Kostik Belousov kostikbel at gmail.com
Wed Sep 3 12:40:00 UTC 2008

On Wed, Sep 03, 2008 at 01:53:52PM +0400, Igor Sysoev wrote:
> Hi,
> could anyone tell what does vfs.ffs.rawreadahead enable ?
> As I understand it's used in DIRECTIO code that allows read data
> directly to an userland buffer bypassing the buffer cache.
> What I can not understand where the read ahead data can be placed in ?

The operation of the ffs_rawread is more accurately described as
bypassing the page cache. It creates the physical buffer that maps
the user pages.

The readahead is performed only when the supplied user memory region
is bigger then blocksize. In this case, two reads are performed
simultaneously, with both buffers mapping consequent blocks from
user-supplied buffers. The read operation looks like footsteps.
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