link-by-hash.diff

   1 After applying this patch and running configure, you MUST run this
   2 command before "make":
   3
   4     make proto
   5
   6 Jason M. Felice writes:
   7
   8 This patch adds the --link-by-hash=DIR option, which hard links received
   9 files in a link farm arranged by MD4 file hash.  The result is that the system
  10 will only store one copy of the unique contents of each file, regardless of
  11 the file's name.
  12
  13
  14 --- orig/Makefile.in    2004-11-03 11:56:03
  15 +++ Makefile.in 2004-07-03 20:20:15
  16 @@ -35,7 +35,7 @@ OBJS1=rsync.o generator.o receiver.o cle
  17         main.o checksum.o match.o syscall.o log.o backup.o
  18  OBJS2=options.o flist.o io.o compat.o hlink.o token.o uidlist.o socket.o \
  19         fileio.o batch.o clientname.o
  20 -OBJS3=progress.o pipe.o
  21 +OBJS3=progress.o pipe.o hashlink.o
  22  DAEMON_OBJ = params.o loadparm.o clientserver.o access.o connection.o authenticate.o
  23  popt_OBJS=popt/findme.o  popt/popt.o  popt/poptconfig.o \
  24         popt/popthelp.o popt/poptparse.o
  25 --- orig/hashlink.c     2004-09-24 16:44:25
  26 +++ hashlink.c  2004-09-24 16:44:25
  27 @@ -0,0 +1,340 @@
  28 +/*
  29 +   Copyright (C) Cronosys, LLC 2004
  30 +
  31 +   This program is free software; you can redistribute it and/or modify
  32 +   it under the terms of the GNU General Public License as published by
  33 +   the Free Software Foundation; either version 2 of the License, or
  34 +   (at your option) any later version.
  35 +
  36 +   This program is distributed in the hope that it will be useful,
  37 +   but WITHOUT ANY WARRANTY; without even the implied warranty of
  38 +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  39 +   GNU General Public License for more details.
  40 +
  41 +   You should have received a copy of the GNU General Public License
  42 +   along with this program; if not, write to the Free Software
  43 +   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  44 +*/
  45 +
  46 +/* This file contains code used by the --link-by-hash option. */
  47 +
  48 +#include "rsync.h"
  49 +
  50 +extern char *link_by_hash_dir;
  51 +
  52 +#if HAVE_LINK
  53 +
  54 +char* make_hash_name(struct file_struct *file)
  55 +{
  56 +       char hash[33], *dst;
  57 +       unsigned char *src;
  58 +       unsigned char c;
  59 +       int i;
  60 +
  61 +       src = (unsigned char*)file->u.sum;
  62 +       for (dst = hash, i = 0; i < 4; i++, src++) {
  63 +               c = *src >> 4;
  64 +               *(dst++) = (c >= 10) ? (c - 10 + 'a') : (c + '0');
  65 +               c = *src & 0x0f;
  66 +               *(dst++) = (c >= 10) ? (c - 10 + 'a') : (c + '0');
  67 +       }
  68 +       *dst++ = '/';
  69 +       for (i = 0; i < 12; i++, src++) {
  70 +               c = *src >> 4;
  71 +               *(dst++) = (c >= 10) ? (c - 10 + 'a') : (c + '0');
  72 +               c = *src & 0x0f;
  73 +               *(dst++) = (c >= 10) ? (c - 10 + 'a') : (c + '0');
  74 +       }
  75 +       *dst = 0;
  76 +
  77 +       asprintf(&dst,"%s/%s",link_by_hash_dir,hash);
  78 +       return dst;
  79 +}
  80 +
  81 +
  82 +void kill_hashfile(struct hashfile_struct *hashfile)
  83 +{
  84 +       if (!hashfile)
  85 +               return;
  86 +       free(hashfile->name);
  87 +       close(hashfile->fd);
  88 +       free(hashfile);
  89 +}
  90 +
  91 +
  92 +void kill_hashfiles(struct hashfile_struct *hashfiles)
  93 +{
  94 +       struct hashfile_struct *iter, *next;
  95 +       if ((iter = hashfiles) != NULL) {
  96 +               do {
  97 +                       next = iter->next;
  98 +                       kill_hashfile(iter);
  99 +                       iter = next;
 100 +               } while (iter != hashfiles);
 101 +       }
 102 +}
 103 +
 104 +
 105 +struct hashfile_struct *find_hashfiles(char *hashname, int64 size, long *fnbr)
 106 +{
 107 +       DIR *d;
 108 +       struct dirent *di;
 109 +       struct hashfile_struct *hashfiles = NULL, *hashfile;
 110 +       STRUCT_STAT st;
 111 +       long this_fnbr;
 112 +
 113 +       *fnbr = 0;
 114 +
 115 +       /* Build a list of potential candidates and open
 116 +        * them. */
 117 +       if ((d = opendir(hashname)) == NULL) {
 118 +               rsyserr(FERROR, errno, "opendir failed: \"%s\"", hashname);
 119 +               free(hashname);
 120 +               return NULL;
 121 +       }
 122 +       while ((di = readdir(d)) != NULL) {
 123 +               if (!strcmp(di->d_name,".") || !strcmp(di->d_name,"..")) {
 124 +                       continue;
 125 +               }
 126 +
 127 +               /* We need to have the largest fnbr in case we need to store
 128 +                * a new file. */
 129 +               this_fnbr = atol(di->d_name);
 130 +               if (this_fnbr > *fnbr)
 131 +                       *fnbr = this_fnbr;
 132 +
 133 +               hashfile = (struct hashfile_struct*)malloc(sizeof(struct hashfile_struct));
 134 +               asprintf(&hashfile->name,"%s/%s",hashname,
 135 +                        di->d_name);
 136 +               if (do_stat(hashfile->name,&st) == -1) {
 137 +                       rsyserr(FERROR, errno, "stat failed: %s", hashfile->name);
 138 +                       kill_hashfile(hashfile);
 139 +                       continue;
 140 +               }
 141 +               if (st.st_size != size) {
 142 +                       kill_hashfile(hashfile);
 143 +                       continue;
 144 +               }
 145 +               hashfile->nlink = st.st_nlink;
 146 +               hashfile->fd = open(hashfile->name,O_RDONLY|O_BINARY);
 147 +               if (hashfile->fd == -1) {
 148 +                       rsyserr(FERROR, errno, "open failed: %s", hashfile->name);
 149 +                       kill_hashfile(hashfile);
 150 +                       continue;
 151 +               }
 152 +               if (hashfiles == NULL)
 153 +                       hashfiles = hashfile->next = hashfile->prev = hashfile;
 154 +               else {
 155 +                       hashfile->next = hashfiles;
 156 +                       hashfile->prev = hashfiles->prev;
 157 +                       hashfile->next->prev = hashfile;
 158 +                       hashfile->prev->next = hashfile;
 159 +               }
 160 +       }
 161 +       closedir(d);
 162 +
 163 +       return hashfiles;
 164 +}
 165 +
 166 +
 167 +struct hashfile_struct *compare_hashfiles(int fd,struct hashfile_struct *files)
 168 +{
 169 +       int amt, hamt;
 170 +       char buffer[BUFSIZ], cmpbuffer[BUFSIZ];
 171 +       struct hashfile_struct *iter, *next, *best;
 172 +       uint32 nlink;
 173 +
 174 +       if (!files)
 175 +               return NULL;
 176 +
 177 +       iter = files; /* in case files are 0 bytes */
 178 +       while ((amt = read(fd, buffer, BUFSIZ)) > 0) {
 179 +               iter = files;
 180 +               do {
 181 +                       /* Icky bit to resync when we steal the first node. */
 182 +                       if (!files)
 183 +                               files = iter;
 184 +
 185 +                       next = iter->next;
 186 +
 187 +                       hamt = read(iter->fd, cmpbuffer, BUFSIZ);
 188 +                       if (amt != hamt || memcmp(buffer, cmpbuffer, amt)) {
 189 +                               if (iter == files) {
 190 +                                       files = files->prev;
 191 +                               }
 192 +                               if (iter->next == iter) {
 193 +                                       files = next = NULL;
 194 +                               } else {
 195 +                                       next = iter->next;
 196 +                                       if (iter == files) {
 197 +                                               /* So we know to resync */
 198 +                                               files = NULL;
 199 +                                       }
 200 +                               }
 201 +                               iter->next->prev = iter->prev;
 202 +                               iter->prev->next = iter->next;
 203 +                               kill_hashfile(iter);
 204 +                       }
 205 +
 206 +                       iter = next;
 207 +               } while (iter != files);
 208 +
 209 +               if (iter == NULL && files == NULL) {
 210 +                       /* There are no matches. */
 211 +                       return NULL;
 212 +               }
 213 +       }
 214 +
 215 +       if (amt == -1) {
 216 +               rsyserr(FERROR, errno, "read failed in compare_hashfiles()");
 217 +               kill_hashfiles(files);
 218 +               return NULL;
 219 +       }
 220 +
 221 +       /* If we only have one file left, use it. */
 222 +       if (files == files->next) {
 223 +               return files;
 224 +       }
 225 +
 226 +       /* All files which remain in the list are identical and should have
 227 +        * the same size.  We pick the one with the lowest link count (we
 228 +        * may have rolled over because we hit the maximum link count for
 229 +        * the filesystem). */
 230 +       best = iter = files;
 231 +       nlink = iter->nlink;
 232 +       do {
 233 +               if (iter->nlink < nlink) {
 234 +                       nlink = iter->nlink;
 235 +                       best = iter;
 236 +               }
 237 +               iter = iter->next;
 238 +       } while (iter != files);
 239 +
 240 +       best->next->prev = best->prev;
 241 +       best->prev->next = best->next;
 242 +       if (files == best)
 243 +               files = files->next;
 244 +       kill_hashfiles(files);
 245 +       return best;
 246 +}
 247 +
 248 +
 249 +int link_by_hash(char *fnametmp,char *fname,struct file_struct *file)
 250 +{
 251 +       STRUCT_STAT st;
 252 +       char *hashname = make_hash_name(file);
 253 +       int first = 0, rc;
 254 +       char *linkname;
 255 +       long last_fnbr;
 256 +
 257 +       if (file->length == 0) {
 258 +               return robust_rename(fnametmp,fname,0644);
 259 +       }
 260 +
 261 +       if (do_stat(hashname, &st) == -1) {
 262 +               char *dirname;
 263 +
 264 +               /* Directory does not exist. */
 265 +               dirname = strdup(hashname);
 266 +               *strrchr(dirname,'/') = 0;
 267 +               if (do_mkdir(dirname, 0755) == -1 && errno != EEXIST) {
 268 +                       rsyserr(FERROR, errno, "mkdir failed: %s", dirname);
 269 +                       free(hashname);
 270 +                       free(dirname);
 271 +                       return robust_rename(fnametmp,fname,0644);
 272 +               }
 273 +               free(dirname);
 274 +
 275 +               if (do_mkdir(hashname, 0755) == -1 && errno != EEXIST) {
 276 +                       rsyserr(FERROR, errno, "mkdir failed: %s", hashname);
 277 +                       free(hashname);
 278 +                       return robust_rename(fnametmp,fname,0644);
 279 +               }
 280 +
 281 +               first = 1;
 282 +               asprintf(&linkname,"%s/0",hashname);
 283 +               rprintf(FINFO, "(1) linkname = %s\n", linkname);
 284 +       } else {
 285 +               struct hashfile_struct *hashfiles, *hashfile;
 286 +
 287 +               if (do_stat(fnametmp,&st) == -1) {
 288 +                       rsyserr(FERROR, errno, "stat failed: %s", fname);
 289 +                       return -1;
 290 +               }
 291 +               hashfiles = find_hashfiles(hashname, st.st_size, &last_fnbr);
 292 +
 293 +               if (hashfiles == NULL) {
 294 +                       first = 1;
 295 +                       asprintf(&linkname,"%s/0",hashname);
 296 +                       rprintf(FINFO, "(2) linkname = %s\n", linkname);
 297 +               } else {
 298 +                       int fd;
 299 +                       /* Search for one identical to us. */
 300 +                       if ((fd = open(fnametmp,O_RDONLY|O_BINARY)) == -1) {
 301 +                               rsyserr(FERROR, errno, "open failed: %s", fnametmp);
 302 +                               kill_hashfiles(hashfiles);
 303 +                               return -1;
 304 +                       }
 305 +                       hashfile = compare_hashfiles(fd, hashfiles);
 306 +                       hashfiles = NULL;
 307 +                       close(fd);
 308 +
 309 +                       if (hashfile) {
 310 +                               first = 0;
 311 +                               linkname = strdup(hashfile->name);
 312 +                               rprintf(FINFO, "(3) linkname = %s\n", linkname);
 313 +                               kill_hashfile(hashfile);
 314 +                       } else {
 315 +                               first = 1;
 316 +                               asprintf(&linkname, "%s/%ld", hashname,
 317 +                                        last_fnbr + 1);
 318 +                               rprintf(FINFO, "(4) linkname = %s\n", linkname);
 319 +                       }
 320 +               }
 321 +       }
 322 +
 323 +       if (!first) {
 324 +               rprintf(FINFO, "link-by-hash (existing): \"%s\" -> %s\n",
 325 +                               linkname, full_fname(fname));
 326 +               robust_unlink(fname);
 327 +               rc = do_link(linkname, fname);
 328 +               if (rc == -1) {
 329 +                       if (errno == EMLINK) {
 330 +                               first = 1;
 331 +                               free(linkname);
 332 +                               asprintf(&linkname,"%s/%ld",hashname,
 333 +                                        last_fnbr + 1);
 334 +                               rprintf(FINFO, "(5) linkname = %s\n", linkname);
 335 +                               rprintf(FINFO,"link-by-hash: max link count exceeded, starting new file \"%s\".\n", linkname);
 336 +                       } else {
 337 +                               rsyserr(FERROR, errno, "link \"%s\" -> \"%s\"",
 338 +                                       linkname, full_fname(fname));
 339 +                               rc = robust_rename(fnametmp,fname,0644);
 340 +                       }
 341 +               } else {
 342 +                       do_unlink(fnametmp);
 343 +               }
 344 +       }
 345 +
 346 +       if (first) {
 347 +               rprintf(FINFO, "link-by-hash (new): %s -> \"%s\"\n",
 348 +                               full_fname(fname),linkname);
 349 +
 350 +               rc = robust_rename(fnametmp,fname,0644);
 351 +               if (rc != 0) {
 352 +                       rsyserr(FERROR, errno, "rename \"%s\" -> \"%s\"",
 353 +                               full_fname(fnametmp), full_fname(fname));
 354 +               }
 355 +               rc = do_link(fname,linkname);
 356 +               if (rc != 0) {
 357 +                       rsyserr(FERROR, errno, "link \"%s\" -> \"%s\"",
 358 +                               full_fname(fname), linkname);
 359 +               }
 360 +       }
 361 +
 362 +       free(linkname);
 363 +       free(hashname);
 364 +       return rc;
 365 +}
 366 +
 367 +#endif
 368 --- orig/options.c      2005-02-20 01:12:42
 369 +++ options.c   2005-02-14 02:52:09
 370 @@ -134,6 +134,7 @@ char *log_format = NULL;
 371  char *password_file = NULL;
 372  char *rsync_path = RSYNC_PATH;
 373  char *backup_dir = NULL;
 374 +char *link_by_hash_dir = NULL;
 375  char backup_dir_buf[MAXPATHLEN];
 376  int rsync_port = 0;
 377  int compare_dest = 0;
 378 @@ -314,6 +315,7 @@ void usage(enum logcode F)
 379    rprintf(F,"     --compare-dest=DIR      also compare destination files relative to DIR\n");
 380    rprintf(F,"     --copy-dest=DIR         ... and include copies of unchanged files\n");
 381    rprintf(F,"     --link-dest=DIR         hardlink to files in DIR when unchanged\n");
 382 +  rprintf(F,"     --link-by-hash=DIR      create hardlinks by hash into DIR\n");
 383    rprintf(F," -z, --compress              compress file data during the transfer\n");
 384    rprintf(F," -C, --cvs-exclude           auto-ignore files the same way CVS does\n");
 385    rprintf(F," -f, --filter=RULE           add a file-filtering RULE\n");
 386 @@ -353,7 +355,7 @@ void usage(enum logcode F)
 387  enum {OPT_VERSION = 1000, OPT_DAEMON, OPT_SENDER, OPT_EXCLUDE, OPT_EXCLUDE_FROM,
 388        OPT_FILTER, OPT_COMPARE_DEST, OPT_COPY_DEST, OPT_LINK_DEST,
 389        OPT_INCLUDE, OPT_INCLUDE_FROM, OPT_MODIFY_WINDOW,
 390 -      OPT_READ_BATCH, OPT_WRITE_BATCH, OPT_TIMEOUT, OPT_MAX_SIZE,
 391 +      OPT_READ_BATCH, OPT_WRITE_BATCH, OPT_TIMEOUT, OPT_MAX_SIZE, OPT_LINK_BY_HASH,
 392        OPT_REFUSED_BASE = 9000};
 393
 394  static struct poptOption long_options[] = {
 395 @@ -422,6 +424,7 @@ static struct poptOption long_options[]
 396    {"compare-dest",     0,  POPT_ARG_STRING, 0, OPT_COMPARE_DEST, 0, 0 },
 397    {"copy-dest",        0,  POPT_ARG_STRING, 0, OPT_COPY_DEST, 0, 0 },
 398    {"link-dest",        0,  POPT_ARG_STRING, 0, OPT_LINK_DEST, 0, 0 },
 399 +  {"link-by-hash",     0,  POPT_ARG_STRING, 0, OPT_LINK_BY_HASH, 0, 0},
 400    {"fuzzy",           'y', POPT_ARG_NONE,   &fuzzy_basis, 0, 0, 0 },
 401    /* TODO: Should this take an optional int giving the compression level? */
 402    {"compress",        'z', POPT_ARG_NONE,   &do_compression, 0, 0, 0 },
 403 @@ -853,6 +856,21 @@ int parse_arguments(int *argc, const cha
 404                         basis_dir[basis_dir_cnt++] = (char *)arg;
 405                         break;
 406
 407 +                case OPT_LINK_BY_HASH:
 408 +#if HAVE_LINK
 409 +                       arg = poptGetOptArg(pc);
 410 +                       if (sanitize_paths)
 411 +                               arg = sanitize_path(NULL, arg, NULL, 0);
 412 +                       link_by_hash_dir = (char *)arg;
 413 +                       break;
 414 +#else
 415 +                       snprintf(err_buf, sizeof err_buf,
 416 +                                "hard links are not supported on this %s\n",
 417 +                                am_server ? "server" : "client");
 418 +                       rprintf(FERROR, "ERROR: %s", err_buf);
 419 +                       return 0;
 420 +#endif
 421 +
 422                 default:
 423                         /* A large opt value means that set_refuse_options()
 424                          * turned this option off. */
 425 @@ -1431,6 +1449,11 @@ void server_options(char **args,int *arg
 426                 }
 427         }
 428
 429 +       if (link_by_hash_dir && am_sender) {
 430 +               args[ac++] = "--link-by-hash";
 431 +               args[ac++] = link_by_hash_dir;
 432 +       }
 433 +
 434         if (files_from && (!am_sender || remote_filesfrom_file)) {
 435                 if (remote_filesfrom_file) {
 436                         args[ac++] = "--files-from";
 437 --- orig/receiver.c     2005-02-20 20:55:51
 438 +++ receiver.c  2005-01-15 21:29:13
 439 @@ -52,6 +52,7 @@ extern int delay_updates;
 440  extern struct stats stats;
 441  extern char *log_format;
 442  extern char *tmpdir;
 443 +extern char *link_by_hash_dir;
 444  extern char *partial_dir;
 445  extern char *basis_dir[];
 446
 447 @@ -206,12 +207,13 @@ static int get_tmpname(char *fnametmp, c
 448
 449
 450  static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 451 -                       char *fname, int fd, OFF_T total_size)
 452 +                       char *fname, int fd, OFF_T total_size, char *md4)
 453  {
 454         static char file_sum1[MD4_SUM_LENGTH];
 455         static char file_sum2[MD4_SUM_LENGTH];
 456         struct map_struct *mapbuf;
 457         struct sum_struct sum;
 458 +       struct mdfour mdfour_data;
 459         int32 len;
 460         OFF_T offset = 0;
 461         OFF_T offset2;
 462 @@ -231,6 +233,9 @@ static int receive_data(int f_in, char *
 463         } else
 464                 mapbuf = NULL;
 465
 466 +       if (md4)
 467 +               mdfour_begin(&mdfour_data);
 468 +
 469         sum_init(checksum_seed);
 470
 471         while ((i = recv_token(f_in, &data)) != 0) {
 472 @@ -247,6 +252,8 @@ static int receive_data(int f_in, char *
 473                         cleanup_got_literal = 1;
 474
 475                         sum_update(data, i);
 476 +                       if (md4)
 477 +                               mdfour_update(&mdfour_data,data,i);
 478
 479                         if (fd != -1 && write_file(fd,data,i) != i)
 480                                 goto report_write_error;
 481 @@ -273,6 +280,8 @@ static int receive_data(int f_in, char *
 482
 483                         see_token(map, len);
 484                         sum_update(map, len);
 485 +                       if (md4)
 486 +                               mdfour_update(&mdfour_data,map,len);
 487                 }
 488
 489                 if (inplace) {
 490 @@ -313,6 +322,8 @@ static int receive_data(int f_in, char *
 491         }
 492
 493         sum_end(file_sum1);
 494 +       if (md4)
 495 +               mdfour_result(&mdfour_data, (unsigned char*)md4);
 496
 497         if (mapbuf)
 498                 unmap_file(mapbuf);
 499 @@ -358,7 +369,7 @@ static void read_gen_name(int fd, char *
 500
 501  static void discard_receive_data(int f_in, OFF_T length)
 502  {
 503 -       receive_data(f_in, NULL, -1, 0, NULL, -1, length);
 504 +       receive_data(f_in, NULL, -1, 0, NULL, -1, length, NULL);
 505  }
 506
 507
 508 @@ -621,8 +632,12 @@ int recv_files(int f_in, struct file_lis
 509                         rprintf(FINFO, "%s\n", safe_fname(fname));
 510
 511                 /* recv file data */
 512 +#if HAVE_LINK
 513 +               if (link_by_hash_dir)
 514 +                       file->u.sum = (char*)malloc(MD4_SUM_LENGTH);
 515 +#endif
 516                 recv_ok = receive_data(f_in, fnamecmp, fd1, st.st_size,
 517 -                                      fname, fd2, file->length);
 518 +                                      fname, fd2, file->length, file->u.sum);
 519
 520                 if (!log_before_transfer)
 521                         log_recv(file, &initial_stats, iflags);
 522 --- orig/rsync.c        2005-02-20 00:02:23
 523 +++ rsync.c     2005-02-19 09:32:31
 524 @@ -41,6 +41,7 @@ extern int keep_dirlinks;
 525  extern int make_backups;
 526  extern struct stats stats;
 527  extern char *backup_dir;
 528 +extern char *link_by_hash_dir;
 529  extern char *log_format;
 530  extern char *backup_suffix;
 531  extern int backup_suffix_len;
 532 @@ -306,7 +307,12 @@ void finish_transfer(char *fname, char *
 533                 rprintf(FINFO, "renaming %s to %s\n",
 534                         safe_fname(fnametmp), safe_fname(fname));
 535         }
 536 -       ret = robust_rename(fnametmp, fname, file->mode & INITACCESSPERMS);
 537 +#if HAVE_LINK
 538 +       if (link_by_hash_dir)
 539 +               ret = link_by_hash(fnametmp, fname, file);
 540 +       else
 541 +#endif
 542 +               ret = robust_rename(fnametmp, fname, file->mode & INITACCESSPERMS);
 543         if (ret < 0) {
 544                 rsyserr(FERROR, errno, "%s %s -> \"%s\"",
 545                     ret == -2 ? "copy" : "rename",
 546 --- orig/rsync.h        2005-02-20 00:02:23
 547 +++ rsync.h     2004-07-03 20:20:15
 548 @@ -620,6 +620,14 @@ struct stats {
 549         int current_file_index;
 550  };
 551
 552 +struct hashfile_struct {
 553 +       struct hashfile_struct *next;
 554 +       struct hashfile_struct *prev;
 555 +       char *name;
 556 +       int fd;
 557 +       uint32 nlink;
 558 +};
 559 +
 560
 561  #include "byteorder.h"
 562  #include "lib/mdfour.h"
 563 --- orig/rsync.yo       2005-02-20 01:12:43
 564 +++ rsync.yo    2005-02-13 06:58:47
 565 @@ -356,6 +356,7 @@ to the detailed description below for a
 566       --compare-dest=DIR      also compare received files relative to DIR
 567       --copy-dest=DIR         ... and include copies of unchanged files
 568       --link-dest=DIR         hardlink to files in DIR when unchanged
 569 +     --link-by-hash=DIR      create hardlinks by hash into DIR
 570   -z, --compress              compress file data during the transfer
 571   -C, --cvs-exclude           auto-ignore files in the same way CVS does
 572   -f, --filter=RULE           add a file-filtering RULE