Update raid6check man page

raid6check.8

@@ -13,48 +13,72 @@ Linux Software RAID
 
 .SH SYNOPSIS
 
-.BI raid6check " <raid6 device> <start stripe> <number of stripes>"
+.BI raid6check " <raid6_device> <start_stripe> <number_of_stripes>"
+.RB [ autorepair ]
+
+.BI raid6check " <raid6_device> " repair " <stripe> <failed_slot_1>"
+.I "<failed_slot_2>"
 
 .SH DESCRIPTION
 RAID6 devices in which one single component drive has errors can use
 the double parity in order to find out which component drive.
-The "raid6check" tool checks, for each stripe, the double parity
-consistency, reports mismatches and, if possible, which
+The "raid6check" tool has two modes: check mode and repair mode.
+
+"raid6check" requires a non-degraded RAID6 MD device as first
+parameter. If the given MD device is not a RAID6, "raid6check" will,
+of course, not continue.
+
+If the RAID6 MD device is degraded, "raid6check" will report
+an error and it will not proceed further.
+
+.SS Check mode
+In the check mode, the "raid6check" tool checks, for each stripe, the
+double parity consistency, reports mismatches and, if possible, which
 component drive has the mismatch.
 Since it works at stripe level, it can report different drives with
 mismatches at different stripes.
 
-"raid6check" requires a non-degraded RAID6 MD device as first
-parameter, a starting stripe (usually 0) and the number of stripes
-to be checked.
+In addition to the MD device, "raid6check" requries a starting stripe
+(usually 0) and the number of stripes to be checked.
 If this third parameter is also 0, it will check the array up to
 the end.
+If the fourth parameter is "autorepair", it will overwrite single-slot
+errors.
 
 "raid6check" will start printing information about the RAID6, then
 for each stripe, it will report the parity rotation status.
 In case of parity mismatches, "raid6check" reports, if possible,
-which component drive could be responsible. Otherwise it reports
-that it is not possible to find the component drive.
-
-If the given MD device is not a RAID6, "raid6check" will, of
-course, not continue.
+which component drive could be responsible.
+Otherwise it reports that it is not possible to find the component drive.
 
-If the RAID6 MD device is degraded, "raid6check" will report
-an error and it will not proceed further.
-
-No write operations are performed on the array or the components.
+No write operations are performed on the array or the components,
+unless "autorepair" is specified.
 Furthermore, the checked array can be online and in use during
 the operation of "raid6check".
 
+.SS Repair mode
+In the repair mode, the "raid6check" tool checks the given stripe.
+If inconsistencies are found, it attempts to repair the strip assuming
+that the two given slots are incorrect.
+
+-1 may be used to specify parity P and -2 parity Q.
+
 .SH EXAMPLES
 
 .B "  raid6check /dev/md0 0 0"
 .br
 This will check /dev/md0 from start to end.
 
-.B "  raid6check /dev/md3 0 1"
+.B "  raid6check /dev/md3 0 1 autorepair"
+.br
+This will check the first stripe of /dev/md3.
+Any error will be reported, and if single-slot, repaired.
+
+.B "  raid6check /dev/md3 0 repair 1 -1 -2"
 .br
 This will check the first stripe of /dev/md3.
+If any inconsistencies are found, repair is performed assuming
+the two parities are incorrect.
 
 .B "  raid6check /dev/md1 1000 0"
 .br
@@ -76,7 +100,10 @@ Furthermore, the sysfs interface is needed in order to find out
 the RAID6 parameters.
 
 .SH BUGS
-Negative parameters can lead to unexpected results.
+Negative stripe parameters can lead to unexpected results due to
+strtoull.
+(Negative slot numbers should work for Q [-1] and P [-2] but
+currently involve compiler-defined behavior.)
 
 It is not clear what will happen if the RAID6 MD device gets
 degraded during the check.