QFS File System
Configurations
There are three different ways you can configure your QFS file system, whether it is QFS with or without SAM. These three configurations are often referred to as different file systems, but their disk-based file system characteristics are actually the same. You can set up a QFS file system as:
· The “standalone” QFS file system which runs on a single host using unshared storage. This is the simplest configuration.
You can also set up QFS in two different ways on shared storage:
The shared file system will be discussed in the Shared QFS File System paper.
Single Writer,
Multiple Reader QFS file system
The single writer, multiple reader QFS file system configuration shares the same file system among multiple hosts exclusively though fibre connections to the data and metadata disks. Each host mounts the file system over fibre channel connections to shared storage, and each has access to all metadata and data devices. However, one host is designated as the “writer” and mounts the file system with the mount option –o writer. The other hosts are designated as “readers”, and mount the file system with the mount option –o reader. No other configuration is required to implement this file system and no special entries are required in the mcf file. You can convert any QFS file system to a Single Writer, Multiple Reader file system simply by 1) Attaching the shared storage to all hosts that will be mounting the file system 2) Unmounting the file system and remounting it with the appropriate mount option on all hosts.
Only the writer is allowed to write to the file system. It is possible to change the writer host simply by having the writer host unmount the file system so one of the readers can unmount it then remount it with the writer option. In some releases of Sun StorageTek SAM-FS, it is actually possible for multiple hosts to mount the file system with the “writer” mount option, but the result is usually file system corruption because there is no file locking provided, so this configuration is not supported.
There is no authentication with the single writer, multiple reader QFS file system configuration, but it can nonetheless be set up as a very secure configuration, and is used by some corporations that allow downloads from their websites. The web servers are all readers on the QFS file system that contains the publicly available files. The writer is not even on the internet. It is only attached to a private network, so it is nearly impossible to hack.
You must set the metadata device stripe width to 1 on this file system configuration. It can be monitored with samcmd m. Look for the “W” in the status field of the mass storage device output to indicate that the file system is mounted “writer” and an “R” to indicate that it is mounted “reader.”
Archiving with the single-writer, multiple-reader file system can be complicated and it does not work well when files are released from disk, so it is not officially supported by Sun Microsystems. In practice, the “writer” system accesses files the same way a standalone system does, but the “reader” system can never stage files, and will not normally be able to access a file that has been released from disk in any way. The “reader” file system should be mounted “nosam” if the Sun StorageTek SAM-FS product has been installed on the reader host.
In addition, the directory /var/opt/SUNWsamfs/archiver/<file system name>/ArchReq is created when the file system is initialized, so if you the “writer” system to perform archiving, the file system must be initialized on the “writer” system. If you want to switch the reader and the writer, the “ArchReq” directory must be created on the reader, which will then be able to perform archiving once the file system has been re-mounted with the option “writer.” No system with the mount option “reader” can ever perform any archiving activity, because that would require metadata writes. (Long and frustrating research by Jonathan Kennedy and Spencer McEwen, Harvard University Library, eventually uncovered this otherwise undocumented behavior).