Create.c: fix uclibc build

[thirdparty/mdadm.git] / md.4

diff --git a/md.4 b/md.4
index aecff38a1d0251a73fa463c36659e5d0973f581a..7a0bc7e636979b6eadfaf2372de0cf94332f85ed 100644 (file)
--- a/md.4
+++ b/md.4
@@ -215,6 +215,19 @@ option or the
 .B "--update=layout-alternate"
 option.
 
+Once you have updated the layout you will not be able to mount the array
+on an older kernel.  If you need to revert to an older kernel, the
+layout information can be erased with the
+.B "--update=layout-unspecificed"
+option.  If you use this option to
+.B --assemble
+while running a newer kernel, the array will NOT assemble, but the
+metadata will be update so that it can be assembled on an older kernel.
+
+No that setting the layout to "unspecified" removes protections against
+this bug, and you must be sure that the kernel you use matches the
+layout of the array.
+
 .SS RAID1
 
 A RAID1 array is also known as a mirrored set (though mirrors tend to
@@ -896,26 +909,40 @@ The list is particularly useful when recovering to a spare.  If a few blocks
 cannot be read from the other devices, the bulk of the recovery can
 complete and those few bad blocks will be recorded in the bad block list.
 
-.SS RAID456 WRITE JOURNAL
+.SS RAID WRITE HOLE
 
-Due to non-atomicity nature of RAID write operations, interruption of
-write operations (system crash, etc.) to RAID456 array can lead to
-inconsistent parity and data loss (so called RAID-5 write hole).
+Due to non-atomicity nature of RAID write operations,
+interruption of write operations (system crash, etc.) to RAID456
+array can lead to inconsistent parity and data loss (so called
+RAID-5 write hole).
+To plug the write hole md supports two mechanisms described below.
 
-To plug the write hole, from Linux 4.4 (to be confirmed),
-.I md
-supports write ahead journal for RAID456. When the array is created,
-an additional journal device can be added to the array through
-.IR write-journal
-option. The RAID write journal works similar to file system journals.
-Before writing to the data disks, md persists data AND parity of the
-stripe to the journal device. After crashes, md searches the journal
-device for incomplete write operations, and replay them to the data
-disks.
+.TP
+DIRTY STRIPE JOURNAL
+From Linux 4.4, md supports write ahead journal for RAID456.
+When the array is created, an additional journal device can be added to
+the array through write-journal option. The RAID write journal works
+similar to file system journals. Before writing to the data
+disks, md persists data AND parity of the stripe to the journal
+device. After crashes, md searches the journal device for
+incomplete write operations, and replay them to the data disks.
 
 When the journal device fails, the RAID array is forced to run in
 read-only mode.
 
+.TP
+PARTIAL PARITY LOG
+From Linux 4.12 md supports Partial Parity Log (PPL) for RAID5 arrays only.
+Partial parity for a write operation is the XOR of stripe data chunks not
+modified by the write. PPL is stored in the metadata region of RAID member drives,
+no additional journal drive is needed.
+After crashes, if one of the not modified data disks of
+the stripe is missing, this updated parity can be used to recover its
+data.
+
+This mechanism is documented more fully in the file
+Documentation/md/raid5-ppl.rst
+
 .SS WRITE-BEHIND
 
 From Linux 2.6.14,