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Try to search in dmesg for errors. For example via
dmesg | less
If there are errors in /etc/fstab the system can't boot properly and usually comes up in rescue mode. In this mode the only drive (root) is mounted as read-only.
Boot into single user mode by selecting your kernel and pressing e to edit and adding either 1, s, or single to the end of the statement. Hit return and then press b to boot. Once you are greeted with a prompt check your /etc/fstab to make sure root is readable. The entry for root should look like this:
UUID=26ab2266-f9f6-40f2-af3f-41f992154ac5 / ext4 defaults 1 1
Additionally check to make sure your other partitions look sane. If those look correct run dmesg to check that it isn't a hard drive problem.
To remount the root volume as RW, to make changes you can use the command:
mount -o remount,rw /
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If you would like remount it to read-write, try to use mount -o remount,rw / |
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Than means that it is important to check if fstab options are correct for new partitions by first un-mounting them them and then trying to mount using
mount -a
Remember that the mount point must already exist, otherwise the entry will not mount on the filesystem
Again, system with incorrect /etc/fstab usually boot to safe mode with root filesystem mounted as read only.
There can be other reasons. Most common are failed disk and a corrupt file system.
In this is a corrupt filesystem case (for example if the server abruptly lost power), in many cases such filesystem can be repaired using fsck utility. To determine which filesystem is affected (and whether this is filesystem related problem at all) useful information usually can be recovered from dmesg command output.
The dmesg command is used to write the kernel messages in Linux and other Unixes to standard output (which by default is the display screen). Here is some recommendation on how to use it (How to use the dmesg command -- by The Linux Information Project (LINFO) ):
The dmesg command is used to write the kernel messages in Linux and other Unix-like operating systems to standard output (which by default is the display screen).
A kernel is the core of an operating system. It is the first part of the operating system that is loaded into memory when a computer boots up (i.e., starts up), and it controls virtually everything on a system. The numerous messages generated by the kernel that appear on the display screen as a computer boots up show the hardware devices that the kernel detects and indicate whether it is able to configure them.
dmesg obtains its data by reading the kernel ring buffer. A buffer is a portion of a computer's memory that is set aside as a temporary holding place for data that is being sent to or received from an external device, such as a hard disk drive (HDD), printer or keyboard. A ring buffer is a buffer of fixed size for which any new data added to it overwrites the oldest data in it.
dmesg can be very useful when troubleshooting or just trying to obtain information about the hardware on a system. Its basic syntax is
dmesg [options]Invoking dmesg without any of its options (which are rarely used) causes it to write all the kernel messages to standard output. This usually produces far too many lines to fit into the display screen all at once, and thus only the final messages are visible. However, the output can be redirected to the less command through the use of a pipe (designated by the vertical bar character), thereby allowing the startup messages to be viewed one screenful at a time:
dmesg | lessless allows the output to be moved forward one screenful at a time by pressing the SPACE bar, backward by pressing the b key and removed by pressing the q key. (The more command could have been used here instead of the less command; however, less is newer than more and has additional functions, including the ability to return to previous pages of the output.)
When a user encounters a problem with the system, it can be convenient to write the output of dmesg to a file and then send that file by e-mail to a system administrator or other knowledgeable person for assistance. For example, the output could be redirected to a file named boot_messages using the output redirection operator (designated by a rightward facing angle bracket) as follows:
dmesg > boot_messagesBecause of the length of the output of dmesg, it can be convenient to pipe its output to grep, a filter which searches for any lines that contain the string (i.e., sequence of characters) following it. The -i option can be used to tell grep to ignore the case (i.e., lower case or upper case) of the letters in the string. For example, the following command lists all references to USB (universal serial bus) devices in the kernel messages:
dmesg | grep -i usbAnd the following tells dmesg to show all serial ports (which are represented by the string tty):
dmesg | grep -i ttyThe dmesg and grep combination can also be used to show how much physical memory (i.e., RAM) is available on the system:
dmesg | grep -i memoryThe following command checks to confirm that the HDD(s) is running in DMA (direct memory access) mode:
dmesg | grep -i dmaThe output of dmesg is maintained in the log file /var/log/dmesg, and it can thus also be easily viewed by reading that file with a text editor, such as vi or gedit, or with a command such as cat, e.g.,
cat /var/log/dmesg | less
Availability of baseline of the configuration is also very helpful.
See also
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Oct 1, 2010 | Server Fault
Tim
Why is my filesystem being mounted read-only in linux?
I am trying to set up a small linux system based on Gentoo on a VirtualBox machine, as a step towards deploying the same system onto a low-spec Single Board Computer. For some reason, my filesystem is being mounted read-only.
In my /etc/fstab, I have:
/dev/sda1 / ext3 defaults 0 0
none /proc proc defaults 0 0
none /sys sysfs defaults 0 0
none /dev/shm tmpfs defaults 0 0However, once booted /proc/mounts shows
rootfs / rootfs rw 0 0
/dev/root / ext3 ro,relatime,errors=continue,barrier=0,data=writeback 0 0
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
udev /dev tmpfs rw,nosuid,relatime,size=10240k,mode=755 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620 0 0
none /dev/shm tmpfs rw,relatime 0 0
usbfs /proc/bus/usb usbfs rw,nosuid,noexec,relatime,devgid=85,devmode=664 0 0
binfmt_misc /proc/sys/fs/binfmt_misc binfmt_misc rw,nosuid,nodev,noexec,relatime 0 0(the above may contain errors: there's no practical way to copy and paste)
The partition at /dev/hda1 is clearly being mounted OK, since I can read all the data, but it's not being mounted as described in fstab. How might I go about diagnosing / resolving this?
Edit: I can remount with mount -o remount,rw / and it works as expected, except that /proc/mounts reports /dev/root mounted at / rather than /dev/sda1 as I'd expect.
If I try to remount with mount -a I get
mount: none already mounted or /sys busy
mount: according to mtab, sysfs is already mounted on /sysEdit 2: I resolved the problem with mount -a (the same error was occuring during startup, it turned out) by changing the sysfs and proc lines to
proc /proc proc [...]
sysfs /sys sysfs [...]Now mount -a doesn't complain, but it doesn't result in a read-write root partition. mount -o remount / does cause the root partition to be remounted, however.
linux boot gentoo fstab
Can you remount the partion rw? mount -o remount,rw / MattyB Oct 1 '10 at 16:58
Do you use an initrd? If you use one, do you pivot the root file system? Mircea Vutcovici Mar 22 '11 at 1:06
2You say /dev/sda1 in some spots, /dev/hda1 in other spots. Mixing the 2 up can cause the issues youre seeing, so make sure everything is referencing the proper device. Patrick Dec 23 '11 at 4:35
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3 Answers 3
activeoldestvotes
up vote4down voteperhaps it is because the disk is unclean, try changing:
/dev/sda1 / ext3 defaults 0 0
to
/dev/sda1 / ext3 defaults 0 1
or at least do an fsck and then reboot
share|improve this answer
answered Oct 19 '11 at 17:54freedenizen
1412Did this answer by freedenizen help you? It's easy to give back. Sign up to get started.
1
Even if this isnt the cause, the change should still be made. Patrick Dec 23 '11 at 6:52
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Oops! I didn't mean to do this.up vote2down vote
You say it's a VirtualBox machine... Does the VirtualBox process have write privileges to the datastore on the host?
mount -a remounts everything as described in /etc/fstab. If this is not behaving as expected, there may be some output in syslog. Check and post here if there is anything relevant.
share|improve this answer
answered Oct 1 '10 at 18:22Aaron Copley
6,67521844
I can't seem to find syslog on the machine, I don't know if it's been disabled in the distro I'm adapting, I'll have a look at re-enabling it. There doesn't seem to be any logging in /var/log, presumably because the device is read-only so nothing can be written. Tim Oct 2 '10 at 5:01
And yes, on a different VirtualBox guest I can mount the device for writing just fine. Tim Oct 2 '10 at 7:07
Oh, right -- /var/log would be read-only. If only you could edit /etc/syslog.conf you can try logging to a remote host. But then, if you could do that you wouldn't have a problem. Have you had any luck since this was originally posted? Aaron Copley Oct 19 '10 at 19:35
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up vote1down vote
If your are running Ubuntu then try first removing ntfs-3g and then install by running - "aptitude install ntfs-config" This usually fixes it. (The problem is that if you installed some other version of ntfs driver (e.g. additional options og parted) then it does not have write capability.)
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answered Mar 4 '12 at 3:40anonyn
111
I must admit that the post about ntfs-3g didn't look like much to me.. But it was accurate enough to lead me to another solution! I just had this same problem after rebooting a working system. In my case, however, I had to unmerge dosfstols. Best wishes to you all! user204239 Jan 3 '14 at 16:23
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It's telling you that you can't write, because the filesystem was mounted read-only. Logically, you want to remount it with writes enabled, if you want to write.
mount -o remount,rw <your_root_partition such as /dev/sda#> /
To just give my feedback, I wanted to use the vi editor to change the fstab file but I couldn't, because of the read only problem.
I tried to remount the volume, but there was something wrong about what I did. No luck either to copy my fstab.bak file back to fstab. The read only curse.
So, I tried the following and it actualy worked much better than I hoped: During the GRUB listing I pressed "e" for editing the commands before booting and I changed the "ro" argument for the drive to rw. Then, I pressed ctrl-x to boot.
I expected to confront the same "waiting to mount" error, but I didn't. The system started as normally did.
I sudo gedit-ed the fstab file back to the original with a smile from one ear to another and rebooted.
The GRUB file was back to the original "ro" (read only) argument and I just came to register to the forum to write about it, since I 've read about your solution by my phone.
I should say I am using karmic koala with GRUB 1.97
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Idea #18468: Be able to boot into recovery mode and fix malformed /etc/fstab
- Description
- Report duplicate
- Help promote this idea!
Written by dcstar the 6 Mar 09 at 03:49. Category: System. Related project: Nothing/Others. Status: New
Rationale If a user make an error modifying the root filesystem line in their /etc/fstab, they will then always be booted up with the root filesystem in "Read Only" mode so they cannot edit the file and boot normally.
The only repair option now is to boot up using Live CD/Recovery media, then mount the filesystem and then edit the /etc/fstab file (using sudo privileges). This can be a bit much to ask of an inexperienced user.
Tags: boot fstab readonly Recovery
