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    SubjectRe: Why does __do_page_cache_readahead submit READ, not READA?
    On Wed, Jul 29, 2009 at 11:18:45PM +0200, Jens Axboe wrote:
    > On Wed, Jul 29 2009, Lars Ellenberg wrote:
    > > I naively assumed, from the "readahead" in the name, that readahead
    > > would be submitting READA bios. It does not.
    > >
    > > I recently did some statistics on how many READ and READA requests
    > > we actually see on the block device level.
    > > I was suprised that READA is basically only used for file system
    > > internal meta data (and not even for all file systems),
    > > but _never_ for file data.
    > >
    > > A simple
    > > dd if=bigfile of=/dev/null bs=4k count=1
    > > will absolutely cause readahead of the configured amount, no problem.
    > > But on the block device level, these are READ requests, where I'd
    > > expected them to be READA requests, based on the name.
    > >
    > > This is because __do_page_cache_readahead() calls read_pages(),
    > > which in turn is mapping->a_ops->readpages(), or, as fallback,
    > > mapping->a_ops->readpage().
    > >
    > > On that level, all variants end up submitting as READ.
    > >
    > > This may even be intentional.
    > > But if so, I'd like to understand that.
    > I don't think it's intentional, and if memory serves, we used to use
    > READA when submitting read-ahead. Not sure how best to improve the
    > situation, since (as you describe), we lose the read-ahead vs normal
    > read at that level. I did some experimentation some time ago for
    > flagging this, see:
    > which should pass down READA properly.

    One of the problems in the past was that reada would fail if there
    wasn't a free request when we actually wanted it to go ahead and wait.
    Or something. We've switched it around a few times I think.


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