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Forum Discussion
XRAID2 vs RAID6, etc.
I've got an RN104 on the way and I'd like to better understand my configuration options. To start with, I have to admit that I'm planning on using 3x2TB Caviar Greens to start with (WD20EARX); I kn...
mdgm got most of your questions, but missed this one:
CoW is an attribute of the btrfs file system. Netgear has chosen not to explicitly control CoW in their UI settings, it is enabled/disabled implicitly as part of the snapshots and bitrot protection features. But Cow can be used w/o RAID (it is still available even if you chose jbod on the RN104). It is the basis of the snapshot feature.
The Bitrot feature in OS 6.2.x is unique to Netgear. They haven't given a lot of details, but there are some hints here and there. btrfs includes optional checksums which it uses to verify file integrity. Apparently when the btrfs checksum fails, Netgear has some proprietary software which attempts to do a RAID repair (even though there was no read error). If one of the physical disks somehow got bad (but readable) data, then the bitrot should be able to restore the original data. So the Bitrot protection couples RAID to the file system checksum feature - so it is not completely independent as I described above. Bitrot protection requires RAID, but it should work with both flexraid and xraid. Netgear chose to enable CoW when you check the bitrot protection box, but they haven't explained why. Perhaps they also might look at snapshots when the checksum fails??? They haven't said.
If I am reading the tea leaves correctly on what Netgear has done, then there are scenarios where the bitrot protection approach wouldn't work. However, it seems like a useful idea to me, and given the high priority you are giving data integrity, you should enable it.
To make things more confusing, BTRFS itself has an experimental mode where a variant of RAID protection is fully integrated into the file system (and not layered at all). This experimental mode has its own bitrot protection feature. However, Netgear isn't using that mode.
Normally a file system (ext, btrfs, ntfs, etc) is built on top of a disk partition. RAID sits between the the file system and multiple physical disks, so it acts like a "logical" disk drive. RAID applies parity protection (single or double) to the physical disks, and can be used with any file system. In fact RAID knows nothing about the file system structure. That is why rebuilding a empty RAID volume takes the same amount of time as a full one. Normally RAID repair is engaged only when there is a read error on the physical disk, or when a disk is replaced.
InteXX wrote:
StephenB wrote: CoW and Bit Rot protection are available in both xraid2 and RAID-6.
As noted above, things seem to pointing to the two as being synonymous; I must be misunderstanding something somewhere. So XRAID2 has CoW and BitRot--how 'bout double parity (in case of failure during a rebuild)?
CoW is an attribute of the btrfs file system. Netgear has chosen not to explicitly control CoW in their UI settings, it is enabled/disabled implicitly as part of the snapshots and bitrot protection features. But Cow can be used w/o RAID (it is still available even if you chose jbod on the RN104). It is the basis of the snapshot feature.
The Bitrot feature in OS 6.2.x is unique to Netgear. They haven't given a lot of details, but there are some hints here and there. btrfs includes optional checksums which it uses to verify file integrity. Apparently when the btrfs checksum fails, Netgear has some proprietary software which attempts to do a RAID repair (even though there was no read error). If one of the physical disks somehow got bad (but readable) data, then the bitrot should be able to restore the original data. So the Bitrot protection couples RAID to the file system checksum feature - so it is not completely independent as I described above. Bitrot protection requires RAID, but it should work with both flexraid and xraid. Netgear chose to enable CoW when you check the bitrot protection box, but they haven't explained why. Perhaps they also might look at snapshots when the checksum fails??? They haven't said.
If I am reading the tea leaves correctly on what Netgear has done, then there are scenarios where the bitrot protection approach wouldn't work. However, it seems like a useful idea to me, and given the high priority you are giving data integrity, you should enable it.
To make things more confusing, BTRFS itself has an experimental mode where a variant of RAID protection is fully integrated into the file system (and not layered at all). This experimental mode has its own bitrot protection feature. However, Netgear isn't using that mode.
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