On 03/20/18 10:07, Vincenzo Di Salvo wrote:
Hi,
probably what you are saying is so important that solves my doubt about a my
misunderstanding about USB protocol and my idea to use the usbdump is totally
wrong and no more usefull.
Foundamentally my goal is only understand if, pluging a usb device, a direct
writing operation can or not occur from the processor into the usb device.
Hi,
You need to distinguish between daemons and the operating system, OS.
When you plug a USB disk device, typically what happens is that the OS
will read from the disk to figure out partition tables and disk slices.
This part is read-only. Later on the OS will tell this information to
user-space, for example hald, and then depending on what software you
have installed, the disk will be automatically mounted, which might
involve a few write requests. mount(8) has a -r option to avoid any
writes to the disk. Usually this is not set for desktop environments.
So, please, let me ask another question to exclude all my misunderstanding.
You wrote:
“A processor runs on the USB device replying to this protocol. There is no
direct hardware access. You need to ask the manufacturer which USB request
returns the counter you are asking for”.
1- If "there is no direct hardware access", this means that the processor can
only do read-only accesses to the USB device. Is it correct ? yes/no:
The USB storage protocol has separate read and write commands. The PC
sends either a read command or a write command to the USB device and a
small CPU interprets this command and replies back the the PC. This
small CPU then makes a new request for the actual disk drive.
2- Speaking of “which USB request returns the counter” do you mean that the counter
increment occurs "internally" following a request of the processor, excluding a
direct writing operation of the processor into the usb device ?. Is it correct ? yes/no:
USB itself doesn't have any counters for disk access. There is only one
running counter, but it doesn't distinguish between read and write
requests. Any request increments the counter by one.
The OS might however keep statistics on how many read/write requests
have been performed. In FreeBSD these statistics is usually available
through the "sysctl -a" tree.
If so, your reply solves my limited knowledges of the usb protocol.
Further, it is common that when you read files, the access time is
updated in the file tables. This cause write requests. Look for the
description of the "noatime" option in the mount manual page.
--HPS
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