Re: USB storage & write barrier support?

[Ric Wheeler <rwheeler@redhat.com>]

On 11/23/2009 03:49 PM, Alan Stern wrote:
> On Mon, 23 Nov 2009, Ric Wheeler wrote:
>
>> What we need is to pass down cache flush commands (ATA_CACHE_FLUSH_EXT is what
>> flushed the cache for ATA/S-ATA devices). Even with a command-queue length of 1,
>> the write cache a USB connected s-ata drive would still loose data on power off
>> without this kind of support.
>
> You can't send ATA commands directly over a USB connection without
> using a passthru protocol of some sort.  Some of the USB interfaces in
> various drive enclosures support such a protocol but a lot of them
> don't.
>
> Of course, you _can_ send SCSI's SYNCHRONIZE CACHE command.  What
> ATA/SATA command (if any) the drive's USB interface will translate it
> into is known only to the interface's manufacturer.  Nevertheless,
> that's what we've been depending on because that's what the sd driver
> uses.
>
> Alan
>

That makes sense - so the usb storage stack does the right thing, what happens 
is up to the SCSI -> ATA translation in the target device.

If I still had occasional access to analyzers, we could always test & verify 
that some of the common enclosures work correctly but I have unfortunately lost 
my toys :-)

Thanks!

Ric