layout parameters for different regions of the file. The Progressive File Layout (PFL) feature uses composite layouts to improve file IO performance over a wider range of workloads, as well as simplify usage and administration. For example, a small PFL file can have a single stripe on flash for low access overhead, while larger files can have many stripes for high aggregate bandwidth and better OST load balancing. The composite layouts are further enhanced in the 2.11 release with the File Level Redundancy (FLR) feature, which allows a file to have multiple overlapping layouts for a file, providing RAID 0+1 redundancy for these files as well as improved read performance. The Lustre 2.11 release also added the Data-on-Metadata (DoM) feature, which allows the first component of a PFL file to be stored directly on the MDT with the inode. This reduces overhead for accessing small files, both in terms of space usage (no OST object is needed) as well as network usage (fewer RPCs needed to access the data). DoM also improves performance for small files if the MDT is SSD-based, while the OSTs are disk-based. In Lustre 2.13 the OST Overstriping feature allows a single component to have multiple stripes on one OST, while the Self-Extending Layout feature allows the component size to be dynamic during write so that it can cope with (flash) OSTs running out of space before the whole filesystem is out of space. Metadata objects and DNE remote or striped directories When a client initially mounts a filesystem, it is provided the 128-bit Lustre File Identifier (FID, composed of the 64-bit Sequence number, 32-bit Object ID, and 32-bit Version) of the root directory for the mountpoint. When doing a filename lookup, the client performs a lookup of each pathname component by mapping the parent directory FID Sequence number to a specific MDT via the FID Location Database (FLDB), and then does a lookup on the MDS managing this MDT using the parent FID and filename. The MDS will return the FID for the requested pathname component along with a DLM lock. Once the MDT of the last parent directory is determined, further directory operations (for non-striped directories) take place exclusively on that MDT, avoiding contention between MDTs. For DNE striped directories, the per-directory layout stored on the parent directory provides a hash function and a list of MDT directory FIDs across which the directory is