[LU-10092] PCC: Lustre Persistent Client Cache

• Type: New Feature
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.13.0
• Labels: patch
• Assignee: Li Xi
• Reporter: Li Xi (Inactive)

Description

PCC is a new framework which provides a group of local cache on Lustre client side. No global namespace will be provided by PCC. Each client uses its own SSD as a local cache for it self. Local file system is used on the SSD to manage the data on local caches. Cached I/O is directed to local file system while normal I/O is directed to OSTs.

PCC uses HSM for data synchronization. It uses HSM copytool to restore file from local caches to Lustre OSTs. Each PCC has a copytool instance running with unique archive number. Any remote access from another Lustre client would trigger the data synchronization. If a client with PCC goes offline, the cached data becomes inaccessible for other client temporally. And after the PCC client reboots and the copytool restarts, the data will be accessible again.

Following is what will happen in different conditions:

►When file is being created on PCC

• A normal file is created on MDT;
• An empty mirror file is created on local cache;
• The HSM status of the Lustre file will be set to archived and released;
• The archive number will be set to the proper value.

►When file is being **prefetched* to PCC*

• An mirror file is copied to local cache;
• The HSM status of the Lustre file will be set to archived and released;
• The archive number will be set to the proper value.

►When file is being accessed from PCC

• Data will be read directly from local cache;
• Metadata will be read from MDT, except file size;
• File size will be got from local cache.

PCC should be able to accelerate some applications with certain I/O patterns.

For more information, please check the presentation during LUG'18 (http://wiki.lustre.org/images/0/04/LUG2018-Lustre_Persistent_Client_Cache-Xi.pdf).

Issue Links

• has to be finished together with: LUDOC-432 Create documentation for PCC Feature
• is blocked by: LU-10499 Readonly Persistent Client Cache support
• is related to: LU-7207 HSM: Add Archive UUID to delete changelog records
• is related to: LU-13137 User process segfaults since 2.13 client upgrade
• is related to: LU-10602 Add file heat support for Persistent Client Cache
• is related to: LU-10606 HSM info as part of LOV layout xattr
• is related to: LU-11333 Using PCC to cache whole "virtual" directories
• is related to: LU-10918 Configurable rule based auto PCC caching for newly created files
• is related to: LU-13924 Document PCC "lfs pcc add" options
• is related to: LU-10114 Feasibility of increasing upper limit of maximum HSM backends registered with MDT