Juan Céspedes - scripts 

Let's say you have a venti server and you would like to mirror its contents to another one. How to do it?

First, you will need to have another venti configured. Please look at the venti(8) manual page to know how to do it, or see my post on how to set up a venti server. Please note that, if you want to run both servers at the same time, and if they are in the same machine, they must listen to different port numbers.

There are several ways to copy the contents of a venti server into another:

• venti-copy

  With venti-copy you can copy an entire tree of blocks from one venti server to another. Both servers must be running in order to copy the blocks.

  If you have the list of scores all the trees in the file scores.txt, you could use the following command to start copying:

  cat scores.txt | while read score ; do venti-copy $venti1 $venti2 $a ; done

  Advantages: you don't need to stop either server; arena partitions in each server can have different geometry

  Disadvantages: it is very slow, and you need to know a priori the list of VtRoot scores you want to copy.

• venti-mirrorarenas

  This command can be used to copy every arena from one arena partition to another. Both servers should be stopped.

  Advantages: it is very fast, and there are several tests to check that the arena geometries are the same.

  Disadvantages: it only works if source and destination have exactly the same number of arenas of the same size; after finishing the copy, you must use venti-buildarenas in the destination server.

• venti-rdarena and venti-wrarena

  venti-rdarena reads one arena from a venti server and writes it to standard output; venti-wrarena writes an arena from a file to a venti server.

  In order to mirror one venti server, you could read all its arenas with venti-rdarena, one by one, and then write them to another server with venti-wrarena. Both servers should be stopped.

  Advantages: it works even if source and destination have different number of arenas; it is faster than venti-copy.

  Disadvantages: it only works if source and destination have arenas of the same size; it is very slow if your venti servers have a lot of arenas (each invocation of these executables read all the arena map); after finishing the copy, you must use venti-buildarenas in the destination server.

• Copy all the arenas by hand

  If you figure out the position in the arena partition of each arena, and its size, you can yust copy the arenas with dd. Both servers should be stopped.

  For example, if you formatted the arena partitions with the default arenasize and blocksize, then the arenas will have 512MB (536870912 bytes), and will be located in the position 794624 + 536870912*n. So, you could use the following loop to copy arenas $start to $end, from file $orig to $dest:

  for i in $( seq $start $end )
  do
    echo "Copying arena $i from $start to $end..."
    skip=$(( 97 + $i * 65536 ))
    dd if="$orig" bs=8192 skip=$skip count=65536 seek=$skip of="$dest"
  done

  Advantages: it is very fast.

  Disadvantages: it only works if source and destination have arenas of the same size; you have to make sure that the arenas are located exactly where you are reading and writting them; after finishing the copy, you must use venti-buildarenas in the destination server.

Sometimes we need to reduce the size of a PDF file, without altering its content and without having access to the source code.

In order to do that, we can use gs this way:

     gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen \
       -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

-dPDFSETTINGS=/screen option is used to specify that we want the PDF optimized to be displayed on a screen, reducing its size while loosing images quality.

We can specify /ebook instead of /screen to have a slightly better image quality, but still reducing file size.

If you want to have a file crypted, so that noone can see its contents unless they have the correct password, you can use "gpg" to cypher or decypher it. However, its use is a bit complicated.

So, I decided to write a small shell script, called "gpg-vi", which asks for a password, and lets you edit a file, symmetrically crypted using GnuPG with that password.

The script will not let anyone else in that machine to see the contents of the file, but warning: the script writes the contents of the file in plain in a file in /tmp, so that your user id, or root, can see that file until the edition is finished (or even later, because the contents may still be there in the disk after deleting the file).