Re: data corruption: revalidating a (removable) hdd/flash on re-insert

Kay Sievers
2008-11-04T20:13:24Z

On Tue, Nov 4, 2008 at 20:57, Pavel Machek <pavel@suse.cz> wrote:
> On Fri 2008-10-31 17:10:26, Kay Sievers wrote:
>> On Fri, Oct 31, 2008 at 16:38, Michael Tokarev <mjt@tls.msk.ru> wrote:
>> > To make a long story short: is there a way to force kernel
>> > to re-validate a replaced usb-connected hard drive (or a
>> > flash) *automatically*?
>> >
>> > Because right now, the kernel does not see that the drive
>> > has been replaced, and uses *some* old cached values, which
>> > results in random data corruption here and there, and other
>> > similar odd things.
>>
>> Maybe your card reader is broken. I can not reproduce this with any of
>> the many readers I have. Usually a media change results in media
>> revalidation with the next access to the device. You can easily
>> reproduce that:
>>
>> Insert the media, and force a validation:
>> $ touch /dev/sdb
>>
>> Start logging of the kernel uevents to the console:
>> $ udevadm monitor --kernel &
>>
>> Access the device:
>> $ touch /dev/sdb
>>
>> Nothing should happen, as the reader/kernel knows it is still valid.
>>
>> Now remove the media and insert it immediately again.
>>
>> Access the device:
>> $ touch /dev/sdb
>> UEVENT[1225468868.803950] change
>> /devices/pci0000:00/0000:00:1d.7/usb5/5-2/5-2:1.0/host8/target8:0:0/8:0:0:0
>> (scsi)
>>
>> and you see the reader told to kernel (scsi unit attention) to
>> revalidate the device.
>>
>> These events happen only when the device is accessed. That's why
>> distros poll removable devices for media changes.
>>
>> Every access to removable media is guarded by this revalidation check.
>> If you don't see these events, you should not trust this reader, and
>> at least never change the media while it is connected.
>
> This is rather nasty data-corrupter.

Sure, it is.

> Could we at least blacklist
> broken device, and force revalidation on each close or something like
> that?

What's your idea of revalidation if the hardware does not tell you?
Get an md5 of the disk content? :)

Kay