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NOTE: This hack is only relevant to Apache 2.0.44 or lower. Starting with Apache 2.0.45, the server contains the DeflateCompressionLevel directive, which allows for user-configured compression levels in the httpd.conf file.

One of the complaints leveled against mod_deflate for Apache 2.0.44 and below has been the lower compression ratio that it produces when compared to mod_gzip for Apache 1.3.x and 2.0.x. This issue has been traced to a decision made by the author of mod_deflate to focus on fast compression versus maximum compression.

In discussions with the author of mod_deflate and the maintainer of mod_gzip, the location of the issue was quickly found. The level of compression can be easily modified by changing the ZLIB compression setting in mod_deflate.c from Z_BEST_SPEED (equivalent to “zip -1″) to Z_BEST_COMPRESSION (equivalent to “zip -9″). These defaults can also be replaced with a numeric value between 1 and 9. A “hacked” version of the mod_deflate.c code is available here. In this file, the compression level has been set to 6, which is regarded as a good balance between speed and compression (and also happens to be ZLIB’s default ratio). Some other variations are highlighted below.

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Original Code

zRC = deflateInit2(&ctx->stream, Z_BEST_SPEED, Z_DEFLATED, c->windowSize, c->memlevel, Z_DEFAULT_STRATEGY);

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Hacked Code

1. zRC = deflateInit2(&ctx->stream, Z_BEST_COMPRESSION, Z_DEFLATED, c->windowSize, c->memlevel, Z_DEFAULT_STRATEGY);

2. zRC = deflateInit2(&ctx->stream, 6, Z_DEFLATED, c->windowSize, c->memlevel, Z_DEFAULT_STRATEGY);

3. zRC = deflateInit2(&ctx->stream, 9, Z_DEFLATED, c->windowSize, c->memlevel, Z_DEFAULT_STRATEGY);

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A change has been made to mod_deflate in Apache 2.0.45 that adds a directive named DeflateCompressionLevel to the mod_deflate options. This will accept a numeric value between 1 (Best Speed) and 9 (Best Compression), with the default set at 6.