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Re: fs-independent quotas
On Thu, Oct 20, 2011 at 06:00:28PM +0200, Manuel Bouyer wrote:
> > > It's certainly less trouble to send back to userland the whole set of
> > > data - especially if what userland wants is the whole set of data
> > > (I can't see what a partial read of quota would be usefull for).
> > No, no it really isn't. Suppose there are, say, 50,000 users, so to
> > send back the whole works you have to accumulate 100,000 quota entries
> > in a gigantic blob... a machine with 50,000 users will have enough RAM
> > for this but that doesn't mean that allocating a contiguous chunk of
> > kernel memory that large is easy or desirable. Far better to read it
> > out a couple hundred at a time.
> We're talking a few MB of ram here, isn't it ? the kernel can certainly
> allocate this without troubles (other subsystems do).
The proplib'd and XMLified complete dump for 50,000 users will
probably make a blob of between 10 and 20 MB. (Note: this is an
estimate; I haven't checked the size by trying it. It might be larger.
I'd be surprised if it were much smaller.)
I don't see why it's desirable to manifest such large objects when
it's easily avoidable.
> > There are two design truisms for database stuff that apply here:
> > first, you always end up wanting cursors, and second, you always end
> > up wanting bulk get (and not just single get) from those cursors. So
> > it's usually a good idea to anticipate this and design it all in up
> > front.
> Maybe ... I know that in the end I want the whole set of data and not
> just a part of it.
Yes, probably. The cursor API I've floated so far is not general
enough to support much else. Although it could be made more general.
> But if you believe it's needed this can easily be added to the
> existing quotactl(2) (it would just be a new command).
Yes, perhaps it could... but why? What's to be gained by using a
baroque proplib encoding of what can otherwise be handled as an array
of simple structs?
I remember asking this question when you first proposed the proplib
interface last spring, and never really got a clear answer.
> > > > The reason to wrap the position in a cursor abstraction is to allow
> > > > flexibility about how the position is represented.
> > >
> > > But then the cursor would still be stored in userland ?
> > That's the idea, like reading a file with pread().
> > I think the kernel should know, or at least be able to know, how many
> > cursors are currently open; but I don't think there's any need to keep
> > the cursor state itself in the kernel.
> So you want a quotaopen/quotaclose, with a file descriptor (or something
> similar) ?
The proposed API already has explicit open and close for cursors; what
I'm saying is that this should be exposed to the kernel. (Open already
has to be, to initialize the cursor position; close should be, so the
filesystem can if necessary know if there are cursors open at any
given time. Otherwise you can get into trouble; see for example nfsd
David A. Holland
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