The Linux kernel does buffered reads and writes using the page cache layer, where the filesystem reads and writes are offloaded to the VM/MM layer. The VM layer does a predictive readahead of data by optionally asking the filesystem to read more data asynchronously than what was requested. The VFS layer maintains a dentry cache which gets populated during access of dentries (either during readdir/getdents or during lookup). This dentries within a directory actually forms the address space for the directory, which is read sequentially during getdents. For network filesystems, the dentries are also looked up during revalidate. During sequential getdents, it makes sense to perform a readahead similar to file reads. Even for revalidations and dentry lookups, there can be some heuristics that can be maintained to know if the lookups within the directory are sequential in nature. With this, the dentry cache can be pre-populated for a directory, even before the dentries are accessed, thereby boosting the performance. This could give even more benefits for network filesystems by avoiding costly round trips to the server. NFS client already does a simplistic form of this readahead by maintaining an address space for the directory inode and storing the dentry records returned by the server in this space. However, this dentry access mechanism is so generic that I feel that this can be a part of the VFS/VM layer, similar to buffered reads of a file. Also, VFS layer is better equipped to store heuristics about dentry access patterns. -- Regards, Shyam
