Hi, Up to now I thought that the Compression Level (0, 1, ... 9) directly corresponds to the "gzip" level (no_compression, -1, ... -9). Suddenly I have found that in : 1. MainEvent.cxx : if comp = 0 no compression at all if comp = 1 event is compressed if comp = 2 same as 1. In addition branches with floats in the TClonesArray are also compressed (nothing mentioned about 3, 4, ... 9) 2. Ebench.html : comp = 0 means: no compression at all comp = 1 means: compress everything if split = 0 comp = 1 means: compress only the Tree branches with integers if split = 1 comp = 2 means: compress everything if split=1 (nothing mentioned about 3, 4, ... 9) What is the trick here ? In case this is really true that the compression level does not directly correspond to the "gzip" compression level could you please describe the "rules" exactly (in which case something is compressed and with which "level", what is not compressed) in the "Users Guide" -> "9 Input/Output" -> "The Pysical Layout of ROOT Files" -> "Compression" (page 159). This also reminds me that on page 159 in chapter "Compression" there are some numbers given for the file sizes of compressed files as a function of the compression level. Could I ask you to add here some numbers of the time needed to "write" and time needed to "read" this file as a function of the compression level ? While looking at the compression question I noticed one more bug. On page 269 in chapter "Event - An Example of a ROOT Application." I can read "You can see that the compressed file reads much slower (4.02 seconds vs. 12.5 seconds)." This was really a magic for me some days ago when I read this text for the first time. Up to now I thought that decompressing does not really introduce any big factor (compressing itself does, depending on the level). Now, after a careful second look, I can see where this "much slower" comes from. In the middle example "box" I can see "Real Time=4.02 ... You READ 5.22 MBytes/Realtime seconds ...", while in the last example "box" I can see "RealTime=12.5 ... You WRITE 1.66 ...". You are comparing 4.02 READ time of uncompressed file with 12.5 WRITE time of COMPRESSED file !!! The last "output" was not copied from the "Event 0 0 20", but, most probably, from the "Event 400 1 1 1". I think that in this place one could add both comparisons : write time uncompressed vs. compressed, and read time uncompressed vs. compressed. Best regards, Jacek.
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