On 2010-Jan-19 00:26:27 +0800, Jesus Cea <j...@jcea.es> wrote: >On 01/18/2010 05:11 PM, David Magda wrote: >> Ext2/3 uses 5% by default for root's usage; 8% under FreeBSD for FFS. >> Solaris (10) uses a bit more nuance for its UFS: > >That reservation is to preclude users to exhaust diskspace in such a way >that ever "root" can not login and solve the problem.
At least for UFS-derived filesystems (ie FreeBSD and Solaris), the primary reason for the 8-10% "reserved" space is to minimise FS fragmentation and improve space allocation performance: More total free space means it's quicker and easier to find the required contiguous (or any) free space whilst searching a free space bitmap. Allowing root to eat into that "reserved" space provided a neat solution to resource starvation issues but was not the justification. >I agree that is a lot of space but only 2% of a "modern disk". My point >is that 32GB is a lot of space to reserve to be able, for instance, to >delete a file when the pool is "full" (thanks to COW). And more when the >minimum reserved is 32MB and ZFS can get away with it. I think that >could be a good thing to put a cap to the maximum implicit reservation. AFAIK, it's also necessary to ensure reasonable ZFS performance - the "find some free space" issue becomes much more time critical with a COW filesystem. I recently had a 2.7TB RAIDZ1 pool get to the point where zpool was reporting ~2% free space - and performance was absolutely abyssmal (fsync() was taking over 16 seconds). When I freed up a few percent more space, the performance recovered. Maybe it would be useful if ZFS allowed the "reserved" space to be tuned lower but, at least for ZFS v13, the "reserved" space seems to actually be a bit less than is needed for ZFS to function reasonably. -- Peter Jeremy
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