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Incremental backups to optical media: tar, dar, or something else?

I'm working on a project to back up my family photos from TrueNas to Blu-Ray disks. I have other, more traditional backups based on restic and zfs send/receive, but I don't like the fact that I could delete every copy using only the mouse and keyboard from my main PC. I want something that can't be ransomwared and that I can't screw up once created.

The dataset is currently about 2TB, and we're adding about 200GB per year. It's a lot of disks, but manageably so. I've purchased good quality 50GB blank disks and a burner, as well as a nice box and some silica gel packs to keep them cool, dark, dry, and generally protected. I'll be making one big initial backup, and then I'll run incremental backups ~monthly to capture new photos and edits to existing ones, at which time I'll also spot-check a disk or two for read errors using DVDisaster. I'm hoping to get 10 years out of this arrangement, though longer is of course better.

I've got most of the pieces worked out, but the last big question I need to answer is which software I will actually use to create the archive files. I've narrowed it down to two options: dar and bog-standard gnu tar. Both can create multipart, incremental backups, which is the core capability I need.

Dar Advantages (that I care about):

  • This is exactly what it's designed to do.
  • It can detect and tolerate data corruption. (I'll be adding ECC data to the disks using DVDisaster, but defense in depth is nice.)
  • More robust file change detection, it appears to be hash based?
  • It allows me to create a database I can use to locate and restore individual files without searching through many disks.

Dar disadvantages:

  • It appears to be a pretty obscure, generally inactive project. The documentation looks straight out of the early 2000s and it doesn't have https. I worry it will go offline, or I'll run into some weird bug that ruins the show.
  • Doesn't detect renames. Will back up a whole new copy. (Problematic if I get to reorganizing)
  • I can't find a maintained GUI project for it, and my wife ain't about to learn a CLI. Would be nice if I'm not the only person in the world who could get photos off of these disks.

Tar Advantages (that I care about):

  • battle-tested, reliable, not going anywhere
  • It's already installed on every single linux & mac PC , and it's trivial to put on a windows pc.
  • Correctly detects renames, does not create new copies.
  • There are maintained GUIs available; non-nerds may be able to access

Tar disadvantages:

  • I don't see an easy way to locate individual files, beyond grepping through snar metadata files (that aren't really meant for that).
  • The file change detection logic makes me nervous - it appears to be based on modification time and inode numbers. The photos are in a ZFS dataset on truenas, mounted on my local machine via SMB. I don't even know what an inode number is, how can I be sure that they won't change somehow? Am I stuck with this exact NAS setup until I'm ready to make a whole new base backup? This many blu-rays aren't cheap and burning them will take awhile, I don't want to do it unnecessarily.

I'm genuinely conflicted, but I'm leaning towards dar. Does anyone else have any experience with this sort of thing? Is there another option I'm missing? Any input is greatly appreciated!

30 comments

  • You can't really easily locate where the last version of the file is located on an append-only media without writing the index in a footer somewhere, and even then if you're trying to pull an older version you'd still need to traverse the whole media.

    That said, you use ZFS, so you can literally just zfs send it. ZFS will already know everything that needs to be known, so it'll be a perfect incremental. But you'd definitely need to restore the entire dataset to pull anything out of it, reapply every incremental one by one, and if just one is unreadable the whole pool is unrecoverable, but so would the tar incrementals. But it'll be as perfect and efficient as possible, as ZFS knows the exact change set it needs to bundle up. It's unidirectional, so that's why you can just zfs send into a file and burn it to a CD.

    Since ZFS can easily tell you the difference between two snapshots, it also wouldn't be too hard to make a Python script that writes the full new version of changed files and catalogs what file and what version is on which disc, for a more random access pattern.

    But really for Blurays I think I'd just do it the old fashioned way and classify it to fit on a disc and label it with what's on it, and if I update it make a v2 of it on the next disc.

    • Ohhh boy, after so many people are suggesting I do simple files directly on the disks I went back and rethought some things. I think I'm landing on a solution that does everything and doesn't require me to manually manage all these files:

      • fd (and any number of other programs) can produce lists of files that have been modified since a given date.
      • fpart can produce lists of files that add up to a given size.
      • xorrisofs can accept lists of files to add to an iso

      So if I fd a list of new files (or don't for the first backup), pipe them into fpart to chunk them up, and then pass these lists into xorrisofs to create ISOs, I've solved almost every problem.

      • The disks have plain files and folders on them, no special software is needed to read them. My wife could connect a drive, pop the disk in, and the photos would be right there organized by folder.
      • Incremental updates can be accomplished by keeping track of whenever the last backup was.
      • The fpart lists are also a greppable index; I can use them to find particular files easily.
      • Corruption only affects that particular file, not the whole archive.
      • A full restore can be accomplished with rsync or other basic tools.

      Downsides:

      • Change detection is naive. Just mtime. Good enough?
      • Renames will still produce new copies. Solution: don't rename files. They're organized well enough, stop messing with it.
      • Deletions will be disregarded. I could solve this with some sort of indexing scheme, but I don't think I care enough to bother.
      • There isn't much rhyme or reason to how fpart splits up files. The first backup will be a bit chaotic. I don't think I really care.
      • If I rsync -a some files into the dataset, which have mtimes older than the last backup, they won't get slurped up in the next one. Can be solved by checking that all files are already in the existing fpart indices, or by just not doing that.

      Honestly those downsides look quite tolerable given the benefits. Is there some software that will produce and track a checksum database?

      Off to do some testing to make sure these things work like I think they do!

    • Woah, that's cool! I didn't know you just zfs send anywhere. I suppose I'd have to split it up manually with split or something to get 50gb chunks?

      Dar has dar_manager which you can use to create a database of snapshots and slices that you can use to locate individual files, but honestly if I'm using this backup it'll almost certainly be a full restore after some cataclysm. If I just want a few files I'll use one of my other, always-online backups.

      Edit: Clicked save before I was finished

      I'm more concerned with robustness than efficiency. Dar will warn you about corruption, which should only affect that particular file and not the whole archive. Tar will allow you to read past errors so the whole archive won't be ruined, but I'm not sure how bad the affects would be. I'm really not a fan of a solution that needs every part of every disk to be read perfectly.

      I could chunk them up manually, but we're talking about 2TB of lumpy data, spread across hundreds of thousands of files. I'll definitely need some sort of tooling to track changes, I'm not doing that manually and I bounce around the photo library changing metadata all the time.

  • This is an interesting problem for the same use case which I've been thinking about lately.

    Are you using standard BluRay, or M-Discs?

    My plan was to simply copy files. These are photos, and IME they don't benefit from compression (I stopped taking raw format pictures when I switched to Fujifilm, and the jpgs coming from the camera were better than anything I could produce from raw in Darktable). Without compression, putting then in tarballs then only adds another level of indirection, and I can just checksum images directly after write, and access them directly when I need to. I was going to use the smallest M-Disc for an index and just copy and modify it when it changed, and version that.

    I tend to not change photos after they've been processed through my workflow, so in my case I'm not as concerned with the "most recent version" of the image. In any case, the index would reflect which disc the latest version of an image lived, if something did change.

    For the years I did shoot raw, I'm archiving those as DNG.

    For the sensitive photos, I have a Rube Goldberg plan that will hopefully result in anyone with the passkey being able to mount that image. There aren't many of those, and that set hasn't been added to in years, so it'll go on one disc with the software necessary to mount it.

    My main objective is accessibility after I'm gone, so having a few tools in the way makes trump over other concerns. I see no value in creating tarballs - attach the device, pop in the index (if necessary), find the disc with the file, pop that in, and view the image.

    Key to this is

    • the data doesn't change over time
    • the data is already compressed in the file format, and does not benefit from extra compression

    • I'm using standard BD-DLs. M-Disks are almost triple the price, and this project is already too costly. I'm not looking for centuries of longevity, I'm using optical media because it's read-only once written. I read that properly stored Blu-Rays should be good for 10 or 20 years, which is good enough for me. I'll make another copy when the read errors start getting bad.

      Copying files directly would work, but my library is real big and that sounds tedious. I have photos going back to the 80s and curating, tagging, and editing them is an ongoing job. (This data is saved in XMP sidecars alongside the original photos). I also won't be encrypting or compressing them for the same reasons you mentioned.

      For me, the benefit of the archive tool is to automatically split it up into disk-sized chunks. That and to automatically detect changes and save a new version; your first key doesn't hold true for this dataset. You're right though, I'm sacrificing accessibility for the rest of the family. I'm hoping to address this with thorough documentation and static binaries on every disk.

      • The densities I'm seeing on M-Discs - 100GB, $5 per, a couple years ago - seemed acceptable to me. $50 for a TB? How big is your archive? Mine still fits in a 2TB disk.

        Copying files directly would work, but my library is real big and that sounds tedious.

        I mean, putting it in an archive isn't going to make it any smaller. Compression on even lossless compressed images doesn't often help.

        And we're talking about 100GB discs. Is squeezing that last 10MB out of the disk by splitting an image across two disks worth it?

        The metadata is a different matter. I'd have to think about how to handle the sidecar data... but that you could almost keep on a DVD-RW, because there's no way that's going to be anywhere near as large as the photos themselves. Is your photo editor DB bigger than 4GB?

        I never change the originals. When I tag and edit, that information is kept separate from the source images - so I never have multiple versions of pictures, unless I export them for printing, or something, and those are ephemeral and can be re-exported by the editor with the original and the sidecar. Music, and photos, I always keep the originals isolated from the application.

        This is good, though; it's helping me clarify how I want to archive this stuff. Right now mine is just backed up on multiple disks and once in B2, but I've been thinking about how to archive for long term storage.

        I think in going to go the M-Disc route, with sidecar data on SSD and backed up to BluRay RW. The trick will be letting DarkTable know that the source images are on different media, but I'm pretty sure I saw an option for that. For sure, we're not the first people to approach this problem.

        The whole static binary thing - I'm going that route with an encrypted share for financial and account info, in case I die, but that's another topic.

30 comments