snapraid/snapraid.d

Name{number}
	snapraid - SnapRAID Backup For Disk Arrays

Synopsis
	:snapraid [-c, --conf CONFIG] [-s, --start BLOCK]
	:	[-Z, --force-zero] [-E, --force-empty]
	:	[-v, --verbose]
	:	COMMAND

	:snapraid [-V, --version] [-h, --help]

Description
	SnapRAID is a backup program for a disk array using redundancy.

	SnapRAID uses a disk of the array to store redundancy information,
	and it allows to recover from a disk failure.

	SnapRAID is mainly targeted for a home media center, where you have
	a lot of big files that rarely change.

	The main features of SnapRAID are:

	* You can start using SnapRAID with already filled disks.
	* The disks of the array can have different sizes.
	* You can add more disks at the array at any time.
	* If you accidentally delete some files in a disk, you can
		recover them.
	* If more than one disk fails, you lose the data only on the
		failed disks. All the data in the other disks is safe.
	* It doesn't lock-in your data. You can stop using SnapRAID at any
		time without the need to reformat or move data.
	* All your data is hashed to ensure data integrity.

	The official site of SnapRAID is:

		:http://snapraid.sourceforge.net

Limitations
	SnapRAID is in between a RAID and a backup program trying to get the best
	benefits of them. Although it also has some limitations that you should
	consider before using it.

	The main one, is that if a disk fail, and you haven't recently synced,
	you may not able to do a complete recover.
	More specifically, you may be unable to recover up to the size of the
	amount of the changed or deleted files, from the last sync operation.
	This happens even if the files changed or deleted are not in the
	failed disk.
	New added files don't prevent the recovering of the already existing
	files. You may only lose the just added files, if they were on the failed
	disk.

	This is the reason because SnapRAID is better suited for data that
	rarely change.

	Other limitations are:
	* You have different file-systems for each disk.
		Using a RAID you have only a big file-system.
	* It doesn't stripe data.
		With RAID you get a speed boost with striping.
	* It doesn't support real-time recovery.
		With RAID you do not have to stop working when a disk fails.
	* It's able to recover damages only from a single disk.
		With a Backup you are able to recover from a complete
		failure of the whole disk array.

Getting Started
	To use SnapRAID you need first select one disk of your disk array
	to dedicate at the redundancy information.

	You have to pick the biggest disk in the array, as the redundancy
	information may grow in size as the biggest data disk in the array.

	This disk will be dedicated to this purpose only and you should
	not store other data on it.

	Suppose now that you have mounted all your disks in the mount points:

		:/mnt/diskpar <- selected disk for parity
		:/mnt/disk1
		:/mnt/disk2
		:/mnt/disk3

	you have to create the configuration file /etc/snapraid.conf with
	the following content:

		:parity /mnt/diskpar/parity
		:content /mnt/diskpar/content
		:disk d1 /mnt/disk1/
		:disk d2 /mnt/disk2/
		:disk d3 /mnt/disk3/

	At this point you are ready to start the "sync" command to build the
	redundancy information.

		:snapraid sync

	This process may take some hours the first time, depending on the size
	of the data already present in the disks. If the disks are empty
	the process is immediate.

	You can stop it at any time pressing Ctrl+C, and at the next run it
	will start where interrupted.

	When this command completes, your data is SAFE.

	At this point you can start using your data as you like, and periodically
	update the redundancy information running the "sync" command.

	To check the integrity of your data you can use the "check" command:

		:snapraid check

	If will read all your data, to check if it's correct.

	If an error is found, you can use the "fix" command to fix it.

		:snapraid fix

	Note that the fix command will revert your data at the state of the
	last "sync" command executed. It works like a snapshot was taken
	in "sync".

	In this regard snapraid is more like a backup program than a RAID
	system. For example, you can use it to recover from an accidentally
	deleted directory, simply running the fix command.

Commands
	SnapRAID provides three simple commands that allow to:

	* Make a backup/snapshot -> "sync"
	* Check for integrity -> "check"
	* Restore the last backup/snapshot -> "fix".

	=sync
		Updates the redundancy information. All the modified files
		in the disk array are read, and the redundancy data is
		updated.
		You can stop this process at any time pressing Ctrl+C,
		without losing the work already done.

	=check
		Checks all the files the redundancy data. All the files
		are hashed and compared with the snapshot saved in the
		previous "sync" command.

	=fix
		Checks and fix all the files. It's like "check" but it
		also tries to fix problems reverting the state of the
		disk array at the previous "sync" command.

Options
	-c, --conf CONFIG
		Selects the configuration file. If not specified is assumed
		the file '/etc/snapraid.conf' in Unix, and 'snapraid.conf' 
		in Windows.

	-s, --start BLOCK
		Starts the processing from the specified
		block number. It could be useful to easy retry to check or
		fix some specific block, in case of a damaged disk.

	-Z, --force-zero
		Forces the insecure operation of syncing a file with zero
		size that before was not empty.
		If SnapRAID detects such condition, it stops proceeding
		unless you specify this option.
		This allows to easily detect when after a system crash,
		some accessed files were zeroed.

	-E, --force-empty
		Forces the insecure operation of syncing an empty disk
		that before was not empty.
		If SnapRAID detects such condition, it stops proceeding
		unless you specify this option.
		This allows to easily detect when a data file-system is not
		mounted.

	-v, --verbose
		Prints more information in the processing.

	-h, --help
		Prints a short help screen.

	-V, --version
		Prints the program version.

Configuration
	SnapRAID requires a configuration file to know where your disk array
	is located, and where storing the redundancy information.

	This configuration file is located in /etc/snapraid.conf and
	it should contains the following options:

	=parity FILE
		Defines the file to use to store the redundancy information.
		It must be placed in a disk dedicated for this purpose with
		as much free space as the biggest disk in the array.
		Leaving the parity disk reserved for only this file, ensures that
		it doesn't get fragmented, improving the performance.
		This option can be used only one time.

	=content FILE
		Defines the file to use to store the content of the redundancy
		organization.
		It can be placed in the same disk of the parity file, or better
		in another disk, but NOT in a data disk of the array.
		This option can be used only one time.

	=disk NAME DIR
		Defines the name and the mount point of the disks of the array.
		NAME is used to identify the disk, and it must be unique.
		DIR is the mount point of the disk in the file-system.
		You can change the mount point as you like, as far you
		keep the NAME fixed.
		You should use one option for each disk of the array.

	=exclude PATTERN
		Defines the file or directory patterns to exclude from the sync
		process. See the PATTERN section for more details in the
		pattern specifications.
		This option can be used many times.

	=block_size SIZE_IN_KIBIBYTES
		Defines the basic block size in kibi bytes of
		the redundancy blocks. Where one kibi bytes is 1024 bytes.
		The default is 256 and it should work for most conditions.
		You increase this value if you do not have enough memory
		to run SnapRAID.
		It requires to run something about TS*24/BS bytes, where TS
		is the total size in bytes of your disk array, and BS is the
		block size in bytes.

		For example with 6 disk of 2 TiB and a block size of 256 KiB
		(1 KiB = 1024 Bytes) you have:

		:memory = (6 * 2 * 2^40) * 24 / (256 * 2^10) = 1.1 GiB

		You should instead decrease this value if you have a lot of
		small files in the disk array. For each file, even if of few
		bytes, a whole block is always allocated, so you may have a lot
		of unused space.

	An example of a typical configuration is:

		:parity /mnt/diskpar/parity
		:content /mnt/diskpar/content
		:disk d1 /mnt/disk1/
		:disk d2 /mnt/disk2/
		:disk d3 /mnt/disk3/
		:exclude *.bak
		:exclude /lost+found/
		:exclude tmp/
		:block_size 256

Pattern
	Patterns are used to define the files and directories to exclude
	from the redundancy process.

	It makes sense to exclude any file not worth to be saved or that
	changes often.

	There are four different types of patterns:

	=FILE
		Excludes any file named as FILE. You can use any globbing
		character like * and ?.
		This pattern is applied only to files and not to directories.

	=DIR/
		Excludes any directory named DIR. You can use any globbing
		character like * and ?.
		This pattern is applied only to directories and not to files.

	=/PATH/FILE
		Excludes the exact specified file path. You can use any
		globbing character like * and ? but they never matches a
		directory slash.
		This pattern is applied only to files and not to directories.

	=/PATH/DIR/
		Excludes the exact specified directory path. You can use any
		globbing character like * and ? but they never matches a
		directory slash.
		This pattern is applied only to directories and not to files.

	For example:
		:# Excludes any file named "*.bak"
		:exclude *.bak
		:# Excludes the root directory "/lost+found"
		:exclude /lost+found/
		:# Excludes any directory named "tmp"
		:exclude tmp/

Content
	SnapRAID creates a content file describing the content of your disk array.

	You should never change it manually, although the format of this file is
	described here.

	=blk_size SIZE
		Defines the size of the block in bytes. It must match the size
		defined in the configuration file.

	=file DISK SIZE TIME INODE PATH
		Defines a file in the specified DISK.
		The INODE number is used to identify the file in the "sync"
		command, allowing to rename or move the file in disk without
		the need to recompute the parity for it.
		The SIZE and TIME information are used to identify if the file
		changed from the last "sync" command, and if there is the need
		to recompute the parity.
		The PATH information is used in the "check" and "fix" commands
		to identify the file.

	=blk BLOCK HASH
		Defines the ordered parity block list used by the last defined file.
		BLOCK is the block position in the "parity" file.
		0 for the first block, 1 for the second one and so on.
		HASH is the md5 of the block. In the last block of the file,
		the HASH is the hash of only the used part of the block.

Parity
	SnapRAID creates a parity file containing the parity information of your disk array.

	It's a binary file, containing the parity information of all the blocks defined
	in the "content" file.
	
	For all the blocks at a given position, the parity information is computed with
	the XOR operator applied to all the blocks.

	When a file block is shorter than the default block size, for example because it's
	the last block of a file, it's assumed filled with 0 at the end in the XOR operation.

Copyright
	This file is Copyright (C) 2011 Andrea Mazzoleni

See Also
	rsync(1)