This patch adds a --slow-down=USECs option that causes the sender to scan
the filelist more slowly, and the generator to scan for deletions more
slowly.  It doesn't do anything to make anyone slow down during the normal
transfer processing, though.

The idea is to lessen rsync's impact on disk I/O.  Unfortunately, there
should really be a way to affect more of rsync's processing, perhaps by
specifying a maximum disk I/O rate (and have that affect a maximum stat()
rate or something like that).

To use this patch, run these commands for a successful build:

    patch -p1 <patches/slow-down.diff
    ./configure                           (optional if already run)
    make

based-on: a01e3b490eb36ccf9e704840e1b6683dab867550
diff --git a/flist.c b/flist.c
--- a/flist.c
+++ b/flist.c
@@ -72,6 +72,7 @@ extern int sender_symlink_iconv;
 extern int output_needs_newline;
 extern int sender_keeps_checksum;
 extern int unsort_ndx;
+extern unsigned long sleep_asec;
 extern struct stats stats;
 extern char *filesfrom_host;
 extern char *usermap, *groupmap;
@@ -1754,6 +1755,9 @@ static void send_directory(int f, struct file_list *flist, char *fbuf, int len,
 		}
 
 		send_file_name(f, flist, fbuf, NULL, flags, filter_level);
+		/* Sleep for a bit, to avoid hammering the disk. */
+		if (sleep_asec)
+			usleep(sleep_asec);
 	}
 
 	fbuf[len] = '\0';
diff --git a/options.c b/options.c
--- a/options.c
+++ b/options.c
@@ -108,6 +108,7 @@ int size_only = 0;
 int daemon_bwlimit = 0;
 int bwlimit = 0;
 int fuzzy_basis = 0;
+unsigned long sleep_asec = 0;
 size_t bwlimit_writemax = 0;
 int ignore_existing = 0;
 int ignore_non_existing = 0;
@@ -776,6 +777,7 @@ void usage(enum logcode F)
   rprintf(F,"     --password-file=FILE    read daemon-access password from FILE\n");
   rprintf(F,"     --list-only             list the files instead of copying them\n");
   rprintf(F,"     --bwlimit=RATE          limit socket I/O bandwidth\n");
+  rprintf(F,"     --slow-down=USECs       sleep N usec while creating the filelist\n");
   rprintf(F,"     --write-batch=FILE      write a batched update to FILE\n");
   rprintf(F,"     --only-write-batch=FILE like --write-batch but w/o updating destination\n");
   rprintf(F,"     --read-batch=FILE       read a batched update from FILE\n");
@@ -964,6 +966,7 @@ static struct poptOption long_options[] = {
   {"itemize-changes", 'i', POPT_ARG_NONE,   0, 'i', 0, 0 },
   {"no-itemize-changes",0, POPT_ARG_VAL,    &itemize_changes, 0, 0, 0 },
   {"no-i",             0,  POPT_ARG_VAL,    &itemize_changes, 0, 0, 0 },
+  {"slow-down",        0,  POPT_ARG_LONG,   &sleep_asec, 0, 0, 0 },
   {"bwlimit",          0,  POPT_ARG_STRING, &bwlimit_arg, OPT_BWLIMIT, 0, 0 },
   {"no-bwlimit",       0,  POPT_ARG_VAL,    &bwlimit, 0, 0, 0 },
   {"backup",          'b', POPT_ARG_VAL,    &make_backups, 1, 0, 0 },