Re: [Qemu-devel] [PATCH 0/3] block: blk_co_pcache

[Kevin Wolf <kwolf@redhat.com>]

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Am 17.06.2019 um 15:09 hat Eric Blake geschrieben:
> On 6/17/19 7:09 AM, Kevin Wolf wrote:
> 
> >>>
> >>> Hmm, don't you think that blk_co_pcache sends NBD_CMD_CACHE if called on nbd driver?
> >>> I didn't implement it. But may be I should..
> >>>
> >>> May aim was only to avoid extra allocation and unnecessary reads. But if we implement
> >>> full-featured io request, what should it do?
> >>>
> >>> On qcow2 with backing it should pull data from backing to top, like in copy-on-read.
> >>> And for nbd it will send NBD_CMD_CACHE?
> >>> These semantics seems different, but why not?
> >>>
> >>
> >> Any opinions here? Should I resend or could we use it as a first step,
> >> not touching external API but improving things a little bit?
> > 
> > I'm not opposed to making only a first step now. The interface seems to
> > make sense to me; the only thing that I don't really like is the naming
> > both of the operation (blk_co_pcache) and of the flag (BDRV_REQ_CACHE)
> > because to me, "cache" doesn't mean "read, but ignore the result".
> > 
> > The operation only results in something being cached if the block graph
> > is configured to cache things. And indeed, the most importatn use case
> > for the flag is not populating a cache, but triggering copy-on-read. So
> > I think calling this operation "cache" is misleading.
> > 
> > Now, I don't have very good ideas for better names either. I guess
> > BDRV_REQ_NO_DATA could work, though it's not perfect. (Also, not sure if
> > a blk_co_preadv_no_read wrapper is even needed when you can just call
> > blk_co_preadv with the right flag.)
> > 
> > I'm open for good naming ideas.
> 
> Would 'prefetch' be a more useful name? The name NBD_CMD_CACHE was
> chosen in the NBD project, but we are not stuck to that name if we think
> something better makes more sense.

Whether 'prefetch' is entirely accurate really depends on the graph
configuration, too. But at least it gives me the right idea of "read
something, but don't return it yet", so yes, I think that would work for
me.

Kevin

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