Re: [gentoo-dev] [News item review] Portage rsync tree verification (v4)

[Andrew Barchuk]

Hi everyone,

> three possible solutions for splitting distfiles were listed:
>
> a. using initial portion of filename,
>
> b. using initial portion of file hash,
>
> c. using initial portion of filename hash.
>
> The significant advantage of the filename option was simplicity.  With
> that solution, the users could easily determine the correct subdirectory
> themselves.  However, it's significant disadvantage was very uneven
> shuffling of data.  In particular, the TeΧ Live packages alone count
> almost 23500 distfiles and all use a common prefix, making it impossible
> to split them further.
>
> The alternate option of using file hash has the advantage of having
> a more balanced split.

There's another option to use character ranges for each directory
computed in a way to have the files distributed evenly. One way to do
that is to use filename prefix of dynamic length so that each range
holds the same number of files. E.g. we would have Ab/, Ap/, Ar/ but
texlive-module-te/, texlive-module-th/, texlive-module-ti/. A similar
but simpler option is to use file names as range bounds (the same way
dictionaries use words to demarcate page bounds): each directory will
have a name of the first file located inside. This way files will be
distributed evenly and it's still easy to pick a correct directory where
a file will be located manually.

I have implemented a sketch of distfiles splitting that's using file
names as bounds in Python to demonstrate the idea (excuse possibly
non-idiomatic code, I'm not very versed in Python):

$ cat distfile-dirs.py
#!/usr/bin/env python3

import sys

"""
Builds list of dictionary directories to split the list of input files
into evenly. Each directory has name of the first file that is located
in the directory. Takes number of directories as an argument and reads
list of files from stdin. The resulting list or directories is printed
to stdout.
"""

dir_num = int(sys.argv[1])
distfiles = sys.stdin.read().splitlines()
distfile_num = len(distfiles)
dir_size = distfile_num / dir_num
# allows adding files in the beginning without repartitioning
dirs = ["0"]
next_dir = dir_size
while next_dir < distfile_num:
    dirs.append(distfiles[round(next_dir)])
    next_dir += dir_size
print("/\n".join(dirs) + "/")

$ cat pick-distfiles-dir.py
#!/usr/bin/env python3

"""
Picks the directory for a given file name. Takes a distfile name as an
argument. Reads sorted list of directories from stdin, name of each
directory is assumed to be the name of first file that's located inside.
"""

import sys

distfile = sys.argv[1]
dirs = sys.stdin.read().splitlines()
left = 0
right = len(dirs) - 1
while left < right:
    pivot = round((left + right) / 2)
    if (dirs[pivot] <= distfile):
        left = pivot + 1
    else:
        right = pivot - 1

if distfile < dirs[right]:
    print(dirs[right-1])
else:
    print(dirs[right])

$ # distfiles.txt contains all the distfile names
$ head -n5 distfiles.txt
0CD9CDDE3F56BB5250D87C54592F04CBC24F03BF-wagon-provider-api-2.10.jar
0CE1EDB914C94EBC388F086C6827E8BDEEC71AC2-commons-lang-2.6.jar
0DCC973606CBD9737541AA5F3E76DED6E3F4D0D0-iri.jar
0ad-0.0.22-alpha-unix-build.tar.xz
0ad-0.0.22-alpha-unix-data.tar.xz

$ # calculate 500 directories to split distfiles into evenly
$ cat distfiles.txt | ./distfile-dirs.py 500 > dirs.txt
$ tail -n5 dirs.txt
xrmap-2.29.tar.bz2/
xview-3.2p1.4-18c.tar.gz/
yasat-700.tar.gz/
yubikey-manager-qt-0.4.0.tar.gz/
zimg-2.5.1.tar.gz

$ # pick a directory for xvinfo-1.0.1.tar.bz2
$ cat dirs.txt | ./pick-distfiles-dir.py xvinfo-1.0.1.tar.bz2
xview-3.2p1.4-18c.tar.gz/

Using the approach above the files will distributed evenly among the
directories keeping the possibility to determine the directory for a
specific file by hand. It's possible if necessary to keep the directory
structure unchanged for very long time and it will likely stay
well-balanced. Picking a directory for a file is very cheap. The only
obvious downside I see is that it's necessary to know list of
directories to pick the correct one (can be mitigated by caching the
list of directories if important). If it's desirable to make directory
names shorter or to look less like file names it's fairly easy to
achieve by keeping only unique prefixes of directories. For example:

xrmap-2.29.tar.bz2/
xview-3.2p1.4-18c.tar.gz/
yasat-700.tar.gz/
yubikey-manager-qt-0.4.0.tar.gz/
zimg-2.5.1.tar.gz/

will become

xr/
xv/
ya/
yu/
z/

Thanks for taking time to consider the suggestion.

---
Andrew