Re: [PATCH, RFC 0/3] Introduce new O_HOT and O_COLD flags

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(4/24/12 2:18 AM), Nick Piggin wrote:
On 23 April 2012 21:47, Nick Piggin<npiggin@xxxxxxxxx>  wrote:
On 23 April 2012 18:23, James Bottomley

Experience has taught me to be wary of fine grained hints: they tend to
be more trouble than they're worth (the definitions are either
inaccurate or so tediously precise that no-one can be bothered to read
them).  A small set of broad hints is usually more useable than a huge
set of fine grained ones, so from that point of view, I like the
O_HOT/O_COLD ones.

So long as the implementations can be sufficiently general that large majority
of "reasonable" application of the flags does not result in a slowdown, perhaps.

But while defining the API, you have to think about these things and not
just dismiss them completely.

Read vs write can be very important for caches and tiers, same for
latency constraints, etc. These things aren't exactly a huge unwieldy matrix. We
already have similar concepts in fadvise and such.

I'm not saying it's necessarily a bad idea as such. But experience
has taught me that if you define an API before having much
experience of the implementation and its users, and without
being able to write meaningful documentation for it, then it's
going to be a bad API.

So rather than pushing through these flags first, I think it would
be better to actually do implementation work, and get some
benchmarks (if not real apps) and have something working
like that before turning anything into an API.

Fully agreed.

I _guess_ O_COLD has an enough real world usefullness because a backup operation
makes a lot of "write once read never" inodes. Moreover it doesn't have a system wide
side effect.

In the other hands, I don't imagine how O_HOT works yet. Beccause of, many apps want
to run faster than other apps and it definitely don't work _if_ all applications turn on
O_HOT for every open operations. So, I'm not sure why apps don't do such intentional
abuse yet.

So, we might need some API design discussions.

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