Hi, mballoc currently causes fragmentation of small size files. The behaviour can be observed by running parallel dd on ext4 file system. A sample test case can be found here. http://www.radian.org/~kvaneesh/ext4/mballoc-frag/fragmentation-analysis This is because for small size request/file mballoc uses group prealloc space. That means if you have singe cpu and multiple threads doing parallel io to the file the block request are served from the same prealloc space. This results in the fragmentation observed above. (Group prealloc space is per cpu ) This problem should go away with delayed allocation because the write out happens with multiple blocks and mballoc places then close together even though they get served by the same group prealloc space. Considering that we don't have delalloc yet, i guess we should push mballoc to 2.6.25 with default disabled and O_DIRECT type workloads can enabled then via mount option. -aneesh - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
