Re: [Qemu-devel] [PATCH] block/iscsi: use 16 byte CDBs only when necessary

[ronnie sahlberg]

On Wed, Sep 3, 2014 at 7:18 AM, Paolo Bonzini <address@hidden> 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.