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    SubjectRe: [PATCH 0/8][RFC] IO latency/throughput fixes
    On 04/08/2009 10:40 PM, Eric Sandeen wrote:
    > Linus Torvalds wrote:
    >> On Mon, 6 Apr 2009, Linus Torvalds wrote:
    >>> thing that we think people would be happiest with.
    >>> I think "ordered" was a reasonable default, but that was at least partly
    >>> because _both_ ordered and writeback sucked (partly in different ways).
    >>> I do think we could make it a config option.
    >> A patch _something_ like this.
    >> A few notes:
    >> - This is UNTESTED (of course)
    >> - If I did this right, this _only_ overrides the data mode if it's not
    >> explicitly specified on disk in the superblock mount options.
    >> IOW, if you have done a
    >> tune2fs -o journal_data_ordered
    >> then this will _not_ override that. Only in the absense of any explicit
    >> flags should this trigger and then make the choice be 'writeback'.
    >> And just to be _extra_ backwards compatible, if you really want the old
    >> behavior, and don't want to set the ordering flag explicitly, just answer
    >> 'y' to the EXT3_DEFAULTS_TO_ORDERED Kconfig question.
    >> What do people think? Anybody want to test?
    > I think this is a terrible idea. I ran the following test with
    > data=writeback on (which doesn't have the rename& truncate
    > hacks, but they would not help in this case, either):
    > tar xvjf linux-; echo b> /proc/sysrq-trigger
    > This simulates a crash on a busy system. I got back 8000+ files
    > containing other people's data.
    > data=ordered isn't just "nicer" behavior than writeback on a crash, it's
    > necessary today for security. Making data=writeback default is a
    > security flaw.
    > Are we really considering (wait, not considering; it's checked in
    > already!) - blowing a huge security hole in the filesystem used on the
    > vast majority of installations in the name of speed?
    > Chris suggested earlier in this thread that we should use the XFS trick
    > of not extending the i_size until io completion, and I agree that it
    > makes sense. Chris even offered to take a stab at it and I hope I can
    > work with him on this. It's a -much- better answer than this
    > reactionary change.
    > -Eric

    I agree - we definitely should work to fix the fsync latency problems,
    but this seems to jump back in time to the early 80's for UNIX file systems.

    Writeback mode is just not a safe default for naive users or even more
    sophisticated users who don't understand the risks here. Definitely not
    a journal mode that any distribution would be able to ship as a default.

    Wouldn't it make much more sense to leave the default at the safe data
    ordered mode and let the few people who understand the tradeoff remount
    file systems in writeback mode?



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