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authorJose Fonseca <jfonseca@vmware.com>2016-10-05 23:06:40 +0100
committerJose Fonseca <jfonseca@vmware.com>2016-10-05 23:06:40 +0100
commit6c3d26705c4c3c671cce271f3645a302f01eaad8 (patch)
treedeb83bc466d651fde04213fea3ea6aed77f19bb4 /docs
parentb5ce01582154680b4d5064751afbf2dc5a47f42b (diff)
docs: Document the Snappy format too.
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diff --git a/docs/FORMAT.markdown b/docs/FORMAT.markdown
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This document specifies the binary format of trace streams.
-Trace streams are not written verbatim to file, but compressed, nowadays with
-snappy (see `lib/trace/trace_file_snappy.cpp` for details). Previously they used
-to be compressed with gzip.
+## Compression ##
+
+Trace streams are not written verbatim to file, but compressed.
+
+By default traces are compressed with [Snappy](https://github.com/google/snappy)
+(see below for details). Previously they used to be compressed with gzip. And
+recently it also possible to have them compressed with
+[Brotli](https://github.com/google/brotli), though this is mostly intended for
+space savings on large databases of trace files.
+
+`apitrace repack` utility can be used to recompress the stream without any loss.
+
+### Snappy ###
+
+The used Snappy format is different from the standard _Snappy framing format_,
+because it predates it.
+
+ file = header chunk*
+
+ header = 'a' 't'
+
+ chunk = compressed_length compressed_data
+
+ compressed_length = uint32 // length of compressed data in little endian
+ compressed_data = byte*
## Versions ##