Distributed File System (Microsoft)
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- This article is about Microsoft's implementation of DFS. For general discussion of the concept and other implementations, see Distributed file system.
The Microsoft Distributed File System, or DFS, is a set of client and server services that allow a large enterprise to organize many distributed SMB file shares into a distributed file system. DFS provides location transparency and redundancy to improve data availability in the face of failure or heavy load by allowing shares in multiple different locations to be logically grouped under one folder, or DFS root.
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[edit] Details
When users try to access a share that exists off the DFS root, the user is really looking at a DFS link and the DFS server transparently redirects them to the correct file server and share.
A DFS root can only exist on a Windows 2000 version that is part of the server family, or on Windows Server 2003. Windows 2000 can only host one DFS root per server, while Windows Server 2003 Enterprise and Datacenter Edition can host multiple DFS roots on the same server. (A Samba server can also host the root of a DFS.)
There are two ways of implementing DFS on Windows 2000 and Windows Server 2003:
- Standalone DFS roots allows for a DFS root that exists only on the local computer, and thus does not use Active Directory. A Standalone DFS can only be accessed on the computer which it is created. It doesn't offer any fault tolerance and cannot be linked to any other DFS.
- Domain-based DFS roots exist within Active Directory and can have their information distributed to other domain controllers within the domain — this provides fault tolerance to DFS. DFS roots that exist on a domain must be hosted on a domain controller. This is to ensure that links with the same target get all their information replicated over the network. The file and root information is replicated via the Microsoft File Replication Service (FRS).
Enhanced DFS management is one of the "branch office server management" features added to Windows Server 2003 in the R2 release in 2006.
Versions 2.6 and later of the Linux kernel come with a SMB client VFS called "cifs" that supports DFS.
[edit] Comparison of DFS to AFS
Microsoft's DFS can be compared to AFS because of the similarities they both share.
[edit] Similarities
- Both Dfs and AFS use a volume location server to present many spatially-disparate server volumes in a single tree
- Both Dfs and AFS support strongly encrypted authentication methods
[edit] Benefits
- Dfs has full incremental snapshot capability instead of AFS's single backup volumes
- Dfs client support is built in and supported by Microsoft on NT-based Windows; the AFS client must be installed and supported separately
- Dfs management tools are built in to Microsoft server operating systems
- Dfs-compatible SAN appliances are readily available
- Dfs has read-write (multi-master) replication and failover; AFS only has read-only replication (though DCE DFS, a derivative of AFS, has read-write replication)
- Dfs volumes can be manually parsed and checked using standard filesystem check tools; AFS uses its own file storage mechanisms (namei and inode) that require the AFS salvager to recovery and verify, and in which files and directories appear as nonsense to someone attempting data recovery
[edit] Drawbacks
- AFS has a unified cross-domain namespace (/afs) and standardized AFSDB record to locate servers in other cells
- AFS encrypts transmitted file data using 'fcrypt', a variant of DES
- AFS employs a local file cache on the client machine; Dfs does not, making it infeasible to run applications or directly manipulate data files that are stored on a network volume
- AFS employs UNIX filesystem semantics with few exceptions (@sys, cross-directory hard links and device files unsupported); Dfs employs Windows filesystem semantics, so UNIX applications cannot create case-differentiated file names, create files corresponding to MS-DOS devices (con, aux, prn, lpt[1-9], com[1-9], nul), or create files with reserved characters in the name (such as a colon character)
- AFS users can create and manage their own ACL groups; Dfs ACL groups can only be created by domain administrators
