While we're talking about ext4 allocator issues, the other problem allocator-related issue that needs looking at is the inode allocator; the spreading algorithm causes us to take a performance degredation sooner than we would otherwise when the filesystem is only partially filled (since access times at the beginning of the disk is faster than at the end of the disk for people who are still using spinning rust platters). I talked to a company while I was at OSCON a few weeks ago, and they noticed a regression using one of their benchmarks. I'm not sure how of a big deal that is, since eventually as the disk gets filled, the access times will average out, but for people who only use 30-50% of their filesystem, we can do better by changing our spreading algorithm. Spreading is still important for making the filesystem less likely to be subject to aging, but we may want to use a spreading algorithm that tries using block groups closer to the beginning of the disk, and then only starts using block groups in the second half of the disk only when first half of the disk is filled. The other thing that we should look at is the hueristics that we use when creating a new subdirectory, to decide whether to put the subdirectory in the current flex_bg, or in a new flex_bg. Currently what I coded up is an extension based on BSD's FFS's Orlov algorithm. The basic idea is that if the number of directories in the flex_bg exceeds average number of directories per flex_bg plus inodes_per_group/16, or the number of free inodes in the flex_bg is less than 75% of the average number of inodes per flex_bg, or the number of free blocks is less than 75% of the average free blocks per flex_bg, then we try the next flex_bg in the filesystem until we find one that meets the Orlov criteria. However, the Orlov criteria hasn't been seriously investigated in over two decades, and distribution of files and depth of directory hierarchies probably have changed somewhat since then. So the decision when we decide to move to a new directory is something that we should look at. So in summary, here are some of aspects of the Orlov algorithm that might stand some tweaking: * How to pick the flex_bg for top-level directories (currently we use random starting point) * Whether to use the current flex_bg for subdirectories (currently we use a fairly complicated set of directories) * How to pick a new flex_bg for subdirectories (currently we use the next flex_bg as a starting point) - Ted -- 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
