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    SubjectRe: safe file systems
        From: Larry McVoy <lm@linux.cobaltmicro.com>
    To: linux-kernel@vger.rutgers.edu
    Subject: safe file systems

    Do you think it would be possible to build a safe, slow file system? By
    safe, I mean that I could hit reset in the middle of 50 parallel un-tars
    and reboot the system and the file system comes up clean (no fsck, but
    data loss)?

    NO fsck - or just a very quick fsck?

    And 'hit the reset' is not the only kind of breakage. Do you mean only hit
    the reset, or do you wish to allow for 'the cat was chewing on the power
    cable'? (Read below).

    Has anyone thought about this very much? If so, is there a mailing list
    or archive that I can browse?

    I thought about it a couple of years back (hmm... actually maybe more) when
    I was stuffing about with Ultrix. (I was started by writing a defragger, and
    wanted to change big chunks of the file system around, without sync'ing and
    still be totally fault tolerant. I never coded a FS, which I'm told for
    Ultrix is a good thing not to do). It's amazing how easy it is to develop
    stuff on a modern Linux box :) It would be painful to go back to a machine
    ten times slower with a fraction on the disk storage and memory....



    If you have ordered writes, then surely its possible with no fsck and
    possibly considerable data-loss... ?

    If you wanted a full fsck you could have much lower data-loss. And there are
    steps in between where a smaller fsck would result in perhaps only moderate
    data-loss.

    I presume the latter is essentially just a journalling file-system.

    Also, your writes have to be very strictly ordered. Some intelligent IO
    subsystems will reorder your write operations on you, which I guess could
    cause lots of really obscure hard to track down funnies if you were unaware
    of you IO doing this.

    A DPT SCSI card with 64MB of ram (probably with less too) I'm fairly sure
    does this. (Create a file several GB long, and then update the ends of the
    file and sync all the time - the HD does not trash). These controllers also
    delay writes, which for performance is really nice - but in the case of
    power-loss could be a pain. (Although if writes are ordered it will just
    mean more data loss that otherwise in this case).

    I think even my cheapish SCSI disks (which have a 1 (maybe 2?) MB cache)
    will do this. I assume here a system reset will not affect them, but
    power-failure did last time I checked. (But you can twiddle with the tables
    on them and modify the way it writes data back, etc).




    -Chris

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