Bug 6432

Summary:	USB stick: repreated message in syslog : "assuming drive cache: write through"
Product:	Drivers	Reporter:	Toralf Förster (toralf.foerster)
Component:	USB	Assignee:	Alan Stern (stern)
Status:	REJECTED INVALID
Severity:	low	CC:	akpm, bunk, greg
Priority:	P2
Hardware:	i386
OS:	Linux
Kernel Version:	2.6.16-gentoo-r3	Subsystem:
Regression:	---	Bisected commit-id:
Bug Depends on:
Bug Blocks:	5089

Description Toralf Förster 2006-04-24 01:17:00 UTC

After plugin an (old) 32 MB USB stick I got the following messages in my syslog,
there is one message repeated again : "assuming drive cache: write through"

Here's the log:
...
Apr 24 10:11:26 n22 usb 2-2: new full speed USB device using uhci_hcd and address 6
Apr 24 10:11:26 n22 usb 2-2: configuration #1 chosen from 1 choice
Apr 24 10:11:27 n22 SCSI subsystem initialized
Apr 24 10:11:27 n22 Initializing USB Mass Storage driver...
Apr 24 10:11:27 n22 scsi0 : SCSI emulation for USB Mass Storage devices
Apr 24 10:11:27 n22 usb-storage: device found at 6
Apr 24 10:11:27 n22 usb-storage: waiting for device to settle before scanning
Apr 24 10:11:27 n22 usbcore: registered new driver usb-storage
Apr 24 10:11:27 n22 USB Mass Storage support registered.
Apr 24 10:11:32 n22 Vendor: IBM       Model: Memory Key        Rev: 4.50
Apr 24 10:11:32 n22 Type:   Direct-Access                      ANSI SCSI
revision: 00
Apr 24 10:11:32 n22 usb-storage: device scan complete
Apr 24 10:11:32 n22 scsi.agent[30376]: disk at
/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0/host0/target0:0:0/0:0:0:0
Apr 24 10:11:32 n22 SCSI device sda: 63424 512-byte hdwr sectors (32 MB)
Apr 24 10:11:32 n22 sda: Write Protect is off
Apr 24 10:11:32 n22 sda: Mode Sense: 45 00 00 08
Apr 24 10:11:32 n22 sda: assuming drive cache: write through
Apr 24 10:11:32 n22 SCSI device sda: 63424 512-byte hdwr sectors (32 MB)
Apr 24 10:11:32 n22 sda: Write Protect is off
Apr 24 10:11:32 n22 sda: Mode Sense: 45 00 00 08
Apr 24 10:11:32 n22 sda: assuming drive cache: write through
Apr 24 10:11:32 n22 sda: sda1
Apr 24 10:11:32 n22 sd 0:0:0:0: Attached scsi removable disk sda
...

n22 /usr/src/linux # uname -a
Linux n22 2.6.16-gentoo-r3 #10 Sun Apr 23 21:04:53 CEST 2006 i686 Intel(R)
Pentium(R) M processor 1700MHz GNU/Linux

Comment 1 Andrew Morton 2006-04-24 01:33:39 UTC

Why is this a problem?

Comment 2 Toralf Förster 2006-04-24 01:45:06 UTC

It's not a problem, but sometimes such a message indicates a hidden problem/bug.
That's why I filed this report with a low severity.
So if this behaviour is wanted and/or not harmful this bug can be closed.

Comment 3 Alan Stern 2006-08-30 12:48:51 UTC

This is not a bug; it is normal behavior.  It has to do with the way the SCSI
and block layers work.  Each new device is revalidated (which involves checking
its total capacity, the write-protect setting, and the cache setting) when it is
first detected and then again when it is registered with the block layer.

The duplicated messages may be a little annoying, but they aren't a problem. 
And believe me, the developers are well aware of them.  I'm going to close out
this bug report.

Comment 4 Toralf Förster 2006-08-30 23:53:12 UTC

Ah, thanks for the comprehensive answer.