[SOLVED] What drives does df query?

RandomTroll · 03-15-2020, 08:51 PM

When I run df, I get

Quote:

df: /usb: Input/output error

Why does df query /usb? It was mounted on /dev/sdb1, but that device is now absent, had been properly unmounted. There was an entry for /usb in /etc/mtab, but I deleted it. When I attach a usb device, it gets /dev/sdc, so the system thinks /dev/sdb isn't available.

ferrari · 03-15-2020, 09:41 PM

What does this command output?

Code:

sudo blkid

berndbausch · 03-15-2020, 10:43 PM

There might be interesting information in the message buffer, for example

Code:

dmesg | grep -C 4 sdb

A reboot should fix that anyway.

Edit: partprobe and partx could be useful tools here as well. Perhaps the kernel thinks that one of the sdb partitions is still in use and needs to be told that this is not so. partx allows editing the kernel's copy of the partition table.

pan64 · 03-16-2020, 12:47 AM

for me that means: the system thinks /usb is still available, was not properly detached. As it was mentioned a reboot should help. Also you need to give more details if it [will] happen again.
You may find additional info in /var/log (about all the events related to it, like mount/umount/eject...)

If I remember well manual editing of /etc/mtab is pointless, but it may depend on your OS.

RandomTroll · 03-16-2020, 08:09 AM

Quote:

Originally Posted by ferrari

What does this command output?

Code:

sudo blkid

Quote:

/dev/sda1: UUID="c4c9a606-76ac-49cc-a7c6-2a32c20978b2" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="136bebb0-01"
/dev/sda2: UUID="20fa2d53-bbf4-4f5c-b6a1-04235aa24480" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="136bebb0-02"
/dev/sr1: BLOCK_SIZE="2048" UUID="2014-03-28-11-03-18-00" LABEL="Mobile Hotspot" TYPE="iso9660" PTTYPE="mac"

Quote:

Originally Posted by berndbausch

There might be interesting information in the message buffer, for example

Code:

dmesg | grep -C 4 sdb

returns a slew of errors. It seems that although umount /usb returned no errors to stderr it failed and recorded errors in logs.

Quote:

Originally Posted by berndbausch

A reboot should fix that anyway.

Yes. I didn't ask for a fix. I asked what df was consulting that made it query /usb. Perhaps a cached version of /etc/mtab? What can I make it do so that it doesn't?

Quote:

Originally Posted by berndbausch

Perhaps the kernel thinks that one of the sdb partitions is still in use and needs to be told that this is not so.

I asked how to tell the kernel that no sdb partition is in use.

Quote:

Originally Posted by berndbausch

partx allows editing the kernel's copy of the partition table.

partx tells me /dev/sdb isn't.

Quote:

Originally Posted by pan64

for me that means: the system thinks /usb is still available, was not properly detached.

So I think. I asked what df consulted.

Quote:

Originally Posted by pan64

As it was mentioned a reboot should help.

I didn't ask for help.

Quote:

Originally Posted by pan64

Also you need to give more details if it [will] happen again.
You may find additional info in /var/log (about all the events related to it, like mount/umount/eject...)

It's happening now.

Quote:

Originally Posted by pan64

If I remember well manual editing of /etc/mtab is pointless, but it may depend on your OS.

It makes mount not query.

pan64 · 03-16-2020, 08:35 AM

Quote:

Originally Posted by RandomTroll

It makes mount not query.

Are you sure about that? mtab is a "virtual" file, actually generated when you want to read it, editing it is just pointless. But its "implementation"/behavior depends on the OS.

MadeInGermany · 03-16-2020, 04:48 PM

Traditionally /etc/mtab is a file created in parallel to the kernel memory, and df looked at it.
In Linux /proc/mounts is an ASCII representation of the kernel memory.
Recent distos make /etc/mtab a symlink to /proc/mounts. (Check with ls -l)
Then is is quite clear: the mount and df commands must use /proc/mounts.

syg00 · 03-16-2020, 05:33 PM

Hmmm - maybe you should run strace over df before making definitive statements.

As the OP wants no help, I'll offer none.

RandomTroll · 03-16-2020, 09:08 PM

Quote:

Originally Posted by pan64

Are you sure about that? mtab is a "virtual" file, actually generated when you want to read it, editing it is just pointless. But its "implementation"/behavior depends on the OS.

Quote:

Originally Posted by MadeInGermany

Traditionally /etc/mtab is a file created in parallel to the kernel memory, and df looked at it.

I have occasional problems with flash drives umounting without complaint but the kernel thinking they're still mounted, reserving the device, e.g., /dev/sdb I have cleared them sometimes by editing /etc/mtab.

Quote:

Originally Posted by MadeInGermany

In Linux /proc/mounts is an ASCII representation of the kernel memory.

/proc/mounts is what I was looking for, thanks.

Quote:

Originally Posted by MadeInGermany

Recent distos make /etc/mtab a symlink to /proc/mounts. (Check with ls -l)

Not the latest Slackware. I can cat them both simultaneously, see the differences, though mtab seems to be a subset of mounts.

Quote:

Originally Posted by MadeInGermany

Then is is quite clear: the mount and df commands must use /proc/mounts.

mount doesn't report /usb being mounted when df complains that it can't query it.

Quote:

Originally Posted by syg00

As the OP wants no help, I'll offer none.

Thanks.

rnturn · 03-17-2020, 11:04 AM

Quote:

Originally Posted by RandomTroll

When I run df, I get Why does df query /usb? It was mounted on /dev/sdb1, but that device is now absent, had been properly unmounted. There was an entry for /usb in /etc/mtab, but I deleted it. When I attach a usb device, it gets /dev/sdc, so the system thinks /dev/sdb isn't available.

I'm not sure if it'll make your USB problem go away but you should at least be able to find out what "df" is looking at to find the USB device by running:

Code:

strace df

Sorry... I don't have any phantom USB devices lingering on my system or I'd post some strace output as an example.

HTH...