From: Sergey Poznyakoff Date: Sun, 6 Nov 2005 11:51:12 +0000 (+0000) Subject: Properly document incremental dumps X-Git-Url: https://git.dogcows.com/gitweb?a=commitdiff_plain;h=d01f65092c3ecf3dc8b5f443a542975249afa86c;p=chaz%2Ftar Properly document incremental dumps --- diff --git a/doc/snapshot.texi b/doc/snapshot.texi index d4e2a6a..ab003f7 100644 --- a/doc/snapshot.texi +++ b/doc/snapshot.texi @@ -5,7 +5,7 @@ @c published by the Free Software Foundation. A @dfn{snapshot file} (or @dfn{directory file}) is created during -incremental backups (@pxref{incremental and listed-incremental}). It +incremental backups (@pxref{Incremental Dumps}). It contains the status of the filesystem at the time of the dump and is used to determine which files were modified since the last backup. diff --git a/doc/tar.texi b/doc/tar.texi index 0340802..ddfbac5 100644 --- a/doc/tar.texi +++ b/doc/tar.texi @@ -240,8 +240,7 @@ Coping with Scarce Resources Performing Backups and Restoring Files * Full Dumps:: Using @command{tar} to Perform Full Dumps -* Inc Dumps:: Using @command{tar} to Perform Incremental Dumps -* incremental and listed-incremental:: The Incremental Options +* Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps * Backup Levels:: Levels of Backups * Backup Parameters:: Setting Parameters for Backups and Restoration * Scripted Backups:: Using the Backup Scripts @@ -2203,7 +2202,7 @@ archive, or if they do not already exist in the archive. Normally when creating an archive, @command{tar} strips an initial @samp{/} from member names. This option disables that behavior. -@FIXME-xref{} +@xref{absolute}. @item --after-date @@ -4592,48 +4591,52 @@ http://www.cs.umd.edu/projects/amanda/amanda.html ftp://ftp.cs.umd.edu/pub/amanda @end smallexample -@ifclear PUBLISH +@FIXME{ Here is a possible plan for a future documentation about the backuping scripts which are provided within the @GNUTAR{} distribution. -@smallexample -.* dumps -. + what are dumps - -. + different levels of dumps -. - full dump = dump everything -. - level 1, level 2 dumps etc, - - A level n dump dumps everything changed since the last level - n-1 dump (?) - -. + how to use scripts for dumps (ie, the concept) -. - scripts to run after editing backup specs (details) - -. + Backup Specs, what is it. -. - how to customize -. - actual text of script [/sp/dump/backup-specs] - -. + Problems -. - rsh doesn't work -. - rtape isn't installed -. - (others?) - -. + the --incremental option of tar - -. + tapes -. - write protection -. - types of media -. : different sizes and types, useful for different things -. - files and tape marks +@itemize @bullet +@item dumps + @itemize @minus + @item what are dumps + @item different levels of dumps + @itemize + + @item full dump = dump everything + @item level 1, level 2 dumps etc + A level @var{n} dump dumps everything changed since the last level + @var{n}-1 dump (?) + @end itemize + @item how to use scripts for dumps (ie, the concept) + @itemize + + @item scripts to run after editing backup specs (details) + @end itemize + @item Backup Specs, what is it. + @itemize + + @item how to customize + @item actual text of script [/sp/dump/backup-specs] + @end itemize + @item Problems + @itemize + + @item rsh doesn't work + @item rtape isn't installed + @item (others?) + @end itemize + @item the @option{--incremental} option of tar + @item tapes + @itemize + + @item write protection + @item types of media, different sizes and types, useful for different things + @item files and tape marks one tape mark between files, two at end. -. - positioning the tape + @item positioning the tape MT writes two at end of write, - backspaces over one when writing again. -@end smallexample - -@end ifclear + backspaces over one when writing again. + @end itemize + @end itemize +@end itemize +} This chapter documents both the provided shell scripts and @command{tar} options which are more specific to usage as a backup tool. @@ -4646,8 +4649,7 @@ called @dfn{dumps}. @menu * Full Dumps:: Using @command{tar} to Perform Full Dumps -* Inc Dumps:: Using @command{tar} to Perform Incremental Dumps -* incremental and listed-incremental:: The Incremental Options +* Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps * Backup Levels:: Levels of Backups * Backup Parameters:: Setting Parameters for Backups and Restoration * Scripted Backups:: Using the Backup Scripts @@ -4681,9 +4683,10 @@ If you want to dump each filesystem separately you will need to use the @value{op-one-file-system} option to prevent @command{tar} from crossing filesystem boundaries when storing (sub)directories. -The @value{op-incremental} option is not needed, since this is a complete -copy of everything in the filesystem, and a full restore from this -backup would only be done onto a completely empty disk. +The @value{op-incremental} (@FIXME-pxref{}) option is not needed, +since this is a complete copy of everything in the filesystem, and a +full restore from this backup would only be done onto a completely +empty disk. Unless you are in a hurry, and trust the @command{tar} program (and your tapes), it is a good idea to use the @value{op-verify} option, to make @@ -4692,199 +4695,183 @@ also detect cases where the file was modified while (or just after) it was being archived. Not all media (notably cartridge tapes) are capable of being verified, unfortunately. -@value{op-listed-incremental} take a file name argument always. If the -file doesn't exist, run a level zero dump, creating the file. If the -file exists, uses that file to see what has changed. - -@value{op-incremental} @FIXME{look it up} - -@value{op-incremental} handle old @acronym{GNU}-format incremental backup. +@node Incremental Dumps +@section Using @command{tar} to Perform Incremental Dumps -This option should only be used when creating an incremental backup of -a filesystem. When the @value{op-incremental} option is used, @command{tar} -writes, at the beginning of the archive, an entry for each of the -directories that will be operated on. The entry for a directory -includes a list of all the files in the directory at the time the -dump was done, and a flag for each file indicating whether the file -is going to be put in the archive. This information is used when -doing a complete incremental restore. +@dfn{Incremental backup} is a special form of @GNUTAR{} archive that +stores additional metadata so that exact state of the filesystem +can be restored when extracting the archive. -Note that this option causes @command{tar} to create a non-standard -archive that may not be readable by non-@acronym{GNU} versions of the -@command{tar} program. +@GNUTAR{} currently offers two options for handling incremental +backups: @value{op-listed-incremental} and @value{op-incremental}. -The @value{op-incremental} option means the archive is an incremental -backup. Its meaning depends on the command that it modifies. +The option @option{--listed-incremental} instructs tar to operate on +an incremental archive with additional metadata stored in a standalone +file, called @dfn{snapshot file}. The purpose of this file is to help +determine what files have been changed, added or deleted since the +last backup, so that the next incremental backup will contain only +modified files. The name of the snapshot file is given as an argument +to the option: -If the @value{op-incremental} option is used with @value{op-list}, -@command{tar} will list, for each directory in the archive, the list -of files in that directory at the time the archive was created. This -information is put out in a format that is not easy for humans to -read, but which is unambiguous for a program: each file name is -preceded by either a @samp{Y} if the file is present in the archive, -an @samp{N} if the file is not included in the archive, or a @samp{D} -if the file is a directory (and is included in the archive). Each -file name is terminated by a null character. The last file is -followed by an additional null and a newline to indicate the end of -the data. - -If the @value{op-incremental} option is used with @value{op-extract}, then -when the entry for a directory is found, all files that currently -exist in that directory but are not listed in the archive @emph{are -deleted from the directory}. - -This behavior is convenient when you are restoring a damaged file -system from a succession of incremental backups: it restores the -entire state of the file system to that which obtained when the backup -was made. If you don't use @value{op-incremental}, the file system will -probably fill up with files that shouldn't exist any more. - -@value{op-listed-incremental} handle new @acronym{GNU}-format -incremental backup. This option handles new @acronym{GNU}-format -incremental backup. It has much the same effect as -@value{op-incremental}, but also the time when the dump is done and -the list of directories dumped is written to the given -@var{file}. When restoring, only files newer than the saved time are -restored, and the directory list is used to speed up operations. - -@value{op-listed-incremental} acts like @value{op-incremental}, but when -used in conjunction with @value{op-create} will also cause @command{tar} to -use the file @var{file}, which contains information about the state -of the filesystem at the time of the last backup, to decide which -files to include in the archive being created. That file will then -be updated by @command{tar}. If the file @var{file} does not exist when -this option is specified, @command{tar} will create it, and include all -appropriate files in the archive. - -The file, which is archive independent, contains the date it was last -modified and a list of devices, inode numbers and directory names. -@command{tar} will archive files with newer mod dates or inode change -times, and directories with an unchanged inode number and device but -a changed directory name. The file is updated after the files to -be archived are determined, but before the new archive is actually -created. - -@node Inc Dumps -@section Using @command{tar} to Perform Incremental Dumps -@UNREVISED +@table @option +@item --listed-incremental=@var{file} +@itemx -g @var{file} + Handle incremental backups with snapshot data in @var{file}. +@end table -@cindex incremental dumps -@cindex dumps, incremental +To create an incremental backup, you would use +@option{--listed-incremental} together with @option{--create} +(@pxref{create}). For example: -Performing incremental dumps is similar to performing full dumps, -although a few more options will usually be needed. +@smallexample +$ @kbd{tar --create \ + --file=archive.1.tar \ + --listed-incremental=/var/log/usr.snar \ + /usr} +@end smallexample -A standard scheme is to do a @emph{monthly} (full) dump once a month, -a @emph{weekly} dump once a week of everything since the last monthly -and a @emph{daily} every day of everything since the last (weekly or -monthly) dump. +This will create in @file{archive.1.tar} an incremental backup of +@file{/usr} filesystem, storing additional metadata in the file +@file{/var/log/usr.snar}. If this file does not exist, it will be +created. The created archive will then be called @dfn{level 0 backup} +(see the next section for more info on backup levels). -Here is a sample script to dump the directory hierarchies @samp{/usr} -and @samp{/var}. +Otherwise, if the file @file{/var/log/usr.snar} exists, it is used to +determine the modified files. In this case only these files will be +stored in the archive. Suppose, for example, that after running the +above command, you delete file @file{/usr/doc/old} and create +directory @file{/usr/local/db} with the following contents: @smallexample -#! /bin/sh -tar --create \ - --blocking-factor=126 \ - --file=/dev/rmt/0 \ - --label="`hostname` /usr /var `date +%Y-%m-%d`" \ - --listed-incremental=/var/log/usr-var.snar \ - --verbose \ - /usr /var +$ @kbd{ls /usr/local/db} +/usr/local/db/data +/usr/local/db/index @end smallexample -This script uses the file @file{/var/log/usr-var.snar} as a snapshot to -store information about the previous tar dump. +Some time later you create another incremental backup. You will +then see: -The blocking factor 126 is an attempt to make the tape drive stream. -Some tape devices cannot handle 64 kB blocks or larger, and require the -block size to be a multiple of 1 kB; for these devices, 126 is the -largest blocking factor that can be used. +@smallexample +$ @kbd{tar --create \ + --file=archive.2.tar \ + --listed-incremental=/var/log/usr.snar \ + /usr} +tar: usr/local/db: Directory is new +usr/local/db/ +usr/local/db/data +usr/local/db/index +@end smallexample -@node incremental and listed-incremental -@section The Incremental Options -@UNREVISED +@noindent +The created archive @file{archive.2.tar} will contain only these +three members. This archive is called @dfn{level 1 backup}. Notice, +that @file{/var/log/usr.snar} will be updated with the new data, so if +you plan to create more @samp{level 1} backups, it is necessary to +create a working copy of the snapshot file before running +@command{tar}. The above example will then be modified as follows: -@value{op-incremental} is used in conjunction with @value{op-create}, -@value{op-extract} or @value{op-list} when backing up and restoring file -systems. An archive cannot be extracted or listed with the -@value{op-incremental} option specified unless it was created with the -option specified. This option should only be used by a script, not by -the user, and is usually disregarded in favor of -@value{op-listed-incremental}, which is described below. - -@value{op-incremental} in conjunction with @value{op-create} causes -@command{tar} to write, at the beginning of the archive, an entry for -each of the directories that will be archived. The entry for a -directory includes a list of all the files in the directory at the -time the archive was created and a flag for each file indicating -whether or not the file is going to be put in the archive. - -Note that this option causes @command{tar} to create a non-standard -archive that may not be readable by non-@acronym{GNU} versions of the -@command{tar} program. - -@value{op-incremental} in conjunction with @value{op-extract} causes -@command{tar} to read the lists of directory contents previously stored -in the archive, @emph{delete} files in the file system that did not -exist in their directories when the archive was created, and then -extract the files in the archive. - -This behavior is convenient when restoring a damaged file system from -a succession of incremental backups: it restores the entire state of -the file system to that which obtained when the backup was made. If -@value{op-incremental} isn't specified, the file system will probably -fill up with files that shouldn't exist any more. - -@value{op-incremental} in conjunction with @value{op-list} and two -@value{op-verbose} options causes @command{tar} to print, for each -directory in the archive, the list of files in that directory at the -time the archive was created. This information is put out in a format -that is not easy for humans to read, but which is unambiguous for a -program: each file name is preceded by either a @samp{Y} if the file -is present in the archive, an @samp{N} if the file is not included in -the archive, or a @samp{D} if the file is a directory (and is included -in the archive). Each file name is terminated by a newline character. -The last file is followed by an additional newline to -indicate the end of the data. - -@value{op-listed-incremental} acts like @value{op-incremental}, but when -used in conjunction with @value{op-create} will also cause @command{tar} -to use the file @var{snapshot-file}, which contains information about -the state of the file system at the time of the last backup, to decide -which files to include in the archive being created. That file will -then be updated by @command{tar}. If the file @var{file} does not exist -when this option is specified, @command{tar} will create it, and include -all appropriate files in the archive. - -The file @var{file}, which is archive independent, contains the date -it was last modified and a list of devices, inode numbers and -directory names. @command{tar} will archive files with newer mod dates -or inode change times, and directories with an unchanged inode number -and device but a changed directory name. The file is updated after -the files to be archived are determined, but before the new archive is -actually created.@FIXME-xref{to the description of the file format}. +@smallexample +$ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1} +$ @kbd{tar --create \ + --file=archive.2.tar \ + --listed-incremental=/var/log/usr.snar-1 \ + /usr} +@end smallexample Incremental dumps depend crucially on time stamps, so the results are unreliable if you modify a file's time stamps during dumping (e.g., with the @option{--atime-preserve} option), or if you set the clock backwards. -Despite it should be obvious that a device has a non-volatile value, NFS -devices have non-dependable values when an automounter gets in the picture. -This led to a great deal of spurious redumping in incremental dumps, -so it is somewhat useless to compare two NFS devices numbers over time. -So @command{tar} now considers all NFS devices as being equal when it comes -to comparing directories; this is fairly gross, but there does not seem +Metadata stored in snapshot files include device numbers, which, +obviously is supposed to be a non-volatile value. However, it turns +out that NFS devices have non-dependable values when an automounter +gets in the picture. This can lead to a great deal of spurious +redumping in incremental dumps, so it is somewhat useless to compare +two NFS devices numbers over time. The solution implemented currently +is to considers all NFS devices as being equal when it comes to +comparing directories; this is fairly gross, but there does not seem to be a better way to go. -@command{tar} doesn't access @var{snapshot-file} when -@value{op-extract} or @value{op-list} are specified, but the -@value{op-listed-incremental} option must still be given. A -placeholder @var{snapshot-file} can be specified, e.g., -@file{/dev/null}. +Note that incremental archives use @command{tar} extensions and may +not be readable by non-@acronym{GNU} versions of the @command{tar} program. + +To extract from the incremental dumps, use +@option{--listed-incremental} together with @option{--extract} +option (@pxref{extracting files}). In this case, @command{tar} does +not need to access snapshot file, since all the data necessary for +extraction are stored in the archive itself. So, when extracting, you +can give whatever argument to @option{--listed-incremental}, the usual +practice is to use @option{--listed-incremental=/dev/null}. +Alternatively, you can use @option{--incremental}, which needs no +arguments. In general, @option{--incremental} (@option{-G}) can be +used as a shortcut for @option{--listed-incremental} when listing or +extracting incremental backups (for more information, regarding this +option, @pxref{incremental-op}). + +When extracting from the incremental backup @GNUTAR{} attempts to +restore the exact state the file system had when the archive was +created. In particular, it will @emph{delete} those files in the file +system that did not exist in their directories when the archive was +created. If you have created several levels of incremental files, +then in order to restore the exact contents the file system had when +the last level was created, you will need to restore from all backups +in turn. Continuing our example, to restore the state of @file{/usr} +file system, one would do@footnote{Notice, that since both archives +were created withouth @option{-P} option (@pxref{absolute}), these +commands should be run from the root filesystem.}: + +@smallexample +$ @kbd{tar --extract \ + --listed-incremental=/dev/null \ + --file archive.1.tar} +$ @kbd{tar --extract \ + --listed-incremental=/dev/null \ + --file archive.2.tar} +@end smallexample + +To list the contents of an incremental archive, use @option{--list} +(@pxref{list}), as usual. To obtain more information about the +archive, use @option{--listed-incremental} or @option{--incremental} +combined with two @option{--verbose} options@footnote{Two +@option{--verbose} options were selected to avoid breaking usual +verbose listing output (@option{--list --verbose}) when using in +scripts. + +Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary +contents of the DUMPDIR header (with terminating nulls) when +@option{--incremental} or @option{--listed-incremental} option was +given, no matter what the verbosity level. This behavior, and, +especially, the binary output it produced were considered incovenient +and were changed in version 1.16}: + +@smallexample +@kbd{tar --list --incremental --verbose --verbose archive.tar} +@end smallexample + +This command will print, for each directory in the archive, the list +of files in that directory at the time the archive was created. This +information is put out in a format which is both human-readable and +unambiguous for a program: each file name is printed as + +@smallexample +@var{x} @var{file} +@end smallexample -@FIXME{this section needs to be written} +@noindent +where @var{x} is a letter describing the status of the file: @samp{Y} +if the file is present in the archive, @samp{N} if the file is not +included in the archive, or a @samp{D} if the file is a directory (and +is included in the archive).@FIXME-xref{dumpdir format}. Each such +line is terminated by a newline character. The last line is followed +by an additional newline to indicate the end of the data. + +@anchor{incremental-op}The option @option{--incremental} (@option{-G}) +gives the same behavior as @option{--listed-incremental} when used +with @option{--list} and @option{--extract} options. When used with +@option{--create} option, it creates an incremental archive without +creating snapshot file. Thus, it is impossible to create several +levels of incremental backups with @option{--incremental} option. @node Backup Levels @section Levels of Backups @@ -4930,8 +4917,8 @@ their use in detail. designed to be used together. While it is possible to restore files by hand from an archive which was created using a backup script, and to create an archive by hand which could then be extracted using the restore script, -it is easier to use the scripts. @value{xref-incremental}, and -@value{xref-listed-incremental}, before making such an attempt. +it is easier to use the scripts. @value{xref-incremental}, before +making such an attempt. @node Backup Parameters @section Setting Parameters for Backups and Restoration @@ -5442,8 +5429,8 @@ positioning.} system if they were not in the file system when the archive was made. @end quotation -@value{xref-incremental}, and @value{ref-listed-incremental}, -for an explanation of how the script makes that determination. +@value{xref-incremental}, for an explanation of how the script makes +that determination. @node Choosing @chapter Choosing Files and Names for @command{tar} @@ -6087,7 +6074,7 @@ arguments. @strong{Please Note:} @value{op-after-date} and @value{op-newer-mtime} should not be used for incremental backups. Some files (such as those in renamed directories) are not selected properly by these options. -@xref{incremental and listed-incremental}. +@xref{Incremental Dumps}. @end quotation @noindent diff --git a/doc/value.texi b/doc/value.texi index a083ec0..41f67c4 100644 --- a/doc/value.texi +++ b/doc/value.texi @@ -155,9 +155,9 @@ @set pxref-ignore-zeros @pxref{Reading} @set op-incremental @option{--incremental} (@option{-G}) -@set ref-incremental @ref{Inc Dumps} -@set xref-incremental @xref{Inc Dumps} -@set pxref-incremental @pxref{Inc Dumps} +@set ref-incremental @ref{Incremental Dumps} +@set xref-incremental @xref{Incremental Dumps} +@set pxref-incremental @pxref{Incremental Dumps} @set op-info-script @option{--info-script=@var{script-name}} (@option{--new-volume-script=@var{script-name}}, @option{-F @var{script-name}}) @set ref-info-script @ref{Multi-Volume Archives} @@ -190,9 +190,7 @@ @set pxref-list @pxref{list} @set op-listed-incremental @option{--listed-incremental=@var{snapshot-file}} (@option{-g @var{snapshot-file}}) -@set ref-listed-incremental @ref{Inc Dumps} -@set xref-listed-incremental @xref{Inc Dumps} -@set pxref-listed-incremental @pxref{Inc Dumps} +@c No special refs for this option, use {*ref-incremental} instead. @set op-mode @option{--mode=@var{permissions}} @set ref-mode @ref{Option Summary}