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Samba Robert Eckstein, David Collier-Brown, Peter Kelly 1st Edition November 1999 1-56592-449-5, Order Number: 4495 416 pages, $34.95 Buy the hardcopy Table of Contents Chapter 5 Browsing and Advanced Disk Shares 5.5 Locks and Oplocks Concurrent writes to a single file are not desirable in any operating system. To prevent this, most operating systems use locks to guarantee that only one process can write to a file at a time. Operating systems traditionally lock entire files, although newer ones allow a range of bytes within a file to be locked. If another process attempts to write to a file (or section of one) that is already locked, it will receive an error from the operating system and will wait until the lock is released. Samba supports the standard DOS and NT filesystem (deny-mode) locking requests, which allow only one process to write to an entire file on a server at a give time, as well as byte-range locking. In addition, Samba supports a new locking mechanism known in the Windows NT world as opportunistic locking - oplock for short. 5.5.1 Opportunistic Locking Opportunistic locking allows a client to notify the Samba server that it will not only be the exclusive writer of a file, but will also cache its changes to that file on its own machine (and not on the Samba server) in order to speed up file access for that client. When Samba knows that a file has been opportunistically locked by a client, it marks its version as having an opportunistic lock and waits for the client to complete work on the file, at which point it expects the client to send the final changes back to the Samba server for synchronization. If a second client requests access to that file before the first client has finished working on it, Samba can send an oplock break request to the first client. This tells the client to stop caching its changes and return the current state of the file to the server so that the interrupting client can use it as it sees fit. An opportunistic lock, however, is not a replacement for a standard deny-mode lock. It is not unheard of for the interrupting process to be granted an oplock break only to discover that the original process also has a deny-mode lock on a file as well. Figure 5.8 illustrates this opportunistic locking process. 171
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the mangledmapoption. This option allows you to specify mapping patterns that can be used before or even in place of name mangling performed by Samba. For example: [data] mangled map =(*.database *.db) (*.class *.cls) Here, Samba is instructed to search each file it encounters for characters that match the first pattern specified in the parenthesis and convert them to the modified second pattern in the parenthesis for display on an 8.3 client. This is useful in the event that name mangling converts the filename incorrectly or to a format that the client cannot understand readily. Patterns are separated by whitespaces. 5.3 File Permissions and Attributes on MS-DOS and Unix 5.5 Locks and Oplocks O Reilly Home | O Reilly Bookstores | How to Order | O Reilly Contacts International | About O Reilly | Affiliated Companies 1999, O Reilly & Associates, Inc. 170
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We recommend that you leave this option alone unless you have a well-justified need to change it. 5.4.2.7 mangling char This share-level option specifies the mangling character used when Samba mangles filenames into the 8.3 format. The default character used is a tilde (~). You can reset it to whatever character you wish, for instance: [data] mangling char = # 5.4.2.8 mangled stack Samba maintains a local stack of recently mangled 8.3 filenames; this stack can be used to reverse map mangled filenames back to their original state. This is often needed by applications that create and save a file, close it, and need to modify it later. The default number of long filename/mangled filename pairs stored on this stack is 50. However, if you want to cut down on the amount of processor time used to mangle filenames, you can increase the size of the stack to whatever you wish, at the expense of memory and slightly slower file access. [global] mangled stack = 100 5.4.2.9 mangled map If the default behavior of name mangling is not sufficient, you can give Samba further instructions on how to behave
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the defaultcaseconfiguration option. The default for this option is no. If you set it to yes, you should be sure that all clients will be able to handle the mangled filenames that result. You can override it per share as follows: [data] mangle case = yes 169
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8.3 operating systems. The default value is yes. You can override it per share as follows: [data] mangled names = no 5.4.2.6 mangle case This option tells Samba whether it should mangle filenames that are not composed entirely of the case specified
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If you specify this value, the names of newly created files will be translated into uppercase, and cannot be overridden in a program. We recommend that you use the default value unless you are dealing with a Windows for Workgroups or other 8.3 client, in which case it should be upper. 5.4.2.3 preserve case This option specifies whether a file created by Samba on behalf of the client is created with the case provided by the client operating system, or the case specified by the defaultcase configuration option above. The default value is yes, which uses the case provided by the client operating system. If it is set to no, the value of the defaultcase option is used. Note that this option does not handle 8.3 file requests sent from the client - see the short preservecaseoption below. You may want to set this option to yesif applications that create files on the Samba server are sensitive to the case used when creating the file. If you want to force Samba, for example, to mimic the behavior of a Windows NT filesystem, you can leave this option to its default, yes. 5.4.2.4 short preserve case This option specifies whether an 8.3 filename created by Samba on behalf of the client is created with the default case of the client operating system, or the case specified by the defaultcase configuration option. The default value is yes, which uses the case provided by the client operating system. You can let Samba choose the case through the defaultcaseoption by setting it as follows: [global] short preserve case = no If you want to force Samba to mimic the behavior of a Windows NT filesystem, you can leave this option set to its default, yes. 5.4.2.5 mangled names This share-level option specifies whether Samba will mangle filenames for 8.3 clients in that share. If the option is set to no, Samba will not mangle the names and (depending on the client), they will either be invisible or appear truncated to those
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an operating system that takes advantage of case-sensitive filenames, you can set this configuration option to yes as shown here: [accounting] case sensitive = yes Otherwise, we recommend that you leave this option set to its default. 5.4.2.2 default case The defaultcaseoption is used with preservecase. This specifies the default case (upper or lower) that Samba will use when it creates a file on one of its shares on behalf of a client. The default case is lower, which means that newly created files will use the mixed-case names given to them by the client. If you need to, you can override this global option by specifying the following: [global] default case = upper 168
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Table 5.7: Name Mangling Options Option Parameters Function Default Scope case sensitive (casesignames) boolean If yes, Samba will treat filenames as case-sensitive (Windows doesn t). no Share default case (upperor lower) Case to assume as default (only used when preserve case is no). Lower Share preserve case boolean If yes, keep the case the client supplied (i.e., do not convert to default case). yes Share short preserve case boolean If yes, preserve case of 8.3-format names that the client provides. yes Share mangle case boolean Mangle a name if it is mixed case. no Share mangled names boolean Mangles long names into 8.3 DOS format. yes Share mangling char string (single character) Gives mangling character. ~ Share mangled stack numerical Number of mangled names to keep on the local mangling stack. 50 Global mangled map string (list of patterns) Allows mapping of filenames from one format into another. None Share 5.4.2.1 case sensitive This share-level option, which has the obtuse synonym casesignames, specifies whether Samba should preserve case when resolving filenames in a specific share. The default value for this option is no, which is how Windows handles file resolution. If clients are
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Windows for Workgroups 3.1, then you probably do not need to change any of these options from their defaults. 5.4.1.1 Representing and resolving filenames with Samba Another item that we should point out is that there is a difference between how an operating system represents a file and how it resolves it. For example, if you ve used Windows 95/98/NT, you have likely run across a file called README.TXT. The file can be represented by the operating system entirely in uppercase letters. However, if you open an MS-DOS prompt and enter the command editreadme.txt, the all-caps file is loaded into the editing program, even though you typed the name in lowercase letters! This is because the Windows 95/98/NT family of operating systems resolves files in a case-insensitive manner, even though the files are represented it in a case-sensitive manner. Unix-based operating systems, on the other hand, always resolve files in a case-sensitive manner; if you try to edit README.TXT with the command vireadme.txt, you will likely be editing the empty buffer of a new file. Here is how Samba handles case: if the preservecaseis set to yes, Samba will always use the case provided by the operating system for representing (not resolving) filenames. If it is set to no, it will use the case specified by the defaultcaseoption. The same is true for short preservecase. If this option is set to yes, Samba will use the default case of the operating system for representing 8.3 filenames; otherwise it will use the case specified by the default caseoption. Finally, Samba will always resolve filenames in its shares based on the value of the casesensitiveoption. 5.4.2 Mangling Options Samba allows you to give it more refined instructions on how it should perform name mangling, including those controlling the case sensitivity, the character inserted to form a mangled name, and the ability to manually map filenames from one format to another. These options are shown in Table 5.7. 167
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[global] include = /ucsr/local/samba/lib/smb.conf.% m This resolves to smb.conf.WfWg when a Window for Workgroups client attaches. Now you can create a file /usr/local/samba/lib/smb.conf.WfWg which might contain these options: [global] case sensitive = no default case = upper preserve case = no short preserve case = no mangle case = yes mangled names= yes If you are not
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