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<title>Release 0.12 (November 20, 2008)</title>
<itemizedlist>
<listitem>
<para>Nix no longer uses Berkeley DB to store Nix store metadata.
The principal advantages of the new storage scheme are: it works
properly over decent implementations of NFS (allowing Nix stores
to be shared between multiple machines); no recovery is needed
when a Nix process crashes; no write access is needed for
read-only operations; no more running out of Berkeley DB locks on
certain operations.</para>
<para>You still need to compile Nix with Berkeley DB support if
you want Nix to automatically convert your old Nix store to the
new schema. If you don’t need this, you can build Nix with the
<filename>configure</filename> option
<option>--disable-old-db-compat</option>.</para>
<para>After the automatic conversion to the new schema, you can
delete the old Berkeley DB files:
<screen>
$ cd /nix/var/nix/db
$ rm __db* log.* derivers references referrers reserved validpaths DB_CONFIG</screen>
The new metadata is stored in the directories
<filename>/nix/var/nix/db/info</filename> and
<filename>/nix/var/nix/db/referrer</filename>. Though the
metadata is stored in human-readable plain-text files, they are
not intended to be human-editable, as Nix is rather strict about
the format.</para>
<para>The new storage schema may or may not require less disk
space than the Berkeley DB environment, mostly depending on the
cluster size of your file system. With 1 KiB clusters (which
seems to be the <literal>ext3</literal> default nowadays) it
usually takes up much less space.</para>
</listitem>
<listitem><para>There is a new substituter that copies paths
directly from other (remote) Nix stores mounted somewhere in the
filesystem. For instance, you can speed up an installation by
mounting some remote Nix store that already has the packages in
question via NFS or <literal>sshfs</literal>. The environment
variable <envar>NIX_OTHER_STORES</envar> specifies the locations of
the remote Nix directories,
e.g. <literal>/mnt/remote-fs/nix</literal>.</para></listitem>
<listitem><para>New <command>nix-store</command> operations
<option>--dump-db</option> and <option>--load-db</option> to dump
and reload the Nix database.</para></listitem>
<listitem><para>The garbage collector has a number of new options to
allow only some of the garbage to be deleted. The option
<option>--max-freed <replaceable>N</replaceable></option> tells the
collector to stop after at least <replaceable>N</replaceable> bytes
have been deleted. The option <option>--max-links
<replaceable>N</replaceable></option> tells it to stop after the
link count on <filename>/nix/store</filename> has dropped below
<replaceable>N</replaceable>. This is useful for very large Nix
stores on filesystems with a 32000 subdirectories limit (like
<literal>ext3</literal>). The option <option>--use-atime</option>
causes store paths to be deleted in order of ascending last access
time. This allows non-recently used stuff to be deleted. The
option <option>--max-atime <replaceable>time</replaceable></option>
specifies an upper limit to the last accessed time of paths that may
be deleted. For instance,
<screen>
$ nix-store --gc -v --max-atime $(date +%s -d "2 months ago")</screen>
deletes everything that hasn’t been accessed in two months.</para></listitem>
<listitem><para><command>nix-env</command> now uses optimistic
profile locking when performing an operation like installing or
upgrading, instead of setting an exclusive lock on the profile.
This allows multiple <command>nix-env -i / -u / -e</command>
operations on the same profile in parallel. If a
<command>nix-env</command> operation sees at the end that the profile
was changed in the meantime by another process, it will just
restart. This is generally cheap because the build results are
still in the Nix store.</para></listitem>
<listitem><para>The option <option>--dry-run</option> is now
supported by <command>nix-store -r</command> and
<command>nix-build</command>.</para></listitem>
<listitem><para>The information previously shown by
<option>--dry-run</option> (i.e., which derivations will be built
and which paths will be substituted) is now always shown by
<command>nix-env</command>, <command>nix-store -r</command> and
<command>nix-build</command>. The total download size of
substitutable paths is now also shown. For instance, a build will
show something like
<screen>
the following derivations will be built:
/nix/store/129sbxnk5n466zg6r1qmq1xjv9zymyy7-activate-configuration.sh.drv
/nix/store/7mzy971rdm8l566ch8hgxaf89x7lr7ik-upstart-jobs.drv
...
the following paths will be downloaded/copied (30.02 MiB):
/nix/store/4m8pvgy2dcjgppf5b4cj5l6wyshjhalj-samba-3.2.4
/nix/store/7h1kwcj29ip8vk26rhmx6bfjraxp0g4l-libunwind-0.98.6
...</screen>
</para></listitem>
<listitem><para>Language features:
<itemizedlist>
<listitem><para>@-patterns as in Haskell. For instance, in a
function definition
<programlisting>f = args @ {x, y, z}: <replaceable>...</replaceable>;</programlisting>
<varname>args</varname> refers to the argument as a whole, which
is further pattern-matched against the attribute set pattern
<literal>{x, y, z}</literal>.</para></listitem>
<listitem><para>“<literal>...</literal>” (ellipsis) patterns.
An attribute set pattern can now say <literal>...</literal> at
the end of the attribute name list to specify that the function
takes <emphasis>at least</emphasis> the listed attributes, while
ignoring additional attributes. For instance,
<programlisting>{stdenv, fetchurl, fuse, ...}: <replaceable>...</replaceable></programlisting>
defines a function that accepts any attribute set that includes
at least the three listed attributes.</para></listitem>
<listitem><para>New primops:
<varname>builtins.parseDrvName</varname> (split a package name
string like <literal>"nix-0.12pre12876"</literal> into its name
and version components, e.g. <literal>"nix"</literal> and
<literal>"0.12pre12876"</literal>),
<varname>builtins.compareVersions</varname> (compare two version
strings using the same algorithm that <command>nix-env</command>
uses), <varname>builtins.length</varname> (efficiently compute
the length of a list), <varname>builtins.mul</varname> (integer
multiplication), <varname>builtins.div</varname> (integer
division).
<!-- <varname>builtins.genericClosure</varname> -->
</para></listitem>
</itemizedlist>
</para></listitem>
<listitem><para><command>nix-prefetch-url</command> now supports
<literal>mirror://</literal> URLs, provided that the environment
variable <envar>NIXPKGS_ALL</envar> points at a Nixpkgs
tree.</para></listitem>
<listitem><para>Removed the commands
<command>nix-pack-closure</command> and
<command>nix-unpack-closure</command>. You can do almost the same
thing but much more efficiently by doing <literal>nix-store --export
$(nix-store -qR <replaceable>paths</replaceable>) > closure</literal> and
<literal>nix-store --import <
closure</literal>.</para></listitem>
<listitem><para>Lots of bug fixes, including a big performance bug in
the handling of <literal>with</literal>-expressions.</para></listitem>
</itemizedlist>
</chapter>
