that disk can still ignore FLUSH CACHE command cached data are small
    >>> enough to be written on power loss, so small FLUSH CACHE time doesn't
    >>> prove disk cheating.
    >> Have you seen a drive yet that writes back blocks after power loss?
    >> I have heard rumors about this, but all OEM manuals I looked at for
    >> drives I bought or recommended simply stated that the block currently
    >> being written at power loss can become damaged (with write cache off),
    >> and that the drive can lose the full write cache at power loss (with
    >> write cache on) so this looks like daydreaming manifested as rumor.
    > Upon power loss, at least one ATA vendor's disks try to write out as
    > much data as possible.
    > Jeff

    Then I suggest you never use such a drive. Anything that does this,
    will end up replacing a good track with garbage. Unless a disk drive
    has a built-in power source such as super-capacitors or batteries, what
    happens during a power-failure is that all electronics stops and
    the discs start coasting. Eventually the heads will crash onto
    the platter. Older discs had a magnetically released latch which would
    send the heads to an inside landing zone. Nobody bothers anymore.

    Many high-quality drives cache data. Fortunately, upon power loss
    these data are NOT attempted to be written. This means that,
    although you may have incomplete or even bad data on the physical
    medium, at least the medium can be read and written. The sectoring
    has not been corrupted (read destroyed).

    If you think about the physical process necessary to write data to
    the medium, you will understand that without a large amount of
    energy storage capacity on the disk, it's just not possible.

    To write a sector, one needs to cache the data in a sector-buffer
    putting on a sector header and trailing CRC, wait for the write-
    splice from the previous sector (could be almost one rotation),
    then write data and sync to the sector. If the disc is too slow,
    these data will be underwrite the sector. Also, if the disc
    was only 5 percent slow, the clock recovery on a subsequent
    read will be off by 5 percent, outside the range of PLL lock-in,
    so you write something that can never be read, a guaranteed bad block.

    Combinations of journalizing on media that can be completely flushed,
    and ordinary cache-intensive discs can result in reliable data
    storage. However a single ATA or SCSI disk just isn't a perfectly
    reliable storage medium although it's usually good enough.

    Dick Johnson
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