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 AFBackup-Webmin-Module  AFBackup  Webmin  D.Wallmeier
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Introduction to AFBackup 
by Albert Flügel



  • Server Identification
  • Storage Devices
  • Cartridges
  • Cartridge Sets
  • Storage units
  • Read and Write
  • Locking
  • Client

  • Full and Incremental Backup, Backup Levels
  • Packing and Unpacking
  • Filelists
  • Minimum restore information
  • Levels of Desaster / data loss
  • Clientside program structure
  • How to use it

  • client side
  • server side
  • Sample configuration files (Linux)
  • Introduction to AF's Backup system


    This file gives a short introduction into AF's backup system, it's architecture (wow !), the ideas behind it and how it can be used to serve the individual needs.


    Yes, it's a client-server system. The server is a machine, that has access to some kind of storage media, usually a tape drive. This server side knows, how to handle this device, how any cartridges are in use and what to do, when a cartridge gets full or other special cases occur. What the serverside does not know is anything, that has to do with directory structures, files, attributes of the stored files or even their names. This is all handled on the client side. The server only knows data streams. The client demands from the server the functionality to pass him e.g. the next bunch of data from the tape, to write a packet to tape or to put in a certain cartridge and the server performs the actions (hopefully) as he can. The client knows, how to pack and unpack the files and directories and on which cartridges and files they are written and can be found. For this purpose he maintains file lists containing the names of all stored filesystem entries in a local directory. These filelists are not saved to the backup if not explicitly configured. Compression is done on the client side to reduce network traffic and to achieve more safety. If the packed stream had been compressed, a single wrong bit in the stream would make the whole rest of the backup useless. Thus the files are compressed individually on the client side.


    Server Identification

    An afbackup server is addressed using one of it's hostnames and a port number. Two services can be configured for each afbackup server: a single stream and a multi stream server, each serving on a separate port. It's up to the client, which service it will use. The multi stream server can handle requests from several clients in parallel (only for saving, not for restore), thus multiplexing the data stream. This multiplexing requires some overhead, so the performance of the multi stream service can be expected worse than the single stream service. For full backups the single stream server might be preferred, but for incremental backups, where a client is spending most of the time searching for modified filesystem entries, the multi stream server might be the service to choose. Then the possibly several clients can scan their disks in parallel, sending the data to backup to the server as it is found.
    Because the two servers are using the same (sets of) tapes, an additional identifier is introduced to enable clients to figure out, that they are speaking to the same server through different services/ports. This together with the strongly recommended use of a host alias to address the server(s), makes for being able to move the backup service from one machine to another. Change the host alias to the other machine, connect the storage media to it, move over the afbackup installation and that's it. Of course the clients must see, that the host alias has changed, so a working name service is necessary.

    Storage Devices

    Any kind of streamer can be used with this backup system. The only restriction is, that it must have a block- or character- device entry in the /dev-directory. The blocksize of the device must be set correctly in the serverside configuration. Besides of streamers also filesystems can be used to save the data to. The configuration to do this is a little special and
    can be taken from the HOWTO.FAQ.DO-DONT 3. With streamers several tape cartridges can and should be used. There are two basic cases: Either you have a robot or have not. If you have no robot, you must change the cartridges manually. The server supplies you with e-mail, whenever another cartridge should be put into the drive. Having done this, a special command (cartready) must be run on the server to let him know, that he can continue now. If you have a robot, but no special software (commands) to tell it, which cartridge to insert into the drive, you must set the robot to sequential mode. That is, the robot puts the next tape into the drive once the current one is ejected. Such a mode is usually supported and then the server ejects the cartridges out of the drive until the right tape is found. If you have commands to set the cartridge directly, you can configure them in the configuration file.


    The cartridges are identified by numbers beginning with 1. Once they are in use, they automatically get a magnetic label, that they keep until overwritten with a special command (label_tape). It is a good idea also to use adhesive labels and to write the numbers onto the tapes, so they can't be mixed up. The server recognizes, if there is a tape in the drive, that has the wrong label. What then happens, is depending on the cases mentioned above. If you have no robot, you get another email, that points you to the problem. If you have a robot in sequential mode, the server tries all cartridges, he has, to find the right one. If it finds a tape with the right label, this is used. If it does not, it stops and writes errors to the serverside error logfile. If you have a robot and commands to set the cartridge and the robot inserts the wrong one, this is considered a severe problem and the server stops, again writing errors to it's error log, waiting for a maintainer to solve the problem and to allow the server to continue by issuing a command, he receives in a mail. Once all tapes of a set (for cartridge sets see the next section) have been used up, the server tries to start over with the first of the set overwriting it. The server tracks, what tapes are needed for clients to restore all their data. When full, these tapes are not overwritten until explicitly permitted or the data on them is no longer in any index file on a client. Tapes can be set to read-only state. The server will never write to tapes in this mode, even not in the variable append operating mode. In variable append mode, any tape of the correct cartridge set can
    be supplied to the streamer device and it will be accepted for writing. If the data on that tape is still needed by clients, the next data will be appended to the already written area on tape, Otherweise the tape will be overwritten. If not in variable append mode, the next writing position is, where writing stopped before. This can be overridden using the cartis command or the clientside option -G. More cartridges than the robot can juggle can be used. For this case see the HOWTO.FAQ.DO-DONT Q6.

    Cartridge Sets

    You can devide your cartridges into sets for several purposes. A set comprises of several cartridges, e.g. the cartridges number 1-3. There may be cartridges, that are not in any set and a cartridge must not be in more than one set. This would be a configuration error. Nonetheless if you have a robot, you must tell the backup system the number of cartridges, the robot is handling. The configuration parameter is named Cartridge-Sets. If you do not configure sets of cartridges, all of the available cartridges will be used as the one and only existing set. Which set to use, can be configured in the client side configuration file or overriding this setting by using commandline options (-S). Client access to tapes is handled on a per-set basis. Restrictions can be configured evaluating the client's official host name. If the server is unable to determine the client's hostname, access is denied, if a restriction is configured. If no restriction is configured, access is granted for any client. Clients must authenticate to operate a server, so this default behaviour is not to be considered a security issue. Attempts of answers to the question, why to use sets, can be found in the HOWTO.FAQ.DO-DONT Q4.

    Storage units

    This is a term i use for the combination of a backup server's hostname, the port number to connect and a valid cartridge set number on this server. Several storage units can be configured on each client. When starting a backup, the client side first checks the servers for availability one after the other and starts the backup to the first available one found. Running a restore the user or administrator does not have to remember, to which storage unit the backup went. This is all handled transparently. For each of the three clientside configuration parameters (server hostname, port number and cartridge set) several entries are allowed now. They are associated by position. For further information, see the HOWTO.FAQ.DO-DONT HOWTO Q9.

    Read and Write

    The read position can be requested by clients without any restriction, assumed, the client is granted access to the tape in question. With the write position it is different. The server by default continues to write at the position, where the most recent write operation ended. This handling is maintained individually for each cartridge set. For each set the writing position is stored. Writing position means not only the cartridge used before, but also the number of the file on tape. Thus not another tape is started for each and every backup. The default behaviour can be changed to variable append mode. In this mode any tape of the right cartridge set with remaining space and not in read-only state can be supplied to the streamer device and be accepted for writing. If the tape is containing data needed by clients for restore, the next data is appended, otherwise the tape is reused and completely overwritten. The data stream is written to the tape in pieces (tape files) of a configurable maximum size. This is done to find the position faster, where certain data has been stored. The client knows for every file, on which cartridge the file is and in which tape file it can be found. Thus a restore of certain requested files is much faster than starting one only file for each backup in the worst case using up a whole cartridge.


    The server process generates a lockfile when starting to prevent several server processes to access the same tape drive. Nontheless if several processes start, the one started later waits a configurable time for the device to become available sending email after configurable intervals. Another lock is set for access to a possibly used autochanger.



    Full and Incremental Backup, Backup Levels

    A full backup means, that everything requested is stored on the server. An incremental backup stores only these files and directories, that save changed or are newer than the start of the previous full or incremental backup. For determining, what has changed, the mtime in the inode is used. Why this is done, can be read from the HOWTO.FAQ.DO-DONT, Q2. Problems may arise, if mechanisms are used that create files and directories while resetting the mtime to an earlier date and time (e.g. using tar -x or cpio -i). Then these files are not saved during the next incremental backup. If a backup level is supplied, when running incr_backup, then everything is stored, that has changed since the most recent run with a higher or equal level. Note, that other backup systems define levels the other way round: the lower the level number, the more is written to backup. The full backup can be divided into several parts. This is useful for example, if the duration for backing up everything is too long for the intended time frame. If the full backup should run during the weekend, but the about 60 hours from friday evening to monday morning are not sufficient, this is a case for several parts. These can be configured listing the files and directories for each part in different clientside configuration parameters. Before the parameter NumBackupParts must be set appropriately.

    Packing and Unpacking

    The client packs the files and directories and sends them to the server or receives a data stream from the server and unpacks it. If compression is requested, a configurable compression program can be used to process each packed file individually. The name of the appropriate uncompress-program is packed into the stream, so it needs not be supplied during restore. Nontheless it must be insured, that compress and uncompress-programs are within the search path of the client processes or supplied with the full path.


    For each stored filesystem entry the client writes a line to a file containing the cartridge number, where it can be found for restore, also the tape file number and (since 2.9) the ID of the user owning the filesystem entry. This listfile resides locally in a filesystem on the client. It serves for determining, where the data for restore can be found on the server and to determine, whether a user is allowed to restore the corresponding file, if this possibility is configured (see HOWTO.FAQ.DO-DONT Q7). Each time a full backup is done, a new filelist is started. Incremental backups result in appending lines to this file. The filelist files are identified by the number of the full backup, that is increased continuously each full backup.
    A configurable number of such filelist files is kept for later use. The restore utility needs these lists, when certain files are requested for restore. If these filelists get lost, you are in trouble. You should use the mechanism described in the next section to avoid this.

    Minimum restore information

    The minimum information to restore everything (the stored data and the filelists) is, to which cartridges and - on those - in which tape files the data can be found. This information is passed each start of a full or incremental backup to a configurable program, that can read this information (a line of a special format) as the standard input. It should write this to a place where it is safe from crashes of the local machine. One possibility is a NFS-mounted directory, another one is to send this information via email. This minimum information can be passed to the restore utility for restoring either everything or only the filelist files. The minimum restore information is not cumulative i.e. only the latest one is required. Without this
    minimum restore information, a tape scanning is necessary to do emergency recovery (get everything back after a complete loss). This takes notably longer and can be avoided having
    this little string available, when needed. If older backups or filelists, that have been deleted, cause they fell out of the configurable number of indexes to keep, should be restored, the older minimum restore information can be used. So it is not a bad idea to store it together with the current date. This possibility requires of course, that the tapes with the older backups have not been overwritten.

    Levels of Desaster / data loss

    There are several stages of recovery from loss of data depending on what information is still abailable and usable.
    * saved filesystem entries are lost on a client, but indexes
      (filelists) are intact:
        - (x)afrestore without options -e, -E
        - uses index
        - end user restore possible, if configured
    * index(es) lost, minimum restore information at hand:
        - afrestore with option -e (probably first -f, if desired)
        - pipe the minimum restore string into the program
        - must be done by administrator (superuser)
        - the minimum restore info is NOT (in) an index file,
          it's the string starting with @@@===--->>>
    * index(es) lost, minimum restore information lost:
        - afrestore with option -E
        - supply number(s) of tapes probably having the most
          recent minimum restore information
        - must be done by administrator (superuser)
        - if afbackup -E does not find anything: try to find the
          minimum restore info on tape using dd(1), strings(1)
          and grep for @@@===--->>> (in quotes, please) on the
    * server broken:
        - same procedure like for the client
        - the server's latest version of the var-directory must
          be restored before doing anything else
    See also HOWTO Q24 and the DO-DONT section for more details.

    Clientside program structure

    There is a three-layer program hierarchy on the client side. The program "afclient" in the client's binary directory is performing the packing, unpacking and the communication with the server side. It does not read the configuration file and does not maintain the filelists described above. This is done by programs with higher functionality, that in turn call the "afclient"-program setting it into different modes. The entry "afbackup" in the same directory is identical with "afclient". These higher level programs are full_backup, incr_backup, afrestore, afverify and a few more. afrestore in turn is used by the X11-frontend xafrestore. See HOWTO Q41 for more information about the architecture.

    How to use it

    First a short summary, which program serves for what purpose, in order of estimated interest:

    client side:

    full_backup runs a backup of all configured files/directories
    incr_backup runs an incremental or level-X backup i.e. only things modified since one of the previous backups are saved
    afrestore restore files/directories whose names are matching given arguments
    xafrestore an X11-frontend for afrestore
    afverify compare the latest or an older backup with the current status of the files/directories
    update_indexes remove entries from the online file index, whose corresponding tapes have been erased on the server(s)
    copy_tape make an identical copy of one tape to another assigning a different secondary label number
    clientconfig program to modify the client configuration interactively, online help available
    xclientconfig dito, but as X11-program

    server side:

    afserver  the server program operating the tape and serving one client at a time
    afmserver the multiplexing frontend for afserver, possibly serving several clients in parallel
    label_tape to put a valid afbackup label to a tape or read it
    cart_ctl in combination with a changer: handle tapes and maintain their locations
    cartis  to change the writing position on tape manually (use carefully !)
    cartready to give manual hints to the server when requested  to perform some administrative action
    serverconfig program to modify the client configuration interactively, online help available
    xserverconfig dito, but as X11-program
    xserverstatus displays the current status of the server and the most recent entries of the serverside log

    For detailed information about the programs see the PROGRAMS file, the manual pages, and the HOWTO-FAQ, e.g. FAQ Q41

    Now a typical case is described. Individual needs can be satisfied reading the rest of the documentation. Below are hints to configuration files for some typical cases, that are coming with the distribution.

    We assume, you have a DAT-cartridge handler with 12 tapes connected to some machine without the possibility to set a cartridge directly via commandline interface. So the first thing to do is set the robot to sequential mode. It is no bad idea to make this machine "fetch" the backup from the clients using a small script, that is started via crond. Thus the first step is to setup this backup server. Unpack the distribution, run the Install script and select installation option number 1 for installing and configuring the server side. The last question to be answered is, whether to run the serverside configuration program. Answer with yes, cause we should do this now. The most entries can be accepted as they are by default. Which device should be used, you must know yourself, you have for sure already done some archiving with tar to the connected streamer. The blocksize can be looked up usually in the man-pages of rmt or tar. Set the parameter "CartridgeHandler" to 1 and the number of cartridges to 12. Cause you must use the sequential mode of the robot, clear the "SetCartridgeCommand", if not already done entering the parameter number and a single space for the value. Set the "ChangeCartridgeCommand" to a command, that ejects a tape from the drive (usually mt -f %d rewoffl - the %d stands for the configured devicename, just for convenience, that it needs not to be typed in several times). The UserToInform-parameter should be set to some administrator or group of administrators, that should receive important informations. The rest of the parameters is quite self-explanatory. Use the online-help, the CONFIG and the HOWTO.FAQ.DO-DONT, if you don't know, what they mean. Especially the CONFIG should give valuable hints. Finish the server configuration entering "ok".
    Now make the robot to have cartridge number 1 present inside the streamer. If you do not want to do this or can't convince the device to do, what you desire, run the following command: /the/path/to/server/bin/cartis <cartno>
    where <cartno> should be the integer number specifying the cartridge currently present in the drive.

    Now install the client side on each client, that should do backups to the server. Again, unpack the distribution and run the Install script, choosing the installation option number 3 for client with remote start possibility. Again, all defaults should be ok, so the last question, whether to run the clientside configuration program can be confirmed. Reasonable defaults are present for each parameter. What you have to configure in any case are the files and directories, that are to be stored. Furthermore, the startup information program should be configured to a reasonable command (see above under "Minimum restore information" for details). If you are upgrading to a newer version and some of the parameters are not set, see the client.config (or server.config, respectively) of the current distribution for defaults, that i consider sufficient for normal cases. This also might give you better ideas, what the parameters are good for.

    Finally write small scripts starting the backup one after the other on the clients (the server may off course also be a client). An example script is provided in the HOWTO.FAQ.DO-DONT under 2, just replace the list of hosts in the shell-variable BACKUPCLIENTS. Now add entries to root's crontab-file. Entering 
    crontab -l > my_crontab
    as root writes the current crontab to the file my_crontab. To run e.g. a full backup each friday evening and incremental backups each evening from monday to thursday (all at 8 pm), add these two lines:

    0 20 * * 5   /usr/backup/client/bin/full_backup_cycle
    0 20 * * 1-4 /usr/backup/client/bin/incr_backup_cycle

    assumed the named programs are the scripts you wrote according to the above description. Then run the command
    crontab my_crontab
    to install the modified crontab file.

    That's it. Maybe, you want to start the first full backup manually to see, whether everything is correctly installed and working. In this case just start the script yourself and watch, what happens. Especially watch the client- and serverside logfiles in
    /path/to/client/var/backup.log and /path/to/server/var/backup.log.

    Sample configuration files (Linux)


    For a streamer on /dev/nst0 without a changer, so tapes must be changed manually


    For a streamer on /dev/nst0, changer on /dev/sg0 (run scsicheck to find correct device), stacker has 7 slots, no loadport, copy changer.conf.mtx to /path/to/server/lib/changer.conf


    For writing the backup to a filesystem (directory /var/backup) using the directory option of (see:) HOWTO Q 3


    For writing the backup to a filesystem (directory /var/backup) using the symlink option of (see:) HOWTO Q 3


    client side configuration file, appropriate for many cases (server hostname or alias must be backuphost, do adapt the parameter DirsToBackup to what is really appropriate)

    If this introduction together with the rest of the documentation is not sufficient, please let me know, but PLEASE give me instructive hints, what you are missing or what should be clarified.

    Albert Fluegel

    May 10 2001

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