Patrick O'Callaghan wrote:
If I am reading this correctly, it is talking about the buffers on the drive itself, if you have write buffering enabled on the drive. To get around this, you would have to send a command to the drive telling it to write its buffers to disk, and have some way to know that it has done this. A work around would be to make sure that write caching is disabled on any removable hard drive.On Thu, 2008-04-24 at 10:28 -0500, Mikkel L. Ellertson wrote:Patrick O'Callaghan wrote:By design, the default action of umount is to fail if there are any open files on the filesystem. I am not sure what potential race condition you are talking about - I would think that proper unmount code would first make sure the file system in not in use, then block opening of files on the file system, write any dirty buffers, and then unmount the file system.You've stopped using the filesystem, you sync to make sure everything's hunky dory, then you unmount. If you can't be sure that activity has stopped (i.e. there are no open files or current directories on the fs) you have to hope that unmount will fail and let you know, but there is a potential race condition if you're not careful. The only way to avoid the race would be if 'unmount' included a 'sync' within the kernel, but the docs don't say that. I imagine the reason it's that way is that unmount can take an indeterminate amount of time, especially for remotely mounted filesystems. Just speculating of course. pocThe potential race is when the last process closes the last file, then unmount goes into action. Unmount will find the fs 'unused' because there are no open files, but the buffer system may not have finished writing blocks to the device. Also, when the 'sync' command returns, all you can be sure of is that the buffer flushing action has been started in the kernel, *not* that the data is physically on the drive.From the BUGS section of sync(2):According to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but may return before the actual writing is done. However, since version 1.3.20 Linux does actually wait. (This still does not guarantee data integrity: modern disks have large caches.)
For some file systems, you can use the sync option - but this does not help with NFS. Then again, any network file system is venerable it network problem...Also, as I've said before, it may be "reasonable" to assume that unmount flushes the buffers, but the docs don't actually say this. We all imagine sync(2) and umount(2) in the context of disks or pendrives, but they are also supposed to work for other kinds of filesystem, including NFS and so on.
As far as the man page goes, it may be in need or re-writing. You are correct that it does not say it say it writes the buffers, but it does not say it does not do this. So we are left guessing. It may be that that the person that wrote the man page didn't feel it needed to be covered. I have run into this before on other man pages.
Mikkel -- Do not meddle in the affairs of dragons, for thou art crunchy and taste good with Ketchup!
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