linux-kernel - Re: generic_make_request fucntion for scsi

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Date:	Sat, 20 Jun 2009 09:53:09 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Ravish Tayal <ravish.tayal@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: generic_make_request fucntion for scsi_disk

Please don't drop the cc list ... part of the point of this is to create
an archive record for people with similar questions to find.

On Sat, 2009-06-20 at 20:11 +0530, Ravish Tayal wrote:
> Thanks James. 
>  
> For simple case when I echo few lines to usb mounted disk (fat file
> system).
>  
> mkdir /var/disk1
> mount /dev/sda /var/disk1
> echo "TESTing " >/var/disk1/test.txt
>  
> in this case will it be echo commands context it will be queue the
> commands to scsiglue.c or in between scsi layer would hold the
> commands.

That echo will end up in the page cache as a dirty page.

> I have noticed on usb analyser the write scsi commands happens for max
> sector size (64K) ... does in this case scsi command would be happen 
> for few bytes ?

Block commands all have to happen at a granularity of the device block
size (512b usually).  The elevators try to build up to larger sizes.

> or in other words is pdflush invokation indirectly control the WRITE
> scsi command length if file is of few MB or scsi layer itself does
> ot ?

The elevators control final request size depending on what inputs they
have and what parameters they were given.  In all cases, the request
will be a multiple of the filesystem block size (which will be the same
or bigger than the device block size).

> I still not able to find where sd register its request queue to file
> system.

It doesn't.  The Filesystem registers with block via sys_mount and SCSI
with block in scsi_lib.c:scsi_alloc_queue().

James

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