On 2016-09-15 22:58, Duncan wrote:
E V posted on Thu, 15 Sep 2016 11:48:13 -0400 as excerpted:

I'm investigating using btrfs for archiving old data and offsite
storage, essentially put 2 drives in btrfs RAID-1, copy the data to the
filesystem and then unmount, remove a drive and take it to an offsite
location. Remount the other drive -o ro,degraded until my systems slots
fill up, then remove the local drive and put it on a shelf. I'd verify
the file md5sums after data is written to the drive for piece of mind,
but maybe a btrfs scrub would give the same assurances. Seem
straightforward? Anything to look out for? Long term format stability
seems good, right? Also, I like the idea of being able to pull the
offsite drive back and scrub if the local drive ever has problems, a
nice extra piece of mind we wouldn't get with ext4. Currently using the
4.1.32 kernel since the driver for the r750 card in our 45 drives system
only supports up to 4.3 ATM.

As described I believe it should work fine.

Btrfs raid1 isn't like normal raid1 in some ways and in particular isn't
designed to be mounted degraded, writable, long term, only temporarily,
in ordered to replace a bad device.  As that's what I thought you were
going to propose when I read the subject line, I was all ready to tell
you no, don't try it and expect it to work, but of course you had
something different in mind, only read-only mounting of the degraded
raid1 (unless needed for scrub, etc), not mounting it writable, and as
long as you are careful to do just that, only mount it read-only, you
should be fine.

While I generally agree with Duncan that this should work if you're careful, I will say that as things stand right now, you almost certainly _SHOULD NOT_ be using BTRFS for archival storage, be it in the way you're talking about, or even just as a back-end filesystem for some other setup. While I consider it stable enough for regular usage, the number of issues is still too significant IMO to trust long term archival data storage to it.

There are lots of other options for high density archival storage, and most of them are probably better than BTRFS at the moment. For reference, here's what I would do if I needed archival storage beyond a few months: 1. Use SquashFS to create a mountable filesystem image containing the data to be archived.
2. Compute and store checksums for the resultant FS image (probably SHA256)
3. Using veritysetup, dm-verity, and the new forward error correction it provides, generate block-level authenticated checksums for the whole image, including enough data to repair reasonable data corruption.
4. Compute and store checksums for the resultant dm-verity data.
5. Compress the data from dm-verity (using the same compression algorithm as used in the SquashFS image). 6. Create a tar archive containing the SquashFS image, the compressed dm-verity data, and a file with the checksums.
7. Store that tar archive in at least two different places.

When restoring data:
1. Collect copies of the tar archive from at least two different places.
2. For both copies:
        1. Extract the tar archive and decompress the dm-verity data.
        2. Verify the checksum of the dm-verity data.
3. If the dm-verity data's checksum is correct, set up a dm-verity target using that and the SquashFS image. 4. If the dm-verity data's checksum is incorrect, verify the checksum of the SquashFS archive. 5. If the SquashFS archive's checksum is correct, use it directly, otherwise discard this copy. 3. Create a read-only dm-raid RAID1 array containing all of the dm-verity backed devices and SquashFS images with in-core sync-logging.
4. Mount the resultant device, and copy any data out.

That will overall give a better level of protection than BTRFS, or ZFS, or almost anything else available on Linux right now can offer, and actually provides better data safety than many commercial solutions. The only down side is that you need recent device-mapper userspace and a recent kernel to create and extract things.
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