linux-kernel - Re: What still uses the block layer?

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Date:	Wed, 17 Oct 2007 19:43:14 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Jeff Garzik <jeff@...zik.org>
CC:	Greg KH <greg@...ah.com>, Rob Landley <rob@...dley.net>,
	James Bottomley <James.Bottomley@...eleye.com>,
	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, Jens Axboe <axboe@...e.de>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Nick Piggin <piggin@...erone.com.au>
Subject: Re: What still uses the block layer?

Jeff Garzik wrote:

>> But again, please remember that these USB devices are really SCSI
>> devices.  Same for SATA devices.  There is a reason they are using the
>> SCSI layer, and it isn't just because the developers felt like it :)
> 
> /somewhat/ true I'm afraid:  libata uses the SCSI layer for ATAPI 
> devices because they are essentially bridges to SCSI devices.  It uses 
> the SCSI layer for ATA devices because the SCSI layer provided a huge 
> amount of infrastructure that would need to have been otherwise 
> duplicated, /then/ massaged into coordinating between <jgarzik's ATA 
> layer> and <SCSI layer> when dealing with ATAPI.
> 
> There is also a detail that was of /huge/ value when introducing a new 
> device class:  distro installers automatically work, if you use SCSI. If 
> you use a new block device type, that behaves differently from other 
> types and is on a different major, you have to poke the distros into 
> action or do it yourself.
> 
> IOW, it was the high Just Works(tm) value of the SCSI layer when it came 
> to ATA (not ATAPI) devices.
> 
> For the future, ATA will eventually be more independent (though the SCSI 
> simulator will be available as an option, for compat), but the value is 
> big enough to put that task on the back-burner.
> 
I remember being told that I didn't understand the problem when I 
suggested using ide-scsi for everything and just hiding the transport. I 
get great pleasure from having been (mostly) right on that one. I still 
have old systems running ZIP drives as scsi...

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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