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347 lines
11 KiB
Text
347 lines
11 KiB
Text
===============================
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SnapRAID Backup For Disk Arrays
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===============================
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1 SYNOPSIS
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==========
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snapraid [-c, --conf CONFIG] [-s, --start BLOCK]
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[-Z, --force-zero] [-E, --force-empty]
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[-v, --verbose]
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COMMAND
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snapraid [-V, --version] [-h, --help]
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2 DESCRIPTION
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=============
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SnapRAID is a backup program for a disk array using redundancy.
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SnapRAID uses a disk of the array to store redundancy information,
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and it allows to recover from a disk failure.
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SnapRAID is mainly targeted for a home media server, where you have
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a lot of big files that rarely change.
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The main features of SnapRAID are:
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* You can start using SnapRAID with already filled disks.
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* The disks of the array can have different sizes.
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* You can add more disks at the array at any time.
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* If you accidentally delete some files in a disk, you can
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recover them.
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* If more than one disk fails, you lose the data only on the
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failed disks. All the data in the other disks is safe.
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* It doesn't lock-in your data. You can stop using SnapRAID at any
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time without the need to reformat or move data.
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The official site of SnapRAID is:
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http://snapraid.sourceforge.net
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3 LIMITATIONS
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=============
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SnapRAID is in between a RAID and a backup program trying to get the best
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benefits of them. Altought it also has some limitations that you should
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consider before using it.
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The main one, is that if a disk fail, and you haven't recently synched,
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you may not able to do a complete recover.
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More specifically, you may be unable to recover up to the size of the
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amount of the changed or deleted files, from the last sync operation.
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This happens even if the files changed or deleted are not in the
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failed disk.
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New added files don't prevent the recovering of the already existing
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files. You may only lose the just added files, if they were on the failed
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disk.
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This is the reason because SnapRAID is better suited for data that
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rarely change.
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Other limitations are:
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* You have different filesystems for each disk.
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Using a RAID you have only a big filesystem.
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* It doesn't stripe data.
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With RAID you get a speed boost with striping.
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* It doesn't support realtime recovery.
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With RAID you do not have to stop working when a disk fails.
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* It's able to recover damages only from a single disk.
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With a Backup you are able to recover from a complete
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failure of the whole disk array.
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4 GETTING STARTED
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=================
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To use SnapRAID you need first to mount all the disks of your disk
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array and select which one to dedicate at the redundancy information.
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This disk will be dedicated to this pourpuse only, and you should
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not store any other data on it.
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You have to pick the biggest disk in the array, as the redundancy
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information may grow in size as the biggest data disk in the array.
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Suppose now that you have mounted all your disks in the mount points:
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/mnt/diskpar
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/mnt/disk1
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/mnt/disk2
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/mnt/disk3
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you have to create the configuration file /etc/snapraid.conf with
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the following content:
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parity /mnt/diskpar/parity
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content /mnt/diskpar/content
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disk d1 /mnt/disk1/
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disk d2 /mnt/disk2/
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disk d3 /mnt/disk3/
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At this point you are ready to start the "sync" command to build the
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redundancy information.
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snapraid sync
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This process will take some hours at best the first time, depending on
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the size of the data already present in the disks.
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If the disks are empty the process is immediate.
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You can stop it at any time pressing Ctrl+C, and at the next run it
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will start where interrupted.
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When this command completes, your data is SAFE.
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At this point you can start using your data as you like, and peridiocally
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update the redundancy information running the "sync" command.
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To check the integrity of your data you can use the "check" command:
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snapraid check
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If will read all your data, to check if it's correct.
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If an error is found, you can use the "fix" command to fix it.
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snapraid fix
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Note that the fix command will revert your data at the state of the
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last "sync" command executed. It works like a snapshot was taken
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in "sync".
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5 COMMANDS
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==========
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SnapRAID provides three simple commands that allow to:
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* Make a backup/snapshot -> "sync"
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* Check for integrity -> "check"
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* Restore the last backup/snapshot -> "fix".
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sync
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Updates the redundancy information. All the modified files
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in the disk array are read, and the redundancy data is
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updated.
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You can stop this process at any time pressing Ctrl+C,
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without losing the work already done.
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check
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Checks all the files the redundancy data. All the files
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are hashed and compared with the snapshot saved in the
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previous "sync" command.
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fix
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Checks and fix all the files. It's like "check" but it
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also tries to fix problems reverting the state of the
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disk array at the previous "sync" command.
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6 OPTIONS
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=========
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-c, --conf CONFIG
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Selects the configuration file. If not specified is assumed
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the file `/etc/snapraid.conf'.
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-s, --start BLOCK
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Starts the processing from the specified
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block number. It could be useful to easy retry to check or
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fix some specific block, in case of a damaged disk.
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-Z, --force-zero
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Forces the insecure operation of synching a file with zero
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size that before was not empty.
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If SnapRAID detects such condition, it stops proceeding
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unless you specify this option.
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This allows to easily detect when after a system crash,
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some accessed files were zeroed.
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-E, --force-empty
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Forces the insecure operation of synching an empty disk
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that before was not empty.
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If SnapRAID detects such condition, it stops proceeding
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unless you specify this option.
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This allows to easily detect when a data filesystem is not
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mounted.
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-v, --verbose
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Prints more information in the processing.
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-h, --help
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Prints a short help screen.
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-V, --version
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Prints the program version.
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7 CONFIGURATION
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===============
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SnapRAID requires a configuration file to know where your disk array
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is located, and where storing the redundancy information.
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This configuration file is located in /etc/snapraid.conf and
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it should contains the following options:
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parity FILE
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Defines the file to use to store the redundancy information.
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It must be placed in a disk dedicated for this porpose with
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as much free space as the biggest disk in the array.
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Leaving the parity disk reserved for only this file, ensures that
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it doesn't get fragmented, improving the performance.
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This option can be used only one time.
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content FILE
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Defines the file to use to store the content of the redundancy
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organization.
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It can be placed in the same disk of the parity file, or better
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in another disk, but NOT in a disk of the array.
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This option can be used only one time.
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disk NAME DIR
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Defines the name and the mount point of the disks of the array.
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NAME is used to identify the disk, and it must be unique.
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DIR is the mount point of the disk in the filesystem.
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You can change the mount point as you like, as far you
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keep the NAME fixed.
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You should use one option for each disk of the array.
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exclude PATTERN
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Defines the file or directory patterns to exclude from the sync
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process. See the PATTERN section for more details in the
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pattern specifications.
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This option can be used many times.
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block_size SIZE_IN_KILOBYTES
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Defines the basic block size in kilo bytes of
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the redundancy blocks. The default is 256 and it should
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work for most conditions.
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You should use this option only if you do not have enough
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memory to run SnapRAID.
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It requires to run TS*24/BS bytes, where TS is the total
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size in bytes of your disk array, and BS is the block size
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in bytes.
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For example with 6 disk of 2 TiB and a block size of 256 KiB
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(1 KiB = 1024 Bytes) you have:
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memory = (6 * 2 * 2^40) * 24 / (256 * 2^10) = 1.1 GiB
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An example of a typical configuration is:
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parity /mnt/diskpar/parity
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content /mnt/diskpar/content
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disk d1 /mnt/disk1/
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disk d2 /mnt/disk2/
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disk d3 /mnt/disk3/
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exclude *.bak
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exclude /lost+found/
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exclude tmp/
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block_size 256
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8 PATTERN
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=========
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Patterns are used to define the files and directories to exclude
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from the redundancy process.
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It makes sense to exclude any file not worth to be saved.
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There are four different types of patterns:
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FILE
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Excludes any file named as FILE. You can use any globbing
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character like * and ?.
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This pattern is applied only to files and not to directories.
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DIR/
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Excludes any directory named DIR. You can use any globbing
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character like * and ?.
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This pattern is applied only to directories and not to files.
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/PATH/FILE
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Excludes the exact specified file path. You can use any
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globbing character like * and ? but they never matches a
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directory slash.
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This pattern is applied only to files and not to directories.
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/PATH/DIR/
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Excludes the exact specified directory path. You can use any
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globbing character like * and ? but they never matches a
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directory slash.
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This pattern is applied only to directories and not to files.
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For example:
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# Excludes any file named "*.bak"
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exclude *.bak
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# Excludes the root directory "lost+found"
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exclude /lost+found/
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# Excludes any directory named "tmp"
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exclude tmp/
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9 CONTENT
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=========
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SnapRAID creates a content file describing the content of your disk array.
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You do not have to to modify it, altought the format of this file is
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described here.
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blk_size SIZE
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Defines the size of the block in bytes. It must match the size
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defined in the configuration file.
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file DISK SIZE TIME INODE PATH
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Defines a file in the specified DISK.
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The INODE information is used to identify the file in the 'sync'
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command, allowing to move the file anywhere in disk without
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the need to recompute the parity for it.
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The SIZE and TIME information are used to identify if the file
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changed from the last 'sync' command, and if there is the need
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to recompute the parity.
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The PATH information is used in the 'check' and 'fix' commands
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to identify the file.
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blk BLOCK HASH
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Defines the ordered blocks used by the last defined file.
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BLOCK is the block position in the parity file.
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HASH is the md5 of the block. In the last block of the file,
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the HASH is the hash only of the used part of the block.
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10 COPYRIGHT
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============
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This file is Copyright (C) 2011 Andrea Mazzoleni
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11 SEE ALSO
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===========
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rsync(1)
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