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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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