Subject: Re: letting userland issue FUA writes
To: Bill Studenmund <wrstuden@NetBSD.org>
From: Steven M. Bellovin <firstname.lastname@example.org>
Date: 03/14/2006 23:41:53
On Tue, 14 Mar 2006 18:35:46 -0800, Bill Studenmund
> I'm interested in adding infrastructure so that applications can issue
> writes that have the FUA (Force Unit Access) bit set. SCSI CDBs handle
> this, as do some RAID cards.
> The main question I have is what kind of user-level interfaces exist for
> this? I certainly could cook up a new system call, but I would much rather
> use prior art. My simplistic attempt at googling for this failed. Does
> anyone else have suggestions?
> Given the systemcall interface, driving it through the kernel isn't hard.
> A flag here and a flag there, and it's set.
Given that it pertains to a file descriptor, shouldn't it be an ioctl?
> The only other key question I see is what do we do with a driver that
> doesn't natively handle FUA? My instinct is to error-out the command, as
> that is what will happen if you use FUA to a SCSI disk that doesn't
> support it. The sub-question is should the driver do it, which means every
> driver needs to catch this flag, or should we expose some sort of
> capability and let some higher level of the queuing/processing system
> catch the error.
> Take care,
--Steven M. Bellovin, http://www.cs.columbia.edu/~smb