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SnapRAID is only one of the available not standard RAID solutions for disk arrays.

The best known others are:

The main factor to categorize them is when the redundancy information is updated. In real-time solutions the parity data is updated in real-time, without an explicit action from the user. Like in standard RAID. In snapshot solutions the parity data is updated only upon an explicit request from the user. Like in backups.

Other important factors are the support of an integrity checksum to identify silent data corruption and the possibility to fix it, if you can use already filled disk, if you can recover your data from not broken disks when you get too many failures to have a full recover, and if all your disks are spinning when reading one file.

SnapRAIDunRAIDFlexRAIDdisParityZFSBtrfsStorage Spaces
Redundancy model
Real-time computes parity at real-time like RAID, snapshot at request time, like Backup.
SnapshotReal-timeReal-time
Snapshot
SnapshotReal-time
Snapshot
Real-time
Snapshot
Real-time
Integrity
If data is validated with some kind of checksum, and which one is the default.
Yes
SpookyHash
128 bit
NoYes
Adler32
32 bit
Yes
MD5
128 bit
Yes
fletcher4
256 bit
Yes
CRC32C
32 bit
No
Fix silent errors
If silent errors are detected and fixed before they can propagate into the parity.
YesNo [1]No [2]NoYes [3]Yes [3]No
Number of failures
How many disk failures are supported? 1 is for RAID5, 2 for RAID6.
1 2 3 4 5 611 2 3 4 5 6+11 2 31 21
Other failures
If more disks fail than the supported redundancy model, is it possible to recover the data in the not failed disks?
YesYesYesYesNoNoNo
Power/Noise
How many disks are spinning when reading a single file?
OneOneOneOneAllAllAll
Filled disk
Can you start using it with already filled disk?
YesNoYesYesNoNoNo
Operating system
Which OS is supported?
Linux
Windows
Mac OS X
OpenIndiana
Solaris
BSD
LinuxWindows
Linux
WindowsLinux [4]
Mac OS X
OpenIndiana
Solaris
BSD
LinuxWindows
Age
The year of the first official release supporting at least RAID5 redundancy.
2011200520082009200520132012
License/Price
Software license and price.
Open Source GPL3
Free
Open Source GPL2
69$/119$
Proprietary
40$/100$
Proprietary
Free
Open Source CDDL
Free
Open Source GPL2
Free
Proprietary
Windows 8
Interface
Which interface is provided? GUI or command line?
command line
or Elucidate GUI, or plugin for for OpenMediaVault
GUIGUIcommand linecommand line
GUI/Plugins [5]
command lineGUI

Notes

[1] - unRAID doesn't have any kind of checksum, and it just ignores silent errors. Even worse, if a parity error is detected as result of a silent error in the data, the parity is automatically recomputed, making impossible to recover the silent error, even manually.

[2] - Flexraid uses checksums to validate files, but such checksums are not verified when data is read to update the parity. This means that any silent error present will propagate into the parity, making impossible to fix it later, even if it can be still detected comparing the file checksum.
You can get in a state where the "Validate" operation reports errors, but the "Verify" one reports no problem in the parity, making impossible to fix the errors.

[3] - ZFS and Btrfs provide a bit-rot protection at the same level of SnapRAID, always checking data before using it. In this regards all the three solutions represent the state-of-the-art.
A cons of ZFS is that the default Fletcher checksum is a choice that favorites speed over quality. The same for the default CRC32C used by Btrfs. The 128 bits SpookyHash used by SnapRAID is instead the state-of-the-art in checksumming quality, without compromising in speed.
Another cons of ZFS is that it lacks a fast RAID implementation in assembler. It only has a C implementation, that is usually from two to four times slower.
ZFS also uses a sub-optimal RAID-Z3 algorithm, that requires double computations than the equivalent SnapRAID's z-parity.
Instead, both SnapRAID and Btrfs use top-notch assembler implementations to compute the RAID parity, always using an optimal RAID algorithm.

[4] - ZFS is also available in Linux at kernel level using ZFS on Linux.

[5] - ZFS has external GUIs, like Napp-it and plugins for FreeNAS and NAS4Free.

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