SnapRAID is only one of the available not standard RAID solutions for disk arrays.
The best known others are:
- unRAID - Commercial and OpenSource GPL2 solution. A modified version of the Linux Reiserfs filesystem with real-time redundancy. No integrity checksum is supported.
FlexRAID- DISCONTINUED Commercial and proprietary C++/Java application for Windows, with some limited support for Linux. It supports both snapshot redundancy and real-time redundancy, with integrity checksum.- ZFS - OpenSource filesystem (but GPL incompatible) with real-time redundancy and integrity checksum.
- Btrfs - OpenSource filesystem GPL2 with real-time redundancy. From Linux 3.9 it supports RAID5/6 redundancy and integrity checksums. Recently various issues were found and the official wiki now states: The parity RAID code has multiple serious data-loss bugs in it. It should not be used for anything other than testing purposes.
- Storage Spaces - The last contender from Microsoft, and integrated into Windows 8. Proprietary with real-time redundancy. Checksumming is not supported. It also has some other limitations.
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.
| SnapRAID | unRAID | FlexRAID | ZFS | Btrfs | Storage Spaces | |
|---|---|---|---|---|---|---|
| Redundancy model Real-time computes parity at real-time like RAID, snapshot at request time, like Backup. |
Snapshot | Real-time | Real-time Snapshot | Real-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 | No [8] | Yes Adler32 32 bit | Yes fletcher4 256 bit | Yes CRC32C 32 bit | No [9] |
| Fix silent errors If silent errors are detected and fixed before they can propagate into the parity. |
Yes | No [1] | No [2] | Yes [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 6 | 1 2 | 1 2 3 4 5 6+ | 1 2 3 | 1 2 | 1 2 |
| Other failures If more disks fail than the supported redundancy model, is it possible to recover the data in the not failed disks? |
Yes | Yes | Yes | No | No | No |
| Power/Noise How many disks are spinning when reading a single file? |
One | One | One | All | All | All |
| Filled disks Can you start with already filled disks? |
Yes | Partial[4] | Yes | No | No | No |
| Add disks Can you add disks at later time? |
Yes | Yes | Yes | No[5] | Yes | Yes |
| Operating system Which OS is supported? |
Linux Windows Mac OS X OpenIndiana Solaris BSD | Linux | Windows Linux | Linux [6] Mac OS X OpenIndiana Solaris BSD | Linux | Windows |
| Age The year of the first official release supporting at least RAID5 redundancy. |
2011 | 2005 | 2008 | 2005 | not yet stable [11] | 2012 |
| License/Price Software license and price. |
Open Source GPL3 Free | Open Source GPL2 59$/129$ | Proprietary 40$/60$ | Open Source CDDL Free | Open Source GPL2 Free | Proprietary Windows 8/10 |
| Interface Which interface is provided? GUI or command line? |
command line or Elucidate GUI, or plugin for for OpenMediaVault | command line GUI | GUI | command line GUI/Plugins [7] | command line GUI[10] | GUI |
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.
ZFS also uses a sub-optimal RAID-Z3 algorithm, that requires double computations than the equivalent SnapRAID's z-parity.
[4] - unRAID allows to use filled disks but only if they are already formatted with the ReiserFS, XFS or Btrfs filesystems. But not ext4 or NTFS, the two most common filesystems used in Linux and Windows.
[5] - ZFS allows to add a new vdev at the pool, but not a disk to a vdev. See The 'Hidden' Cost of Using ZFS for Your Home NAS.
[6] - ZFS is also available in Linux at kernel level using ZFS on Linux.
[7] - ZFS has external GUIs, like Napp-it and plugins for FreeNAS and NAS4Free.
[8] - unRAID can have integrity checksum using a plugin like Dynamix File Integrity, Checksum Suite or bunker but they are all independent at the parity processing and not used to help the recovering process. For example, when recovering a single failed disk with double parity, the checksum can be used to identify additional silent errors, and still be able to recover.
[9] - Storage Spaces can be used with ReFS that support integrity checksum, but not in the Parity mode: "Although, you may be able to select Simple (no resiliency) or Parity, both of these options will fail the process".
[10] - Btrfs has external GUIs, like Rockstor.
[11] - Btrfs supports RAID5 from 2013, but recently some serious issues in the RAID5/6 support were found. At present they aren't fixed yet. The official wiki says (2023): "There are some implementation and design deficiencies that make it unreliable for some corner cases and the feature should not be used in production, only for evaluation or testing".
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