diff options
authorDavid Sterba <>2015-11-26 17:56:45 +0100
committerDavid Sterba <>2016-01-04 11:28:12 +0100
commitbbb0c589fc675d1a132b66a10be2234ffb4bd5eb (patch)
parent1d37645dc35bdd6152399be88d8300318b0651de (diff)
btrfs-progs: docs: mkfs, implications of DUP on devices
We offer DUP but still depend on the hardware, to do the right thing. Signed-off-by: David Sterba <>
2 files changed, 29 insertions, 5 deletions
diff --git a/Documentation/mkfs.btrfs.asciidoc b/Documentation/mkfs.btrfs.asciidoc
index c9ba314c..0b145c7a 100644
--- a/Documentation/mkfs.btrfs.asciidoc
+++ b/Documentation/mkfs.btrfs.asciidoc
@@ -50,7 +50,9 @@ mkfs.btrfs uses the entire device space for the filesystem.
*-d|--data <profile>*::
Specify the profile for the data block groups. Valid values are 'raid0',
-'raid1', 'raid5', 'raid6', 'raid10' or 'single', (case does not matter).
+'raid1', 'raid5', 'raid6', 'raid10' or 'single' or dup (case does not matter).
*-m|--metadata <profile>*::
Specify the profile for the metadata block groups.
@@ -60,13 +62,12 @@ Valid values are 'raid0', 'raid1', 'raid5', 'raid6', 'raid10', 'single' or
A single device filesystem will default to 'DUP', unless a SSD is detected. Then
it will default to 'single'. The detection is based on the value of
`/sys/block/DEV/queue/rotational`, where 'DEV' is the short name of the device.
-This is because SSDs can remap the blocks internally to a single copy thus
-deduplicating them which negates the purpose of increased metadata redunancy
-and just wastes space.
Note that the rotational status can be arbitrarily set by the underlying block
device driver and may not reflect the true status (network block device, memory-backed
SCSI devices etc). Use the options '--data/--metadata' to avoid confusion.
+See 'DUP PROFILES ON A SINGLE DEVICE' for more details.
Normally the data and metadata block groups are isolated. The 'mixed' mode
@@ -265,6 +266,29 @@ There are the following block group types available:
another one is added, but *mkfs.btrfs* will not let you create DUP on multiple
+The mkfs utility will let the user create a filesystem with profiles that write
+the logical blocks to 2 physical locations. Whether there are really 2
+physical copies highly depends on the underlying device type.
+For example, a SSD drive can remap the blocks internally to a single copy thus
+deduplicating them. This negates the purpose of increased redunancy and just
+wastes space.
+The duplicated data/metadata may still be useful to statistically improve the
+chances on a device that might perform some internal optimizations. The actual
+details are not usually disclosed by vendors. As another example, the widely
+used USB flash or SD cards use a translation layer. The data lifetime may
+be affected by frequent plugging. The memory cells could get damaged, hopefully
+not destroying both copies of particular data.
+The traditional rotational hard drives usually fail at the sector level.
+In any case, a device that starts to misbehave and repairs from the DUP copy
+should be replaced! *DUP is not backup*.
diff --git a/utils.c b/utils.c
index c20966c1..32e6e399 100644
--- a/utils.c
+++ b/utils.c
@@ -2505,7 +2505,7 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile,
warning_on(!mixed && (data_profile & BTRFS_BLOCK_GROUP_DUP) && ssd,
- "DUP have no effect if your SSD have deduplication function");
+ "DUP may not actually lead to 2 copies on the device, see manual page");
return 0;