Re: writev to scsi disks

[James Bottomley]

On Wed, 2005-04-20 at 08:01 -0700, Dheeraj Pandey wrote:
> I was wondering if I did a simple writev to a SCSI disk, does it take
> the sg path to the device? I am guessing sg (REQ_SPECIAL) is only
> true for character devices (and ioctl's) and not block devices.

?  I think you misunderstand how writev works.  It's design is to take a
list of scattered buffers in a user program and consolidate them into a
single write.  This (in the current implementation) is a separate entity
from the block level Scatter Gather.

If by sg write path, if you mean scatter-gather write path, then yes,
that single write would be split up again into a sg list based on the
device parameters if you mean does the writev sg list control where on
the disk the data ends up, then no, if you use a disk device as a simple
file, writev consolidates all writes to the current file position.

James




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