On Tue, Nov 29, 2005 at 09:47:57AM -0600, Dave Sherohman wrote: > As seen in `man mke2fs`: > -m reserved-blocks-percentage > Specify the percentage of the filesystem blocks > reserved for the super-user. This value defaults > to 5%. > > The purpose of this is basically to ensure that user processes are > not able to bring the system down by filling up the disk, preventing > root-owned daemons from being able to function.
I did some investigation (thanks, Google!) and found out that while the reservation does allow root (and some daemons) to keep running when a user has filled the disk, the main reason it was set at 5% was because ext2/ext3 have problems with fragmentation and start thrashing at the 95% full mark. Filesystems that don't have fragmentation problems reserve little or no space for root. If you are only using an ext2/ext3 partition to store large files then you can use tune2fs to reduce the reserved space. This may or may not be a red herring, though. It depends on how MythTV's auto-expire is checking the free space. The only safe way is to use the statfs call and check the f_bavail value as it corrects for the reservation while f_bfree does not. The values reported in 'df' demonstrate the space lost to the reservation. I created a 8Mb file and formatted it with an ext2 filesystem with '-m 50' to reserve half of the space. After mounting it via '-o loop' the df results are: Filesystem 1K-blocks Used Available Use% Mounted on /home/chris/test.bin 7931 1 3880 1% /home/chris/test The file was large enough to hold 8192 blocks, but some was skipped to leave room for a boot sector and superblock. Of the remaining 7931 blocks, 1 holds the filesystem's root directory, 3880 are free, and 4050 are "missing" since only root can use them. -- Joke template: Three guys walk into a bar. One of them is a wee bit stupid, and the whole scene unfolds with a tedious inevitability.
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