On Wed, Sep 3, 2014 at 7:18 AM, Paolo Bonzini <pbonz...@redhat.com> wrote: > Il 03/09/2014 16:17, ronnie sahlberg ha scritto: >> I think (a) would be best. >> But I would suggest some small modifications: >> >> Set the default max to something even smaller, like 256 sectors. This >> should not have much effect on throughput since the client/initiator >> can just send several concurrent i/os instead of one large i/o. > > I disagree. 256 sectors are 128k, that's a fairly normal I/O size. >
Fair enough. But maybe a command line argument to set/override the max? This would be useful when using scsi-passthrough to usb-scsi disks. Linux kernel has I think a hard limit on 120k for the usb transport, which means when doing SG_IO to a usb disk you are limited to this as the max transfer length since anything larger will break in the usb layer. This limit also I think is not easily discoverable by an application since * the actual device still reports, in most cases, max_transfer_length == unlimited * and it is semi-hard to discover whether a /dev/sg* device is on a usb bus or not.
