From d4b35b895cdf157e49609b59ec89ab648dafb524 Mon Sep 17 00:00:00 2001 From: John Terpstra Date: Wed, 13 Apr 2005 04:04:36 +0000 Subject: More updates. (This used to be commit 20f8bde1d0a2b2e42efedcdac21778fe34c0ab79) --- .../TOSHARG-HighAvailability.xml | 414 +++++++++++++++++++++ 1 file changed, 414 insertions(+) create mode 100644 docs/Samba-HOWTO-Collection/TOSHARG-HighAvailability.xml (limited to 'docs/Samba-HOWTO-Collection/TOSHARG-HighAvailability.xml') diff --git a/docs/Samba-HOWTO-Collection/TOSHARG-HighAvailability.xml b/docs/Samba-HOWTO-Collection/TOSHARG-HighAvailability.xml new file mode 100644 index 0000000000..385646d91f --- /dev/null +++ b/docs/Samba-HOWTO-Collection/TOSHARG-HighAvailability.xml @@ -0,0 +1,414 @@ + + + + + &author.jht; + &author.jeremy; + + +High Availability + + +Features and Benefits + + +Network administrators are often concerned about the availability of file and print +services. Network users are inclined toward intolerance of the services they depend +on to perform vital task responsibilities. + + + +A sign in a computer room served to remind staff of their responsibilities. It read: + + +
+ +All humans fail, in both great and small ways we fail continually. Machines fail too. +Computers are machines that are managed by humans, the fallout from failure +can be spectacular. Your responsibility is to deal with failure, to anticipate it +and to eliminate it as far as is humanly and economically wise to achieve. +Are your actions part of the problem or part of the solution? + +
+ + +If we are to deal with failure in a planned and productive manner, then first we must +understand the problem. That is the purpose of this chapter. + + + +Parenthetically, in the following discussion there are seeds of information on how to +provision a network infrastructure against failure. Our purpose here is not to provide +a lengthy dissertation on the subject of high availability. Additionally, we have made +a conscious decision to not provide detailed working examples of high availability +solutions; instead we present an overview of the issues in the hope that someone will +rise to the challenge of providing a detailed document that is focused purely on +presentation of the current state of knowledge and practice in high availability as it +applies to the deployment of Samba and other CIFS/SMB technologies. + + + + + +Technical Discussion + + +The following summary was part of a presentation by Jeremy Allison at the SambaXP 2003 +conference that was held at Goettingen, Germany, in April 2003. Material has been added +from other sources, but it was Jeremy who inspired the structure that follows. + + + + The Ultimate Goal + + + All clustering technologies aim to achieve one or more of the following: + + + + Obtain the maximum affordable computational power. + Obtain faster program execution. + Deliver unstoppable services. + Avert points of failure. + Exact most effective utilization of resources. + + + + A clustered file server ideally has the following properties: + + + + All clients can connect transparently to any server. + A server can fail and clients are transparently reconnected to another server. + All servers server out the same set of files. + All file changes are immediately seen on all servers. + Requires a distributed file system. + Infinite ability to scale by adding more servers or disks. + + + + + + Why Is This So Hard? + + + In short, the problem is one of state. + + + + + + All TCP/IP connections are dependent on state information. + + + The TCP connection involves a packet sequence number. This + sequence number would need to be dynamically updated on all + machines in the cluster to effect seamless TCP fail-over. + + + + + CIFS/SMB (the Windows networking protocols) uses TCP connections. + + + This means that from a basic design perspective, fail-over is not + seriously considered. + + + All current SMB clusters are fail-over solutions + &smbmdash; they rely on the clients to reconnect. They provide server + fail-over, but clients can lose information due to a server failure. + + + + + + + Servers keep state information about client connections. + + CIFS/SMB involves a lot of state. + Every file open must be compared with other file opens + to check share modes. + + + + + + + The Front-End Challenge + + + To make it possible for a cluster of file servers to appear as a single server that has one + name and one IP address, the incoming TCP data streams from clients must be processed by the + front end virtual server. This server must de-multiplex the incoming packets at the SMB protocol + layer level and then feed the SMB packet to different servers in the cluster. + + + + One could split all IPC$ connections and RPC calls to one server to handle printing and user + lookup requirements. RPC Printing handles are shared between different IPC4 sessions &smbmdash; it is + hard to split this across clustered servers! + + + + Conceptually speaking, all other servers would then provide only file services. This is a simpler + problem to concentrate on. + + + + + + De-multiplexing SMB Requests + + + De-multiplexing of SMB requests requires knowledge of SMB state information, + all of which must be held by the front-end virtual server. + This is a perplexing and complicated problem to solve. + + + + Windows XP and later have changed semantics so state information (vuid, tid, fid) + must match for a successful operation. This makes things simpler than before and is a + positive step forward. + + + + SMB requests are sent by vuid to their associated server. No code exists today to + affect this solution. This problem is conceptually similar to the problem of + correctly handling requests from multiple requests from Windows 2000 + Terminal Server in Samba. + + + + One possibility is to start by exposing the server pool to clients directly. + This could eliminate the de-multiplexing step. + + + + + + The Distributed File System Challenge + + +Distributed File Systems + There exists many distributed file systems for UNIX and Linux. + + + + Many could be adopted to backend our cluster, so long as awareness of SMB + semantics is kept in mind (share modes, locking and oplock issues in particular). + Common free distributed file systems include: +NFS +AFS +OpenGFS +Lustre + + + + NFS + AFS + OpenGFS + Lustre + + + + The server pool (cluster) can use any distributed file system backend if all SMB + semantics are performed within this pool. + + + + + + Restrictive Constraints on Distributed File Systems + + + Where a clustered server provides purely SMB services, oplock handling + may be done within the server pool without imposing a need for this to + be passed to the backend file system pool. + + + + On the other hand, where the server pool also provides NFS or other file services, + it will be essential that the implementation be oplock aware so it can + interoperate with SMB services. This is a significant challenge today. A failure + to provide this will result in a significant loss of performance that will be + sorely noted by users of Microsoft Windows clients. + + + + Last, all state information must be shared across the server pool. + + + + + + Server Pool Communications + + + Most backend file systems support POSIX file semantics. This makes it difficult + to push SMB semantics back into the file system. POSIX locks have different properties + and semantics from SMB locks. + + + + All smbd processes in the server pool must of necessity communicate + very quickly. For this, the current tdb file structure that Samba + uses is not suitable for use across a network. Clustered smbd's must use something else. + + + + + + Server Pool Communications Demands + + + High speed inter-server communications in the server pool is a design prerequisite + for a fully functional system. Possibilities for this include: + + + + + Proprietary shared memory bus (example: Myrinet or SCI [Scalable Coherent Interface]). + These are high cost items. + + + + Gigabit ethernet (now quite affordable). + + + + Raw ethernet framing (to bypass TCP and UDP overheads). + + + + + We have yet to identify metrics for performance demands to enable this to happen + effectively. + + + + + + Required Modifications to Samba + + + Samba needs to be significantly modified to work with a high-speed server inter-connect + system to permit transparent fail-over clustering. + + + + Particular functions inside Samba that will be affected include: + + + + + The locking database, oplock notifications, + and the share mode database. + + + + Failure semantics need to be defined. Samba behaves the same way as Windows. + When oplock messages fail, a file open request is allowed, but this is + potentially dangerous in a clustered environment. So how should inter-server + pool failure semantics function and how should this be implemented? + + + + Should this be implemented using a point-to-point lock manager, or can this + be done using multicast techniques? + + + + + + + + + A Simple Solution + + + Allowing fail-over servers to handle different functions within the exported file system + removes the problem of requiring a distributed locking protocol. + + + + If only one server is active in a pair, the need for high speed server interconnect is avoided. + This allows the use of existing high availability solutions, instead of inventing a new one. + This simpler solution comes at a price &smbmdash; the cost of which is the need to manage a more + complex file name space. Since there is now not a single file system, administrators + must remember where all services are located &smbmdash; a complexity not easily dealt with. + + + + The virtual server is still needed to redirect requests to backend + servers. Backend file space integrity is the responsibility of the administrator. + + + + + + High Availability Server Products + + + Fail-over servers must communicate in order to handle resource fail-over. This is essential + for high availability services. The use of a dedicated heartbeat is a common technique to + introduce some intelligence into the fail-over process. This is often done over a dedicated + link (LAN or serial). + + + +SCSI + Many fail-over solutions (like Red Hat Cluster Manager, as well as Microsoft Wolfpack) + can use a shared SCSI of Fiber Channel disk storage array for fail-over communication. + Information regarding Red Hat high availability solutions for Samba may be obtained from: + www.redhat.com. + + + + The Linux High Availability project is a resource worthy of consultation if your desire is + to build a highly available Samba file server solution. Please consult the home page at + www.linux-ha.org/. + + + + Front-end server complexity remains a challenge for high availability as it needs to deal + gracefully with backend failures, while at the same time it needs to provide continuity of service + to all network clients. + + + + + + MS-DFS: The Poor Man's Cluster + + +MS-DFS +DFSMS-DFS, Distributed File Systems + MS-DFS links can be used to redirect clients to disparate backend servers. This pushes + complexity back to the network client, something already included by Microsoft. + MS-DFS creates the illusion of a simple, continuous file system name space, that even + works at the file level. + + + + Above all, at the cost of complexity of management, a distributed (pseudo-cluster) can + be created using existing Samba functionality. + + + + + + Conclusions + + + Transparent SMB clustering is hard to do! + Client fail-over is the best we can do today. + Much more work is needed before a practical and manageable high + availability transparent cluster solution will be possible. + MS-DFS can be used to create the illusion of a single transparent cluster. + + + + + + -- cgit