On Thu, Mar 26, 2009 at 3:09 PM, Ilmari Karonen nospam@vyznev.net wrote:
ERSEK Laszlo wrote:
** 4. Thanassis Tsiodras' offline reader, available under
http://users.softlab.ece.ntua.gr/~ttsiod/buildWikipediaOffline.htmlhttp://users.softlab.ece.ntua.gr/%7Ettsiod/buildWikipediaOffline.html
uses, according to section "Seeking in the dump file", bzip2recover to split the bzip2 blocks out of the single bzip2 stream. The page states
This process is fast (since it involves almost no CPU calculationsWhile this may be true relative to other dump-processing operations, bzip2recover is, in fact, not much more than a huge single threaded bit-shifter, which even makes two passes over the dump. (IIRC, the first pass shifts over the whole dump to find bzip2 block delimiteres, then the second pass shifts the blocks found previously into byte-aligned,
separate
bzip2 streams.)
Hmm? Admittedly, I don't know the bzip2 format very well, but as far as I understand it, there should be no bit-shifting involved: each block in the stream is a completely independent, self-contained sequence of bytes.
I'm not sure about the bit-shifting, but the second pass of bzip2recover adds in the file headers.
The thing is, the second pass of bzip2recover is unnecessary if all you want to do is build a file index. And the source code of the first pass of bzip2recover is really simple. I've managed to hack it to make a single pass outputting the starting bytes of each block, and have been able to use the index to dd out the block I want to access (plus a few bytes padding on each end because I was too lazy to find out the exact location), run the real bzip2recover on just that block, and then uncompress the recovered block. That could be made into a lot less of a hack if I wanted to take the time to figure out how exactly to rebuild the header, but I haven't bothered.
Anyway, the point being that it's not necessary to actually run bzip2recover and deal with millions of little files, if you build an index. That way you don't need twice the space, and don't need to mess with reiserfs.