DiskImages/hdiutil: provide an imagekey to set LZMA compression preset level for ULMO-format disk images

Number:rdar://FB8739821 Date Originated:2020-09-24
Status:Open Resolved:
Product:macOS Product Version:10.15.7 19H2
Classification:Suggestion Reproducible:
macOS 10.15 introduced the new ULMO disk image format, which can be created with “hdiutil convert -format ULMO”. This format uses LZMA compression, which generally produces the smallest results of all of the compression algorithms supported by DiskImages.

Other compressed disk image formats allow the compression level to be tuned. UDZO (zlib compression) supports the “zlib-level” image key, which may be specified with “hdiutil convert -format UDZO -imagekey zlib-level=9”. Similarly, UDBZ (bzip2 compression, introduced in 10.4) supports the “bzip2-level” image key, introduced in 10.5. (see my radar report 8132167).

Please add an lama-level image key to customize the LZMA compression level used for ULMO disk image compression.


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