Deduplication Nightmares: What to Use When TAR Slows You Down

If you've ever tried to push massive amounts of similar backup data into a deduplication system, say something like Cohesity, you've probably butted heads with the limits of traditional archiving tools. For years, TAR has been the go-to format for bundling files before shipping them off to storage. But when deduplication comes into play, especially on appliances that compress and encrypt after ingest, things get messy, and fast.

A user in a popular data storage forum recently laid it out plainly. They're trying to back up thousands of OS and application binary files (SAP, Oracle, HANA, the heavy hitters) using TAR, but deduplication isn't playing nice. The files aren't compressed client-side, to keep deduplication possible, and they're being shipped to Cohesity via NFS. The kicker is that copying files individually is painfully slow, on the order of 5 hours versus 10 minutes.

That led to the question: is there an archive format better than TAR when deduplication is on the table?

The tug-of-war between deduplication and archive format

First, it's worth noting that TAR isn't inherently bad for deduplication. As some commenters pointed out, TAR is just a container that bundles files in raw format with headers. The issue comes when you compress those TAR files. Compression obfuscates the underlying data patterns that deduplication engines rely on to detect redundancy, and the same goes for encryption. So if you're TARing and gzipping (.tgz), you're basically feeding your dedup engine garbage, as far as it's concerned.

The original user avoided that trap, with no compression and no encryption before ingest. But dedup quality wasn't the only problem; performance was too. Small files slow down Cohesity's ingest pipeline over NFS, which isn't surprising. Most storage systems, especially those optimized for large sequential I/O, hate being peppered with tiny file operations.

That still leaves a gnarly problem. You can't copy the files one by one, and TAR works but messes with dedup over time due to shifting content positions. What else is out there?

Why dedup struggles with changing TARs

Every time you make a tiny change in a directory and re-TAR it, the whole structure of the resulting .tar file shifts. Because TAR is just a stream of concatenated files, the relative positions of unchanged files move. To a block-level deduplication engine, even a 1-byte shift could mean an entirely new chunk that it has to store, with no dedup win.

Some commenters referenced a 2011 paper that warned about this exact issue: TAR isn't dedup-friendly because changes ripple through the archive. Others pushed back, pointing out that modern dedup systems use smarter chunking algorithms (e.g., variable length, sliding windows) that can still find the redundancies, even inside shifted TARs.

The truth probably lives somewhere in the middle. If your backup strategy involves regular updates to huge archives, you're setting yourself up for a deduplication headache.

So what's the alternative?

There's no silver bullet here. But some strategies can help, depending on what you value more: restore granularity, dedup gains, or performance.

1. Use chunk-aligned archive tools

A few archive tools like DAR (Disk ARchive) or Bacula allow for chunking or segmenting backups in a more dedup-friendly way. They're a bit more complex than good old TAR, but they can preserve file-level context while still bundling data in a way that helps dedup engines do their job.

2. Split archives based on content type

If you have lots of small, mostly unchanging files (like OS binaries), group them together. The more uniform the contents of a TAR, the better deduplication tends to be, especially if files are updated at similar frequencies.

3. Roll your own dedup-aware format

One user hinted that TAR is still fine if you control the backup software and it understands how deduplication at the target works. That's huge. If your backup tool tracks changed blocks, aligns them with known chunks in the dedup table, and packages them accordingly, you can cheat the system a bit. Not every team has that luxury, though.

4. Avoid pre-compression or encryption

This can't be overstated: if you compress or encrypt before sending your data to the dedup target, you're torching your deduplication benefits. Some systems like Cohesity handle compression and encryption after dedup, which is exactly how it should be. Don't get ahead of yourself.

"Just use the agent" isn't always the answer

Several people chimed in suggesting the Cohesity agent and scheduled jobs. That makes sense in theory, since the agent can handle changed blocks and deduplication context and can even handle restores cleanly. In the real world, it's never that simple.

In this case, users who own the backed-up systems need to do restores themselves, but they don't have access to the Cohesity GUI. So the team chose a scriptable approach, with TAR archives stored on disk and restores via shell. It's simple and fast, with no GUI drama.

And that's important. You can have the most elegant storage architecture in the world, but if your restore process is slow or locked behind an admin gate, users will revolt.

A word of caution from the trench

One storage admin laid it bare: dedup sounds cool but rarely delivers big in the enterprise. He'd seen compression offer far more savings than deduplication, and he even warned against relying too heavily on dedup because of potential chunk database issues, especially when the storage system takes a hit.

That comes from experience, and I wouldn't dismiss it as paranoia. If you're building a strategy around deduplication savings, you'd better understand exactly what kind of files you're backing up, how often they change, and how the dedup engine works. Otherwise, you're chasing theoretical savings.

So what should you do?

TAR isn't the villain, but it isn't a miracle format for dedup-aware backups either. Avoid compression and encryption before ingest and let your storage system handle that. Test different strategies, since some users have had luck with split TARs, chunked archives, or tweaking the dedup settings on the storage appliance. And don't obsess over deduplication: if compression gives you 50% and dedup gives you 3%, you know where the savings are.

Maybe it's also time to rethink your archive format if restore times and dedup hits are giving you grief. The tools are out there. You just have to pick the one that fits your pain points instead of your habits.