Re: [PATCH] USB: storage: add "no SYNCHRONIZE CACHE" quirk

[James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>]

On Fri, 2015-06-26 at 11:43 +0200, Stefan Richter wrote:
> On Jun 22 James Bottomley wrote:
> > On Mon, 2015-06-22 at 13:30 -0400, Alan Stern wrote:
> > > On Mon, 22 Jun 2015, James Bottomley wrote:
> > > > Obviously, for a disk with a writeback cache that can't do flush, that
> > > > window is much wider and the real solution should be to try to switch
> > > > the cache to write through.
> > > 
> > > I agree.  Doing the switch manually (by writing to the "cache_type" 
> > > attribute file) works, but it's a nuisance to do this when you have a 
> > > portable USB drive that gets moved among a bunch of machines.
> > 
> > Perhaps it might be wise to do this to every USB device ... for external
> > devices, the small performance gain doesn't really make up for the
> > potential data loss.
> 
> Just a small note on the assumption of externally (and in extension,
> temporarily) attached devices:  Not all USB-attached devices are external,
> and not all external devices are used as removable devices.

The problems don't depend on the connection type: internal devices which
have a writeback cache and don't accept flush commands have data
integrity problems too.  I can't really think of many situations where
you'd be willing to sacrifice data integrity for performance.

James

> (For example, I am using 2 internal CD-ROMs via USB-2 + usb-storage and 2
> internal HDDs via USB 3 + uas in a PC with too few SATA ports and no PCIe
> slot to spare for a controller add-on card, but plenty of USB headers
> available on the mainboard.  Similarly, some NASes have their operating
> system located on a USB-attached device.  Small offices use USB-attached
> disks for backup and won't detach such a disk until rotation for off-site
> deposit.  Not to mention embedded computers with USB-attached but fixed
> disks.)



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