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Overview
Comment: | Initial work on documentation of the VFS. |
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5654d9aebdbc1eda14cbc79cf0edd59f |
User & Date: | drh 2011-05-20 20:41:41.072 |
Context
2011-05-20
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23:21 | Further tightening of the validity rules for valid HTTP requests in althttpd.c. (check-in: 73bc374298 user: drh tags: trunk) | |
20:41 | Initial work on documentation of the VFS. (check-in: 5654d9aebd user: drh tags: trunk) | |
2011-05-19
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17:14 | Merge the 3.7.6.3 changes into trunk. (check-in: 6800ff0968 user: drh tags: trunk) | |
Changes
Added images/vfs1.gif.
cannot compute difference between binary files
Added pages/vfs.in.
> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 | <title>The OS Backend (VFS) To SQLite</title> <tcl>hd_keywords VFS VFSes {OS backend}</tcl> <h1 align="center"> The SQLite OS Interface or "VFS" </h1> <p> This article describes the SQLite OS portability layer or "VFS" - the thin module that is at the bottom of the SQLite implementation stack and which provides portability across operating systems. </p> <img src="images/vfs1.gif" align="right" hspace="10"> <h2>1.0 The VFS In Relation To The Rest Of SQLite</h2> <p> The internal organization of the SQLite library can be viewed as a stack of modules such as shown to the right. The Tokenizer, Parser, and Code Generator components are used to process SQL statements and convert them into executable programs in a virtual machine language or byte code. Roughly speaking, these top three layers implement [sqlite3_prepare_v2()]. The byte code generated by the top three layers is a [prepared statement]. The Virtual Machine module is responsible for running the SQL statement byte code. The B-Tree module organizes a database file into multiple key/value stores with ordered keys and logarithmic performance. The Pager module is responsible for loading pages of the database file into memory, for implementing and controlling transactions, and for creating and maintaining the journal files that prevent database corruption following a crash or power failure. The OS Interface is a thin abstraction that provides a common set of routines for adapting SQLite to run on different operating systems. Roughly speaking, the bottom four layers implement [sqlite3_step()]. </p> <p> This article is about the bottom layer. </p> <p>The OS Interface - also called the "VFS" - is what makes SQLite portable across operating systems. Whenever any of the other modules in SQLite needs to communicate with the filesystem, or the operating system, they invoke methods in the VFS to communicate with the operating system on their behalf. Hence, porting SQLite to a new operating system is simply a matter of writing a new OS interface layer or "VFS".</p> <h2>2.0 Multiple VFSes</h2> <p> The standard SQLite source tree contains built-in VFSes for os/2, unix, and windows. Alternative VFSes can be added at start-time or run-time using the [sqlite3_vfs_register()] interface. </p> <p> Multiple VFSes can be registered at the same time. Each VFS has a unique names. Separate [database connections] within the same process can be using different VFSes at the same time. For that matter, if a single database connection has multiple database files open using the [ATTACH] command, then each database might be using a different VFS. </p> <p> Unix builds come with multiple VFSes built-in. The default VFS for unix is called "unix" and is what is used in an overwhelming majority of applications. Other VFSes that can be found in unix may include: </p> <ol> <li><p><b>unix-dotfile</b> - uses dot-file locking rather than POSIX advisory locks. <li><p><b>unix-excl</b> - obtains and holds an exclusive lock on database files, preventing other processes from accessing the database. Also keeps the [wal-index] in heap rather than in shared memory. <li><p><b>unix-none</b> - all file locking operations are no-ops. <li><p><b>unix-namedsem</b> - uses named semaphores for file locking. VXWorks only. </ol> <p> The various unix VFSes differ only in the way they handle file locking - they share most of the implementation in common with one another and are all located in the same SQLite source file: "<tt>os_unix.c</tt>". Note that because they do use different locking strategies, they are subtle imcompatible with one another. If two processes are accessing the same SQLite database using different unix VFSes, they will probably not see each others locks and may end up interfering with one another, resulting in database corruption. The "unix-none" VFS in particular does no locking at all and will easily result in database corruption if used to access a database by two or more database connections at a time. </p> <h2>2.1 Specifying Which VFS To Use</h2> <p> There is always one VFS which is the default VFS. On unix systems, the "unix" VFS comes up as the default and on windows it is "win32". If no other actions are taken, new database connections will make use of the default VFS. </p> <p> The default VFS can be changed by registering or re-registering the VFS using the [sqlite3_vfs_register()] interface with a second parameter of 1. Hence, of a (unix) process to make the "unix-nolock" VFS the default in place of "unix", the following code would work: </p> <blockquote><pre> sqlite3_vfs_register(sqlite3_vfs_find("unix-nolock"), 1); </pre></blockquote> <p> An alternate VFS can also be specified as the 4th parameter to the [sqlite3_open_v2()] function. For example: </p> <blockquote><pre> int rc = sqlite3_open_v2("demo.db", &db, SQLITE_OPEN_READWRITE, "unix-nolock"); </pre></blockquote> <p> Finally, if [URI filenames] have been enabled, then the alternative VFS can be specified using the "vfs=" parameter on the URI. This technique works with [sqlite3_open()], [sqlite3_open16()], [sqlite3_open_v2()], and when a new database is [ATTACH]-ed to an existing database connection. For example: </p> <blockquote><pre> ATTACH 'file:demo2.db?vfs=unix-none' AS demo2; </pre></blockquote> <p> The VFS specified by a URI has the highest priority. After that comes a VFS specified as the fourth argument to [sqlite3_open_v2()]. The default VFS is used if no VFS is specified otherwise. </p> <h2>2.2 VFS Shims</h2> <p> From the point of view of the uppers layers of the SQLite stack, there is only one VFS in use at a time. But in practice, a particular VFS might just be a thin wrapper around another VFS does the real work. We call a wrapper VFS a "shim". </p> <p> A simple example of a shim is the "vfstrace" VFS. This is a VFS (imnplemented in the [http://www.sqlite.org/src/finfo?name=src/test_vfstrace.c | test_vfstrace.c] source file) that writes a message associated with each VFS method call into a log file, then passes control off to another VFS to do the actual work. </p> <h2>2.3 Other Example VFSes</h2> <p> The following are other VFS implementations available in the public SQLite source tree: </p> <ul> <li><p> <b>test_demovfs.c</b> - This file implements a very simple VFS named "demo" that uss POSIX functions such as open(), read(), write(), fsync(), close(), fsync(), sleep(), time(), and so forth. This VFS only works on unix systems. But it is not intended as a replacement for the standard "unix" VFS used by default on unix platforms. The "demo" VFS is deliberately kept very simple so that it can be used as a learning aid or as template for building other VFSes or for porting SQLite to new operating systems. <li><p> <b>test_quota.c</b> - This file implements a shim called "quota" that enforces cumulative file size limits on a collection of database files. An auxiliary interface is used to define "quote groups". A quota group is a set of files (database files, journals, and temporary files) whose names all match a [GLOB] pattern. The sum of the sizes of all files in each quota group is tracked, and if that sum exceeds a threshold defined for the quota group, a callback function is invoked. That callback can either increase the threshold or cause the operation that would have exceeded the quota to fail with an [SQLITE_FULL] error. One of the uses of this shim is used to enforce resource limits on application databases in Firefox. <li><p> <b>test_multiplex.c</b> - This file implements a shim that allows database files to exceed the maximum file size of the underlying filesystem. This shim presents an interface to the upper six layers of SQLite that makes it look like very large files are being used, when in reality each such large file is split up into many smaller files on the underlying system. This shim has been used, for example, to allow databases to grow larger than 2 gibibytes on FAT16 filesystems. <li><p> <b>test_onefile.c</b> - This file implements a demonstration VFS named "fs" that shows how SQLite can be used on an embedded device that lacks a filesystem. Content is written directly to the underlying media. A VFS derived from this demonstration code could be used by a gadget with a limited amount of flash memory to make SQLite behave as the filesystem for the flash memory on the device. <li><p> <b>test_journal.c</b> - This file implements a shim used during SQLite testing that verifies that the database and rollback journal are written in the correct order and are "synced" at appropriate times in order to guarantee that the database can recover from a power lose are hard reset at any time. The shim checks several invariants on the operation of databases and rollback journals and raises exceptions if any of those invariants are violated. These invariants, in turn, assure that the database is always recoverable. Running a large suite of test cases using this shim provides added assurance that SQLite databases will not be damaged by unexpected power failures or device resets. <li><p> <b>test_vfs.c</b> - This file implements a shim that can be used to simulate filesystem faults. This shim is used during testing to verify that SQLite responses sanely to hardware malfunctions or to other error conditions such as running out of filesystem space that are difficult to test on a real system. </ul> <p> There are other VFS inplementations both in the core SQLite source code library and in available extensions. The list above is not meant to be exhaustive but merely representative of the kinds of features that can be realized using the VFS interface. </p> <h2>3.0 VFS Implementations</h2> <p> A new VFS is implemented by subclassing three objects: </p> <ul> <li>[sqlite3_vfs] <li>[sqlite3_io_methods] <li>[sqlite3_file] </ul> <p> An [sqlite3_vfs] object defines the name of the VFS and the core methods that implement the interface to the operating system, such as checking for existance of files, deleting files, creating files and opening and for reading and/or writing, converting filenames into their canonical form. The [sqlite3_vfs] object also contains methods for obtaining randomness from the operating system, for suspending a process (sleeping) and for finding the current date and time. </p> <p> The [sqlite3_file] object represents an open file. The xOpen method of [sqlite3_vfs] constructs an [sqlite3_file] object when the file is opened. The [sqlite3_file] keeps track of the state of the file while it is opened. </p> <p> The [sqlite3_io_methods] object holds the methods used to interact with an open file. Each [sqlite3_file] contains a pointer to an [sqlite3_io_methods] object that is appropriate for the file it represents. The [sqlite3_io_methods] object contains methods to do things such as read and write from the file, to truncate the file, to flush any changes to persistent storage, to find the size of the file, to lock and unlock the file, and to close file and destroy the [sqlite3_file] object. </p> <p> Writing the code for a new VFS involves constructing a subclass for the [sqlite3_vfs] object and then registering that VFS object using a call to [sqlite3_vfs_register()]. The VFS implementation also provides subclasses for [sqlite3_file] and [sqlite3_io_methods] but those objects are not registered directly with SQLite. Instead, the [sqlite3_file] object is returned from the xOpen method of [sqlite3_vfs] and the [sqlite3_file] object points to an instance of the [sqlite3_io_methods] object. </p> <h2>To Be Continued...</h2> |