* New command `nix-store --optimise' to reduce Nix store disk space
usage by finding identical files in the store and hard-linking them to each other. It typically reduces the size of the store by something like 25-35%. This is what the optimise-store.pl script did, but the new command is faster and more correct (it's safe wrt garbage collection and concurrent builds).
This commit is contained in:
Eelco Dolstra
parent
27a0662828
commit
a8629de827
5 changed files with 188 additions and 99 deletions
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@ -1,91 +0,0 @@
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#! /usr/bin/perl -w
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use strict;
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use File::Basename;
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my @paths = ("/nix/store");
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print "hashing...\n";
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my $hashList = "/tmp/nix-optimise-hash-list";
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system("find @paths -type f -print0 | xargs -0 md5sum -- > $hashList") == 0
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or die "cannot hash store files";
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print "sorting by hash...\n";
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system("sort $hashList > $hashList.sorted") == 0
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or die "cannot sort list";
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sub atomicLink {
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my $target = shift;
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my $new = shift;
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my $tmpNew = "${new}_optimise.$$";
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# Make the directory writable temporarily.
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my $dir = dirname $new;
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my @st = stat $dir or die;
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chmod ($st[2] | 0200, $dir) or die "cannot make `$dir' writable: $!";
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link $target, $tmpNew or die "cannot create hard link `$tmpNew': $!";
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rename $tmpNew, $new or die "cannot rename `$tmpNew' to `$new': $!";
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chmod ($st[2], $dir) or die "cannot restore permission on `$dir': $!";
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utime ($st[8], $st[9], $dir) or die "cannot restore timestamp on `$dir': $!";
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}
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print "hard-linking...\n";
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open LIST, "<$hashList.sorted" or die;
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my $prevFile;
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my $prevHash;
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my $prevInode;
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my $prevExec;
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my $totalSpace = 0;
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my $savedSpace = 0;
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while (<LIST>) {
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/^([0-9a-f]*)\s+(.*)$/ or die;
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my $curFile = $2;
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my $curHash = $1;
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my @st = stat $curFile or die;
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next if ($st[2] & 0222) != 0; # skip writable files
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my $fileSize = $st[7];
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$totalSpace += $fileSize;
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my $isExec = ($st[2] & 0111) == 0111;
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if (defined $prevHash && $curHash eq $prevHash
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&& $prevExec == $isExec)
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{
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if ($st[1] != $prevInode) {
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print "$curFile = $prevFile\n";
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atomicLink $prevFile, $curFile;
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$savedSpace += $fileSize;
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}
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} else {
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$prevFile = $curFile;
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$prevHash = $curHash;
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$prevInode = $st[1];
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$prevExec = ($st[2] & 0111) == 0111;
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}
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}
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print "total space = $totalSpace\n";
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print "saved space = $savedSpace\n";
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my $savings = ($savedSpace / $totalSpace) * 100.0;
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print "savings = $savings %\n";
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close LIST;
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@ -174,7 +174,7 @@ void copyPath(const Path & src, const Path & dst, PathFilter & filter)
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}
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static void _canonicalisePathMetaData(const Path & path)
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static void _canonicalisePathMetaData(const Path & path, bool recurse)
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{
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checkInterrupt();
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@ -223,17 +223,17 @@ static void _canonicalisePathMetaData(const Path & path)
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}
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if (S_ISDIR(st.st_mode)) {
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if (recurse && S_ISDIR(st.st_mode)) {
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Strings names = readDirectory(path);
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for (Strings::iterator i = names.begin(); i != names.end(); ++i)
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_canonicalisePathMetaData(path + "/" + *i);
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_canonicalisePathMetaData(path + "/" + *i, true);
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}
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}
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void canonicalisePathMetaData(const Path & path)
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{
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_canonicalisePathMetaData(path);
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_canonicalisePathMetaData(path, true);
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/* On platforms that don't have lchown(), the top-level path can't
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be a symlink, since we can't change its ownership. */
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@ -625,7 +625,7 @@ void LocalStore::exportPath(const Path & path, bool sign,
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consistent metadata. */
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Transaction txn(nixDB);
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addTempRoot(path);
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if (!isValidPath(path))
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if (!isValidPathTxn(txn, path))
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throw Error(format("path `%1%' is not valid") % path);
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HashAndWriteSink hashAndWriteSink(sink);
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@ -950,6 +950,121 @@ void verifyStore(bool checkContents)
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}
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typedef std::map<Hash, std::pair<Path, ino_t> > HashToPath;
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static void toggleWritable(const Path & path, bool writable)
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{
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struct stat st;
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if (lstat(path.c_str(), &st))
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throw SysError(format("getting attributes of path `%1%'") % path);
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mode_t mode = st.st_mode;
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if (writable) mode |= S_IWUSR;
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else mode &= ~(S_IWUSR | S_IWGRP | S_IWOTH);
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if (chmod(path.c_str(), mode) == -1)
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throw SysError(format("changing writability of `%1%'") % path);
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}
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static void hashAndLink(bool dryRun, HashToPath & hashToPath,
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OptimiseStats & stats, const Path & path)
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{
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struct stat st;
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if (lstat(path.c_str(), &st))
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throw SysError(format("getting attributes of path `%1%'") % path);
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/* Sometimes SNAFUs can cause files in the Nix store to be
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modified, in particular when running programs as root under
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NixOS (example: $fontconfig/var/cache being modified). Skip
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those files. */
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if (S_ISREG(st.st_mode) && (st.st_mode & S_IWUSR)) {
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printMsg(lvlError, format("skipping suspicious writable file `%1%'") % path);
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return;
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}
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/* We can hard link regular files and symlinks. */
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if (S_ISREG(st.st_mode) || S_ISLNK(st.st_mode)) {
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/* Hash the file. Note that hashPath() returns the hash over
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the NAR serialisation, which includes the execute bit on
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the file. Thus, executable and non-executable files with
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the same contents *won't* be linked (which is good because
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otherwise the permissions would be screwed up).
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Also note that if `path' is a symlink, then we're hashing
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the contents of the symlink (i.e. the result of
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readlink()), not the contents of the target (which may not
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even exist). */
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Hash hash = hashPath(htSHA256, path);
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stats.totalFiles++;
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printMsg(lvlDebug, format("`%1%' has hash `%2%'") % path % printHash(hash));
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std::pair<Path, ino_t> prevPath = hashToPath[hash];
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if (prevPath.first == "") {
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hashToPath[hash] = std::pair<Path, ino_t>(path, st.st_ino);
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return;
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}
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/* Yes! We've seen a file with the same contents. Replace
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the current file with a hard link to that file. */
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stats.sameContents++;
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if (prevPath.second == st.st_ino) {
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printMsg(lvlDebug, format("`%1%' is already linked to `%2%'") % path % prevPath.first);
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return;
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}
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printMsg(lvlTalkative, format("linking `%1%' to `%2%'") % path % prevPath.first);
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Path tempLink = (format("%1%.tmp-%2%-%3%")
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% path % getpid() % rand()).str();
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toggleWritable(dirOf(path), true);
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if (link(prevPath.first.c_str(), tempLink.c_str()) == -1)
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throw SysError(format("cannot link `%1%' to `%2%'")
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% tempLink % prevPath.first);
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/* Atomically replace the old file with the new hard link. */
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if (rename(tempLink.c_str(), path.c_str()) == -1)
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throw SysError(format("cannot rename `%1%' to `%2%'")
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% tempLink % path);
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/* Make the directory read-only again and reset its timestamp
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back to 0. */
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_canonicalisePathMetaData(dirOf(path), false);
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stats.filesLinked++;
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stats.bytesFreed += st.st_size;
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}
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if (S_ISDIR(st.st_mode)) {
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Strings names = readDirectory(path);
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for (Strings::iterator i = names.begin(); i != names.end(); ++i)
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hashAndLink(dryRun, hashToPath, stats, path + "/" + *i);
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}
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}
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void LocalStore::optimiseStore(bool dryRun, OptimiseStats & stats)
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{
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HashToPath hashToPath;
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Paths paths;
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PathSet validPaths;
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nixDB.enumTable(noTxn, dbValidPaths, paths);
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for (Paths::iterator i = paths.begin(); i != paths.end(); ++i) {
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addTempRoot(*i);
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if (!isValidPath(*i)) continue; /* path was GC'ed, probably */
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startNest(nest, lvlChatty, format("hashing files in `%1%'") % *i);
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hashAndLink(dryRun, hashToPath, stats, *i);
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}
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}
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/* Upgrade from schema 1 (Nix <= 0.7) to schema 2 (Nix >= 0.8). */
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static void upgradeStore07()
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{
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@ -21,6 +21,20 @@ const int nixSchemaVersion = 4;
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extern string drvsLogDir;
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struct OptimiseStats
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{
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unsigned long totalFiles;
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unsigned long sameContents;
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unsigned long filesLinked;
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unsigned long long bytesFreed;
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OptimiseStats()
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{
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totalFiles = sameContents = filesLinked = 0;
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bytesFreed = 0;
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}
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};
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class LocalStore : public StoreAPI
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{
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private:
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@ -83,6 +97,10 @@ public:
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void collectGarbage(GCAction action, const PathSet & pathsToDelete,
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bool ignoreLiveness, PathSet & result, unsigned long long & bytesFreed);
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/* Optimise the disk space usage of the Nix store by hard-linking
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files with the same contents. */
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void optimiseStore(bool dryRun, OptimiseStats & stats);
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};
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@ -21,6 +21,7 @@ Operations:
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--init: initialise the Nix database
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--verify: verify Nix structures
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--optimise: optimise the Nix store by hard-linking identical files
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--version: output version information
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--help: display help
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@ -466,6 +466,13 @@ static void opCheckValidity(Strings opFlags, Strings opArgs)
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}
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static string showBytes(unsigned long long bytes)
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{
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return (format("%d bytes (%.2f MiB)")
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% bytes % (bytes / (1024.0 * 1024.0))).str();
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}
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struct PrintFreed
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{
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bool show, dryRun;
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@ -477,9 +484,9 @@ struct PrintFreed
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if (show)
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cout << format(
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(dryRun
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? "%d bytes would be freed (%.2f MiB)\n"
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: "%d bytes freed (%.2f MiB)\n"))
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% bytesFreed % (bytesFreed / (1024.0 * 1024.0));
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? "%1% would be freed\n"
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: "%1% freed (%.2f MiB)\n"))
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% showBytes(bytesFreed);
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}
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};
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@ -614,6 +621,43 @@ static void opVerify(Strings opFlags, Strings opArgs)
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}
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static void showOptimiseStats(OptimiseStats & stats)
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{
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printMsg(lvlError,
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format("%1% freed by hard-linking %2% files; there are %3% files with equal contents out of %4% files in total")
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% showBytes(stats.bytesFreed)
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% stats.filesLinked
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% stats.sameContents
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% stats.totalFiles);
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}
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/* Optimise the disk space usage of the Nix store by hard-linking
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files with the same contents. */
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static void opOptimise(Strings opFlags, Strings opArgs)
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{
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if (!opArgs.empty())
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throw UsageError("no arguments expected");
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for (Strings::iterator i = opFlags.begin();
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i != opFlags.end(); ++i)
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throw UsageError(format("unknown flag `%1%'") % *i);
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LocalStore * store2(dynamic_cast<LocalStore *>(store.get()));
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if (!store2) throw Error("you don't have sufficient rights to use --optimise");
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OptimiseStats stats;
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try {
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store2->optimiseStore(true, stats);
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} catch (...) {
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showOptimiseStats(stats);
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throw;
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}
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showOptimiseStats(stats);
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}
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/* Scan the arguments; find the operation, set global flags, put all
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other flags in a list, and put all other arguments in another
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list. */
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@ -659,6 +703,8 @@ void run(Strings args)
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op = opInit;
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else if (arg == "--verify")
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op = opVerify;
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else if (arg == "--optimise")
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op = opOptimise;
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else if (arg == "--add-root") {
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if (i == args.end())
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throw UsageError("`--add-root requires an argument");
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