snapraid/snapraid.html

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<title>SnapRAID Backup For Disk Arrays</title>
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<center><h2>SnapRAID Backup For Disk Arrays</h2></center>
<h2><a name="1">1</a> 
Synopsis
</h2>
<pre>
snapraid [-c, --conf CONFIG] [-s, --start BLOCK]
&nbsp;&nbsp;&nbsp;&nbsp;[-f, --force] [-v, --verbose] [-e, --exclude PATTERN]
&nbsp;&nbsp;&nbsp;&nbsp;COMMAND
</pre>
<p>
<pre>
snapraid [-V, --version] [-h, --help]
</pre>
<h2><a name="2">2</a> 
Description
</h2>
SnapRAID is a backup program for a disk array using redundancy.
<p>
SnapRAID uses a disk of the array to store redundancy information,
and it allows to recover from a disk failure.
<p>
SnapRAID is mainly targeted for a home media server, where you have
a lot of big files that rarely change.
<p>
The main features of SnapRAID are:
<ul>
<li>
You can start using SnapRAID with already filled disks.
</li>
<li>
The disks of the array can have different sizes.
</li>
<li>
You can add more disks at the array at any time.
</li>
<li>
If you accidentally delete some files in a disk, you can
recover them.
</li>
<li>
If more than one disk fails, you lose the data only on the
failed disks. All the data in the other disks is safe.
</li>
<li>
It doesn't lock-in your data. You can stop using SnapRAID at any
time without the need to reformat or move data.
</li>
</ul>
<p>
The official site of SnapRAID is:
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
<a href="http://snapraid.sourceforge.net">http://snapraid.sourceforge.net</a>
</pre>
</td></tr></table>
<h2><a name="3">3</a> 
Limitations
</h2>
SnapRAID is in between a RAID and a backup program trying to get the best
benefits of them. Altought it also has some limitations that you should
consider before using it.
<p>
The main one, is that if a disk fail, and you haven't recently synched,
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.
<p>
This is the reason because SnapRAID is better suited for data that
rarely change.
<p>
Other limitations are:
* You have different filesystems for each disk.
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
Using a RAID you have only a big filesystem.
</td></tr></table>
<ul>
<li>
It doesn't stripe data.
With RAID you get a speed boost with striping.
</li>
<li>
It doesn't support realtime recovery.
With RAID you do not have to stop working when a disk fails.
</li>
<li>
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.
</li>
</ul>
<h2><a name="4">4</a> 
Getting Started
</h2>
To use SnapRAID you need first to mount all the disks of your disk
array and select which one to dedicate at the redundancy information.
<p>
This disk will be dedicated to this pourpuse only, and you should
not store any other data on it.
<p>
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.
<p>
Suppose now that you have mounted all your disks in the mount points:
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
/mnt/diskpar
/mnt/disk1
/mnt/disk2
/mnt/disk3
</pre>
</td></tr></table>
<p>
you have to create the configuration file /etc/snapraid.conf with
the following content:
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
parity /mnt/diskpar/parity
content /mnt/diskpar/content
disk d1 /mnt/disk1/
disk d2 /mnt/disk2/
disk d3 /mnt/disk3/
</pre>
</td></tr></table>
<p>
At this point you are ready to start the "sync" command to build the
redundancy information.
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
snapraid sync
</pre>
</td></tr></table>
<p>
This process will take some hours at best 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.
<p>
When this command completes, your data is SAFE.
<p>
At this point you can start using your data as you like, and peridiocally
update the redundancy information running the "sync" command.
<p>
To check the integrity of your data you can use the "check" command:
<p>
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<pre>
snapraid check
</pre>
</td></tr></table>
<p>
If will read all your data, to check if it's correct.
<p>
If an error is found, you can use the "fix" command to fix it.
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
snapraid fix
</pre>
</td></tr></table>
<p>
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".
<h2><a name="5">5</a> 
Configuration
</h2>
SnapRAID requires a configuration file to know where your disk array
is located, and where storing the redundancy information.
<p>
This configuration file is located in /etc/snapraid.conf and
it should contains the following options:
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
parity FILE
</strong></td></tr><tr><td>
Defines the file to use to store the redundancy information.
It must be placed in a disk dedicated for this porpose with
as much free space as the biggest disk in the array.
This option can be used only one time.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
content FILE
</strong></td></tr><tr><td>
Defines the file to use to store the content of the redundancy
organization.
It must be placed in the same disk of the parity file, or in
another location, but not in a disk of the array.
This option can be used only one time.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
disk NAME DIR
</strong></td></tr><tr><td>
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 filesystem.
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.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
exclude GLOB_PATTERN
</strong></td></tr><tr><td>
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.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
block_size SIZE_IN_KILOBYTES
</strong></td></tr><tr><td>
Defines the basic block size in kilo bytes of
the redundancy blocks. The default is 256 and it should
work for most conditions.
You should use this option only if you do not have enough
memory to run SnapRAID.
It requires to run TS*24/BS bytes, where TS is the total
size in bytes of your disk array, and BS is the block size
in bytes.
</td></tr>
</table>
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
For example with 6 disk of 2 TB and a block size of 256 kB you
have:
</td></tr></table>
<p>
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<pre>
memory = (6 * 2 * 2^40) * 24 / (256 * 2^10) = 1.1 GB
</pre>
</td></tr></table>
<p>
An example of a typical configuration is:
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
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
</pre>
</td></tr></table>
<h2><a name="6">6</a> 
Commands
</h2>
SnapRAID provides three simple commands that allow to:
<ul>
<li>
Make a backup/snapshot -&gt; "sync"
</li>
<li>
Check for integrity -&gt; "check"
</li>
<li>
Restore the last backup/snapshot -&gt; "fix".
</li>
</ul>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
sync
</strong></td></tr><tr><td>
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.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
check
</strong></td></tr><tr><td>
Checks all the files the redundancy data. All the files
are hashed and compared with the snapshot saved in the
previous "sync" command.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
fix
</strong></td></tr><tr><td>
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.
</td></tr>
</table>
<h2><a name="7">7</a> 
Pattern
</h2>
Patterns are used to define the files and directories to exclude
from the redundancy process.
<p>
It makes sense to exclude any file not worth to be saved.
<p>
There are four different types of patterns:
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
FILE
</strong></td></tr><tr><td>
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.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
DIR/
</strong></td></tr><tr><td>
Excludes any directory named DIR. You can use any globbing
character like * and ?.
This pattern is applied only to directories and not to files.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
/PATH/FILE
</strong></td></tr><tr><td>
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.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
/PATH/DIR/
</strong></td></tr><tr><td>
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.
</td></tr>
</table>
<p>
For example:
<p>
<table width="100%" border="0" cellspacing="0" cellpadding="2"><tr><td width="5%"></td><td width="95%">
<pre>
# Excludes any file named "*.bak"
exclude *.bak
# Excludes the root directory "lost+found"
exclude /lost+found/
# Excludes any directory named "tmp"
exclude tmp/
</pre>
</td></tr></table>
<h2><a name="8">8</a> 
Options
</h2>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
-c, --conf CONFIG
</strong></td></tr><tr><td>
Selects the configuration file. If not specified is assumed
the file `/etc/snapraid.conf'.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
-s, --start BLOCK
</strong></td></tr><tr><td>
Starts the processing from the specified
block number. It could be useful to easy retry to fix
some specific block, in case of a damaged disk.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
-f, --force
</strong></td></tr><tr><td>
Forces insecure operations. If snapraid detects
an unsafe operation, it stops the execution asking you
to use this option to force the operation.
For example, it happens if all the files in a disk are
missing.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
-v, --verbose
</strong></td></tr><tr><td>
Prints more information in the processing.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
-h, --help
</strong></td></tr><tr><td>
Prints a short help screen.
</td></tr>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="2">
<tr valign="top" align="left"><td><strong>
-V, --version
</strong></td></tr><tr><td>
Prints the program version.
</td></tr>
</table>
<h2><a name="9">9</a> 
Copyright
</h2>
This file is Copyright (C) 2011 Andrea Mazzoleni
<h2><a name="10">10</a> 
See Also
</h2>
rsync(1)
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