Re: dummy scsi read or scsi command periodically for external USB Hard Disk

["Bryn M. Reeves" <bmr@redhat.com>]

On Mon, Jul 07, 2014 at 11:39:05PM +0800, loody wrote:
> hi David:
> 
> 2014-07-07 23:06 GMT+08:00 David Laight <David.Laight@aculab.com>:
> > From: Lars Melin
> > ...
> >> sgread is not included in BusyBox but you should have "touch".
> >> Create a dummy file on the disk and let cron touch it every 4 minutes.
> >
> > You don't need 'touch' a shell redirect eg ": >file" will do open(..., O_CREAT|O_TRUNC).
> > However that still might not force an actual disc access.
> >
> > In any case you really only want to do a read, doing a write will kill the NAND memory.
> 
> actually I have searched the scsi/usb layer for possible dummy read,
> even read sector 0 is fine, but in vain.
> 
> I found the read
> a. determined by VFS -> block layer,
> b. Block layer put it in queue
> c. call scsi pre-queue function to usb layer.
> 
> That mean if I try to read sector from usb devices, I have to create a
> queue and follow above b) and c) rule.
> is there any already kernel API I can use?
> 
> sincerely appreciate all yours help,

If you don't want to put sg_read into your image you could just use a dd;
busybox includes an implementation that should be good enough.

Just make sure you use the right flags to use O_DIRECT access or you'll
just end up hammering on the page cache. Iirc that's "iflags=direct" (check
the busybox docs to make sure it's the same).

Regards,
Bryn.