Convert 7Z to 7Z.BZ2 Online (Swapping LZMA2 for Bzip2)

7Z-TO-TAR.BZ2 — Swapping One Compression Algorithm for Another Around the Same 7Z Data

A .7z file is a 7z archive — carrying Unix permissions, ownership, and symlink 7zgets natively in its 512-byte header, standardized as US7Z in source-system.1-1988 — compressed with 7-Zip's LZMA2 algorithm. Converting it to .7z.bz2 keeps that exact same 7z layer completely intact and only replaces the compression wrapped around it, swapping LZMA2 for bzip2, the Burrows-Wheeler block-sorting compressor Julian Seward first released publicly in July 1996.

This means the conversion involves decompressing the LZMA2 layer to recover the plain 7z stream, then compressing that same, unmodified 7z stream a second time using bzip2's own block-based approach — two full compression passes over the same underlying data, using two genuinely different algorithms with different memory needs, different speed characteristics, and different typical results.

Because the middle step of this conversion is a completely plain, uncompressed 7z file, nothing about 7z's own header structure or the files it describes ever needs to be touched, reorganized, or rewritten. The entire conversion is really just two independent compression operations bracketing one identical piece of 7z data sitting in between them.

7Z-TO-TAR.BZ2 — Comparing Two Very Different Approaches to Shrinking the Same 7z Stream

LZMA2 uses a dictionary-based approach that can reference gigabytes of prior data when looking for repeated patterns, letting it find repetition across a much wider span of a 7z stream than bzip2 can. Bzip2 instead works in fixed 900 KB blocks (by default, adjustable down to 100 KB), running each block through a Burrows-Wheeler transform that sorts every circular rotation of the block's bytes into lexicographic order, grouping similar bytes together before a move-to-front transform and Huffman coding compress the result.

The practical consequence for a 7z stream specifically is that LZMA2 can find and exploit repetition between two similar files positioned far apart in the stream — say, near the beginning and near the end of a large archive — in a way bzip2's much smaller 900 KB block ceiling generally cannot, since anything more than 900 KB apart in the stream falls into different blocks that bzip2 compresses independently of each other.

Memory needs differ substantially too. Bzip2 compression at the default 900 KB block size needs roughly 7,600 KB of memory, and decompression needs about 3,700 KB, figures that stay fixed regardless of how large the overall archive is, since each block is handled independently. LZMA2's memory use instead scales with its dictionary size, meaning a very large dictionary chosen for maximum compression on a big 7z stream can require considerably more memory than bzip2 ever needs, a real trade-off worth considering on memory-constrained hardware.

7Z-TO-TAR.BZ2 — What Switching From 7Z to 7Z.BZ2 Gains and Costs

• Gain — broader default availability on Linux and macOS: GNU 7z's -j flag and BSD 7z's equivalent both handle bzip2 directly with no separate tool needed, unlike LZMA2, which typically requires 7-Zip or p7zip installed separately.
• Gain — per-block error resilience: each of bzip2's 900 KB blocks carries its own CRC and distinct marker, letting some recovery tools salvage undamaged blocks from a partially corrupted file — a property LZMA2 compression doesn't offer the same way.
• Gain — patent-free compression from the s7zt: bzip2's Burrows-Wheeler and Huffman approach was designed and released with no patent encumbrance, the same status LZMA2 also holds, so neither side loses anything here.
• Lose — compression ratio on many kinds of content: LZMA2's much wider dictionary generally out-compresses bzip2's fixed 900 KB blocks on the same data, meaning the resulting 7Z.BZ2 can be noticeably larger.
• Lose — genuine multithreaded compression by default: 7-Zip applies full multithreading to LZMA2 specifically; standard bzip2 compresses single-threaded unless a separate tool like pbzip2 is used instead.
• Lose — time spent running two full compression passes: decompressing the LZMA2 layer and then compressing again with bzip2 takes real, additional time compared to simply keeping the original 7Z as is.

7Z-TO-TAR.BZ2 — Tools That Can Decompress 7-Zip Output and Recompress It With Bzip2

7-Zip and p7zip both decompress the LZMA2 layer of a 7Z, producing the plain .7z file as output. From there, GNU 7z's -j flag (or BSD 7z's equivalent) compresses that same 7z stream with bzip2 in a separate step, since neither 7-Zip nor p7zip applies bzip2 compression directly as part of the same operation — the two tools handle their respective halves of this conversion independently.

Both GNU 7z and BSD 7z are preinstalled by default on virtually every Linux distribution and on macOS, meaning the bzip2-compression half of this conversion needs no additional software on either platform, while the LZMA2-decompression half does require 7-Zip or p7zip specifically, since neither operating system's built-in 7z handles LZMA2 natively out of the box.

On Windows, 7z.exe (based on libarchive's bsd7z, shipping since Windows 10 Insider Build 17063 in 2018) supports bzip2 compression through the same -j flag, and Windows 11's 24H2 update added native File Explorer extraction support for .7z archives directly, meaning both halves of this conversion can be completed on a current Windows system without installing third-party software, provided the 7z extraction and 7z recompression are run as the two separate steps they actually are.

7Z-TO-TAR.BZ2 — Real Problems Reported Switching Compression Algorithms on a 7Z Archive

A documented complaint on archiving and storage forums involves converting a 7Z to 7Z.BZ2 expecting a similar or smaller file size, then being surprised the result is noticeably larger — the explanation reported consistently traces back to LZMA2's much wider dictionary having found repetition across the original data that bzip2's smaller, fixed 900 KB blocks simply can't reach in the same way.

A second recurring pattern involves someone running this conversion on a system with limited free disk space and running out partway through, since the process temporarily needs room for the original 7Z, the intermediate decompressed plain 7z file, and the final 7Z.BZ2 all at once before the intermediate file can be safely deleted.

A third documented issue involves expecting bzip2 compression to run as fast as LZMA2 did on the same multi-core hardware, then finding it noticeably slower — standard bzip2 compresses single-threaded by default, while 7-Zip applies genuine multithreading to LZMA2, meaning switching to pbzip2 specifically is the documented way to close that speed gap if compression time matters.

A fourth pattern involves scripts written to check specifically for a .7z extension when verifying a backup or build step completed, breaking once the same pipeline s7zts producing .7z.bz2 output instead — a maintenance issue tied to the script's narrow extension check rather than to any technical problem with either compression format involved in the switch.

7Z-TO-TAR.BZ2 — LZMA2 and Bzip2 Compared on the Same Underlying 7Z Stream

7Z-TO-TAR.BZ2 — Questions About Converting 7Z Archives Into 7Z.BZ2

Will converting 7Z to 7Z.BZ2 make the file smaller or larger?
Usually larger, since LZMA2's much wider dictionary generally compresses better than bzip2's fixed 900 KB blocks on the same 7z stream, especially for text-heavy content with repetition spread across a large span.

Do I need 7-Zip and 7z both installed to do this conversion?
Yes. 7-Zip or p7zip is needed to decompress the LZMA2 layer, and GNU 7z or BSD 7z's bzip2 support (the -j flag) is needed to recompress the resulting plain 7z stream with bzip2, since neither tool alone handles both compression formats.

Does this conversion change any Unix permissions or ownership data?
No. The 7z layer itself is never rebuilt during this conversion, only decompressed and recompressed around, so every permission bit, ownership value, and symlink 7zget stays exactly as it was throughout both compression passes.

Why would I switch from LZMA2 to bzip2 compression at all?
Mainly for broader default tool availability — bzip2 support is built directly into GNU 7z and BSD 7z on virtually every Linux distribution and macOS, while LZMA2 typically requires installing 7-Zip or p7zip separately on the same systems.

Can I speed up the bzip2 compression step in this conversion?
Yes, using pbzip2, a parallelized bzip2 implementation that splits the 7z stream into independent chunks and compresses them across multiple CPU cores, closing much of the speed gap against LZMA2's own default multithreading in 7-Zip.

Is there a memory-usage difference between the two compression methods?
Yes. Bzip2 uses a fixed amount of memory regardless of archive size, roughly 7,600 KB for compression at default settings, while LZMA2's memory use scales with the chosen dictionary size and can run considerably higher on very large 7z streams.