Re: [PATCH 1/2 V2] kvm tools: Add scatter-gather variants of IO functions

[Sasha Levin <levinsasha928@gmail.com>]

On Sun, 2011-04-17 at 14:34 +0400, Konstantin Khlebnikov wrote:
> Sasha Levin wrote:
> > On Sun, 2011-04-17 at 14:03 +0400, Konstantin Khlebnikov wrote:
> >> Avi Kivity wrote:
> >>> What if nr is nonzero when the loop terminates?  You need to update the
> >>> first iovec entry so you don't redo that segment.
> >>
> >> Oh yes, it is simple (hunk above), so "const" from prototype must be removed,
> >> while original syscalls declare iov argument as const.
> >
> > The problem here is that if we start changing the internals of the iovec
> > struct it forces us to copy it before calling the I/O functions since if
> > we change it within the functions it's unusable to the caller anymore.
> >
> > Currently it's passed as a const because we only move pointers outside
> > the struct.
> >
> > Basically the trade-off here is copying iovec before every read/write vs
> > having to sometimes read data again (<  1 block), so I've went with the
> > latter.
> >
> > Do you think it's better to go with the first alternative?
> >
> >
> 
> Or we can call one non-vectored operation for the rest of last io-verctor.

I don't think it's worth delaying the next readv() to run through
read_in_full, and whats more, the part we're going to read again is
probably sitting in the file cache anyway.

-- 

Sasha.