Spanning

When a source is too big for a single tape, Canister's Spanning engine comes in handy. This feature makes it quick and easy to Archive datasets that are larger than a single tape.

Getting Started

macOS

When Canister for Mac is presented with a source that won't fit on the mounted tape, it enters Spanning mode. You'll be prompted to set a Spanning Label, making it easy to locate your data later in the Catalog.

In turn, Canister will group the tapes for each Spanning part like this:

From here, rinse and repeat for as many tapes as it takes to complete the Archive.

Windows

When Canister for Windows is presented with a source that won't fit on the mounted tape, it archives as much as will fit, then prompts you to load the next tape via the Windows Notification Center.

Enter the next tape and hit the format button to proceed. Rinse and repeat for as many tapes as it takes to complete the Archive.

Adding new data to spanned tapes

It's likely that you won't fill up the last tape in a spanning set. Therefore, you can add anything you want to that last tape at a later time - no previously archived data will be affected. If the to-be-archived files fit, Canister will add it to the tape. If not, you'll reenter the Spanning flow described above.

Pro Tip: If the data belongs to an existing Spanned Archive, Mac users might find it useful to set the same Spanning Label as before. This way, the newly added data is grouped in the same Catalog.

Spanning Questions

Spanning is an advanced feature that tends to lead to a few workflow questions. Be sure to check the FAQs below before reaching out to support.

Do I need to preformat my tapes?

No, Canister will ask you to format at the relevant points in the Spanning workflow. Insert the next tape, then follow the prompts.

How should I name my tapes?

This comes up quite a bit; the best bet is to name each tape sequentially, for example:

1. Archive_pt1

2. Archive_pt2

3. Archive_pt3

Is there a tape/item limit when Spanning?

Canister doesn't enforce any limits, however Spanning is best used with dataset that requires between 2-4 tapes.

Why? Because Spanning requires Canister keeping the entire source index in memory, which is a heavy lift. Also, the chance of something going wrong on the hardware side of LTO increases exponentially the longer your LTO drive operates. As an upper limit we recommend Spanned Archives containing less than one million items.

How do I redo Spanning if something goes wrong?

As with all things LTO, hiccups happen. While with hard disks it's easy to redo something, due to LTO's linear nature, errors tend to be showstoppers - at least temporarily. With Spanning, the chance that a tape runs into an issue due to a hardware trouble is, of course, compounded.

Canister gets around this problem by using its Dupe Detection implementation to the fullest: in case an error happens with any tape during a spanning archive, simply start over. It's important to feed Canister the tapes in the order they were previously finished, then let Duplicate Detection skip items already existing on each part.

Can I incrementally update a Spanned Archive?

Although Canister supports Dupe Detection, due to LTO's linear nature, we don't recommend making incremental updates to a Spanned Archive. File changes are likely to end up distributed over multiple tapes, which will get complicated when retrieving.

Why does Canister leave free space at the end of each Spanning Part?

Splitting files between multiple tapes is not a standard LTFS feature. As a result, each tape is likely to have a small pocket of space at the end. Tape compression can also play a part here, so it's best to disable at the point of formatting for the best results.

Can I perform Simultaneous Spanning?

Canister for Mac does not support Spanning while creating simultaneous archives. Canister for Windows follows a different flow, which allows the next tape to be inserted for each drive.