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The storage efficiency potential of inode inlining can be significant for data sets consisting of large numbers of small files, which would have required a separate inode and data blocks for housing these files prior to OneFS 9.3.
Latency-wise, the write performance for inlined file writes is typically comparable or slightly better as compared to regular files, because OneFS does not have to allocate extra blocks and protect them. This is also true for reads, too, since OneFS does not have to search for and retrieve any blocks beyond the inode itself. This also frees up space in the OneFS read caching layers, as well as on disk, in addition to requiring fewer CPU cycles.
The following diagram illustrates the levels of indirection a file access request takes to get to its data. Unlike a standard file, an inline file will skip the later stages of the path which involve the inode metatree redirection to the remote data blocks.
Access starts with the Superblock, which is located at multiple fixed block addresses on every drive in the cluster. The Superblock contains the address locations of the LIN Master block, which contains the root of the LIN B+ Tree (LIN table). The LIN B+Tree maps logical inode numbers to the actual inode addresses on disk, which, in the case of an inlined file, also contains the data. This saves the overhead of finding the address locations of the file’s data blocks and retrieving data from them.
For hybrid nodes with sufficient SSD capacity, using the metadata-write SSD strategy will automatically place inlined small files on flash media. However, since the SSDs on hybrid nodes default to 512byte formatting, when using metadata read/write strategies, these SSD metadata pools will need to have the ‘--force-8k-inodes’ flag set in order for files to be inlined. This can be a useful performance configuration for small file HPC workloads, such as EDA, for data that is not residing on an all-flash tier.
Note: Keep in mind that forcing 8KB inodes on a hybrid pool’s SSDs will result in a considerable reduction in available inode capacity than would be available with the default 512 byte inode configuration.