readahead: don't monopolize IO when replaying

TODO

@@ -20,6 +20,9 @@ Bugfixes:
 * properly handle .mount unit state tracking when two mount points are stacked one on top of another on the exact same mount point.
 
 Features:
+
+* document the exit codes when services fail before they are exec()ed
+
 * use the rtc which has: /sys/class/rtc/*/hctosys == "1" as the system RTC
 
 * rework namespace support, don't use pivot_root, and mount things after creating the namespace, not before

src/readahead/readahead-replay.c

@@ -186,7 +186,13 @@ static int replay(const char *root) {
         if (on_ssd)
                 prio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_IDLE, 0);
         else
-                prio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_RT, 7);
+                /* We are not using RT here, since we'd starve IO that
+                we didn't record (which is for example blkid, since
+                its disk accesses go directly to the block device and
+                are thus not visible in fallocate) to death. However,
+                we do ask for an IO prio that is slightly higher than
+                the default (which is BE. 4) */
+                prio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 2);
 
         if (ioprio_set(IOPRIO_WHO_PROCESS, getpid(), prio) < 0)
                 log_warning("Failed to set IDLE IO priority class: %m");