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What does Decompression error or “Data Error -6. Data is corrupt” mean?

If MultiWalk stops during optimization or walkforward with a message that mentions “Decompression error” or “Data Error -6. Data is corrupt.”, this article explains what it means and how to fix it. In almost every case this is not a problem with your strategy or your results file – it points to your computer’s memory (RAM), and it is usually straightforward to resolve.

What the error looks like

You will see something similar to this in the MultiWalk log or in an error message:

Iteration error from optimization file '...(MW).dat'.  Is file corrupt?  Try to rerun optimization to correct.
Could not read optimized record (size = 28078)
Error: Decompression error
Data Error -6.  Data is corrupt.

The exact record size and file name will vary, and the error may appear at a different point each time you run.

What it actually means

To keep result files small, MultiWalk compresses the data it saves during optimization and walkforward. Every compressed block also carries a small integrity check. When MultiWalk reads a block back, it recalculates that check and compares it. “Data Error -6. Data is corrupt.” simply means the data that came back did not match its integrity check – in other words, a few bytes changed somewhere between when the block was written and when it was read back.

In one sense this is a good thing: MultiWalk is catching a problem that most software would silently ignore.

It is not a size limit

There is no record-size or file-size limit involved here. The record sizes shown in these messages (often tens or hundreds of kilobytes) are well within what MultiWalk handles every day. A large number in the message is not the cause.

It is not your strategy or your data

The same strategy, data, and settings will usually run fine on the next attempt, or on a different machine. That is an important clue about the real cause.

What causes it

In almost every case I have seen, this traces back to system memory (RAM) that is not perfectly stable.

MultiWalk’s compression works memory much harder than a typical program does – it moves a large amount of data through RAM very rapidly, much like a memory stress test. Because of that, MultiWalk is often the first program on a PC to reveal memory that is running slightly beyond what it can reliably handle. Everyday software may simply never push the memory hard enough to notice.

A very common trigger is an XMP or EXPO memory profile enabled in the computer’s BIOS. These profiles run the RAM faster than its guaranteed baseline speed – essentially a factory overclock – and they are not always fully stable, especially on newer DDR5 systems.

Why the error moves around

If this were a genuinely damaged file, it would fail in exactly the same place every single time. Instead, the error is a moving target: it happens on one run and not the next, and rarely in the same spot twice. That unpredictable, non-repeating behavior is the tell-tale signature of a memory stability issue rather than a real problem with your file.

How to fix it

These are the fixes that have resolved this for users, in the order worth trying.

1. Adjust your RAM settings (the most common fix)

If your RAM is running on an XMP or EXPO profile, that is the first thing to change:

  1. Restart the PC and enter the BIOS/UEFI setup (usually by pressing Delete, F2, or F12 during startup – check your PC or motherboard maker’s instructions).
  2. Find the memory or overclocking section, and either disable the XMP/EXPO profile or set the memory to a lower speed than the profile’s rating.
  3. Save and exit, then re-run your MultiWalk project.

This is exactly what I did to fix it on my own machine. My PC had DDR5-6000 memory running on its XMP profile and kept producing this error. Disabling XMP and setting the RAM to 4800 MHz stopped the errors completely – and I honestly could not tell any difference in speed.

2. Update your BIOS and drivers (for prebuilt or OEM PCs)

Some prebuilt machines (for example, certain Dell models) do not expose the memory settings in the BIOS. In that case, updating the BIOS/firmware and the manufacturer’s chipset and system drivers has helped.

In one case, a user updated their BIOS to the newest version and it made about 98% of the problem go away for them. On another prebuilt PC, updating the manufacturer’s device drivers resolved it.

3. Re-run the optimization

Re-running the project regenerates the affected file from scratch. Once your memory is on a stable setting, a run that previously failed will normally complete without any errors. If you have not changed any settings yet, a simple re-run may get you past it once – but the underlying cause is still worth addressing so it does not return.

If the error continues

If you have stabilized your memory and still see the error, please email me at dave@multiwalk.net and I will help you sort it out from there.

In short

The “Data Error -6 / Data is corrupt” message is MultiWalk catching a momentary memory glitch, not a damaged strategy or a file that is too large. Getting your system memory onto a stable setting – most often by disabling XMP/EXPO or lowering the RAM speed – resolves it for good.

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