The other subtlety comes if we add fsync() suppression to laptop mode
--- which is something that Bart Samwel is very interested in doing
and I talked to him at FOSDEM about this. As Jeff Garzik recently
pointed out, however, if we let the system reorder writes across
fsync() boundaries, or if we combine two writes to the same block
separated by an fsync(), and the system crashes in the middle of
pushing all of these blocks out to the disk, we can end up trashing
the consistency guarantees of a database such as mysql or postgres.
It's a good point, but it only applies if we add fsync() suppression
to laptop mode --- which we haven't done yet.
I've got absolutely no idea why anyone would want fsync() to stop
meaning "Put my data on the disk please". laptop-mode isn't intended to
reduce data integrity - it's intended to batch disk write-outs such that
there's a lower risk of needing to perform further write-outs in future.
It makes sense for applications which really desperately want
information on disk to fsync() (for instance, saving a file in
OpenOffice).
laptop-mode is something that makes sense as a default behaviour under a
lot of circumstances. Adding fsync() suppression means it's utterly
impossible to use it in that way. An additional mode would be perfectly
reasonable, as long as it's made clear that it's really a request for
data to be discarded at some point. The current mode isn't.