Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait /notify + callback chains

[Stephen C. Tweedie]

Hi,

On Mon, Feb 05, 2001 at 11:06:48PM +0000, Alan Cox wrote:
> > do you then tell the application _above_ raid0 if one of the
> > underlying IOs succeeds and the other fails halfway through?
> 
> struct 
> {
>       u32 flags;      /* because everything needs flags */
>       struct io_completion *completions;
>       kiovec_t sglist[0];
> } thingy;
> 
> now kmalloc one object of the header the sglist of the right size and the
> completion list. Shove the completion list on the end of it as another
> array of objects and what is the problem.

XFS uses both small metadata items in the buffer cache and large
pagebufs.  You may have merged a 512-byte read with a large pagebuf
read: one completion callback is associated with a single sg fragment,
the next callback belongs to a dozen different fragments.  Associating
the two lists becomes non-trivial, although it could be done.

--Stephen
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