Kalpak Shah wrote:
Hi,
There have been reported instances of a filesystem having been
mounted at 2 places at the same time causing a lot of damage to the
filesystem. This patch reserves superblock fields and an INCOMPAT
flag for adding multiple mount protection(MMP) support within the
ext4 filesystem itself. The superblock will have a block number
(s_mmp_block) which will hold a MMP structure which has a sequence
number which will be periodically updated every 5 seconds by a
mounted filesystem. Whenever a filesystem will be mounted it will
wait for s_mmp_interval seconds to make sure that the MMP sequence
does not change. To further make sure, we write a random sequence
number into the MMP block and wait for another s_mmp_interval secs.
If the sequence no. doesn't change then the mount will succeed. In
case of failure, the nodename, bdevname and the time at which the MMP
block was last updated will be displayed. tune2fs can be used to set
s_mmp_interval as desired.
What would the default value of s_mmp_interval be? 5 seconds? more?
If I am not reading this wrong a mount will take more than
's_mmp_interval' seconds to complete. Wouldn't this be too much of a
penalty during boot up if the system has many 'mount at boot' filesystems?
Also, I am curious about this. Is there a test case for mounting the
same filesystem multiple times? Does this use different paths to reach
the device? Or is there a race? Or does it happen on a device shared by
multiple hosts?
-Manoj
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