Document gzip VSL records

doc/sphinx/reference/index.rst

@@ -19,6 +19,7 @@ The Varnish Reference Manual
 	varnishtop.rst
 	shmem.rst
 	vmod.rst
+	vsl.rst
 
 .. todo::
         The programs:

doc/sphinx/reference/vsl.rst

+.. _reference-vsl:
+
+=====================
+Shared Memory Logging
+=====================
+
+Gzip records
+~~~~~~~~~~~~
+
+A Gzip record is emitted for each instance of gzip or gunzip work
+performed.
+Worst case, an ESI transaction stored in gzip'ed objects but delivered
+gunziped, will run into many of these.
+
+The format is::
+
+
+	%c %c %c %d %d %d %d %d
+	|  |  |  |  |  |  |  |
+	|  |  |  |  |  |  |  +- Bit length of compressed data
+	|  |  |  |  |  |  +---- Bit location of 'last' bit
+	|  |  |  |  |  +------- Bit location of first deflate block
+	|  |  |  |  +---------- Bytes output
+	|  |  |  +------------- Bytes input
+	|  |  +---------------- 'E' = ESI, '-' = Plain object
+	|  +------------------- 'F' = Fetch, 'D' = Deliver
+	+---------------------- 'G' = Gzip, 'U' = Gunzip, 'u' = Gunzip-test
+
+Which in practice could look like::
+
+	U F E 182 159 80 80 1392
+	G F E 159 173 80 1304 1314