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SubjectRe: fsync on large files


On Wed, 17 Feb 1999, Stephen C. Tweedie wrote:
>
> The atomic write behaviour is a by-product of fixing an
> entirely different problem (concurrent truncate, for which making it
> atomic does make a lot of sense).

No. The atomic write behaviour was implemented because we couldn't come up
with any other reasonable scheme to handle truncation sanely, but that was
after we had extended tons of effort on making a "normal" write do the
right thing.

I believe that we should just rip out the code that tried to handle
concurrent normal writes, and know they can't happen.

For concurrent writes, it may be entirely acceptable to completely _drop_
the write semaphore at well-defined places and retry, but that's going to
be a per-filesystem thing: the VFS layer is going to basically grab the
semaphore and default to complete atomicity. That way it's up to the
low-level filesystem to handle concurrency, if the filesystem writer is
100% sure he knows what he is doing.

We had exactly this problem with block device drivers, where the only sane
way to handle it is to let the upper levels do the locking so that the
lower levels don't have to know (and more importantly: that way we can
_change_ the locking setup without changing every single driver or
filesystem - for example, the device driver lock is a global thing, but
can be made into a per-device thing without having to completely rewrite
all drivers).

Linus


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