continue to run, experiencing a delay while the backup server takes over the storage. Lustre MDSes are configured as an active/passive pair exporting a single MDT, or one or more active/active MDS pairs with DNE exporting two or more separate MDTs, while OSSes are typically deployed in an active/active configuration exporting separate OSTs to provide redundancy without extra system overhead. In single-MDT filesystems, the standby MDS for one filesystem is the MGS and/or monitoring node, or the active MDS for another file system, so no nodes are idle in the cluster. HSM (Hierarchical Storage Management) Lustre provides the capability to have multiple storage tiers within a single filesystem namespace. It allows traditional HSM functionality to copy (archive) files off the primary filesystem to a secondary archive storage tier. The archive tier is typically a tape-based system, that is often fronted by a disk cache. Once a file is archived, it can be released from the main filesystem, leaving only a stub that references the archive copy. If a released file is opened, the Coordinator blocks the open, sends a restore request to a copytool, and then completes the open once the copytool has completed restoring the file. In addition to external storage tiering, it is possible to have multiple storage tiers within a single filesystem namespace. OSTs of different types (e.g. HDD and SSD) can be declared in named storage pools. The OST pools can be selected when specifying file layouts, and different pools can be used within a single PFL file layout. Files can be migrated between storage tiers either manually or under control of the Policy Engine. Since Lustre 2.11, it is also possible to mirror a file to different OST pools with a FLR file layout, for example to pre-stage files into flash for a computing job. HSM includes some additional Lustre components to manage the interface between the primary filesystem and the archive: Coordinator: receives archive and restore requests and dispatches them to agent nodes. Agent: runs a copytool to copy data from primary storage to the archive and vice versa. Copytool: handles data motion and metadata updates. There are different copytools to interface with different archive systems. A generic POSIX copytool is available for archives that provide a POSIX-like front-end interface. Copytools are also available for the High Performance Storage System (HPSS), Tivoli Storage Manager (TSM), Amazon S3, and Google Drive. Policy Engine: watches filesystem