vfs.ffs.rawreadahead

[Scott Long]

On Wed, 3 Sep 2008, Igor Sysoev wrote:
> On Wed, Sep 03, 2008 at 03:39:55PM +0300, Kostik Belousov wrote:
>
>> 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.
>
> Nice!
>
> As I understand the size limit of one read operation is MAXPHYS, which is
> equal to 128K due to LBA28 ATA limit. On SCSI, SATA, and LBA48 ATA this limit
> can be increased. Is it safe ?
>
>

The value of MAXPHYS is unrelated to capabilities or limitations of ATA.
It was chosen based on the needs to prevent an excessive amount of
parallel I/O from exhausting the kernel address space and system memory. 
In fact, the concern was with SCSI, not with ATA.

MAXPHYS can be raised, especially on 64bit platforms, but doing so also
bloats the sizes of a few key data structures.  I've been looking at a
solution for this, and I'd rather that people keep their MAXPHYS changes
confined to their local trees rather than changing FreeBSD unless they
also solve the associated side effects.

SCott