On Sat, 13 Jan 2007, Michael Tokarev wrote:
>
> (No, really - this load isn't entirely synthetic. It's a typical database
> workload - random I/O all over, on a large file. If it can, it combines
> several I/Os into one, by requesting more than a single block at a time,
> but overall it is random.)
My point is that you can get basically ALL THE SAME GOOD BEHAVIOUR without
having all the BAD behaviour that O_DIRECT adds.
For example, just the requirement that O_DIRECT can never create a file
mapping, and can never interact with ftruncate would actually make
O_DIRECT a lot more palatable to me. Together with just the requirement
that an O_DIRECT open would literally disallow any non-O_DIRECT accesses,
and flush the page cache entirely, would make all the aliases go away.
At that point, O_DIRECT would be a way of saying "we're going to do
uncached accesses to this pre-allocated file". Which is a half-way
sensible thing to do.
But what O_DIRECT does right now is _not_ really sensible, and the
O_DIRECT propeller-heads seem to have some problem even admitting that
there _is_ a problem, because they don't care.
A lot of DB people seem to simply not care about security or anything
else.anything else. I'm trying to tell you that quoting numbers is
pointless, when simply the CORRECTNESS of O_DIRECT is very much in doubt.
I can calculate PI to a billion decimal places in my head in .1 seconds.
If you don't care about the CORRECTNESS of the result, that is.
See? It's not about performance. It's about O_DIRECT being fundamentally
broken as it behaves right now.
Linus
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