@@ -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).
++
+See 'DUP PROFILES ON A SINGLE DEVICE' for more.
*-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.
*-M|--mixed*::
Normally the data and metadata block groups are isolated. The 'mixed' mode
@@ -265,6 +266,29 @@ PROFILES
another one is added, but *mkfs.btrfs* will not let you create DUP on multiple
devices.
+DUP PROFILES ON A SINGLE DEVICE
+-------------------------------
+
+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*.
+
KNOWN ISSUES
------------
@@ -2504,8 +2504,9 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile,
return 1;
}
- warning_on(!mixed && (data_profile & BTRFS_BLOCK_GROUP_DUP) && ssd,
- "DUP have no effect if your SSD have deduplication function");
+ /* warning_on(!mixed && (data_profile & BTRFS_BLOCK_GROUP_DUP) && ssd, */
+ warning_on(!mixed && (data_profile & BTRFS_BLOCK_GROUP_DUP),
+ "DUP may not actually lead to 2 copies on the device, see manual page");
return 0;
}
We offer DUP but still depend on the hardware, to do the right thing. Signed-off-by: David Sterba <dsterba@suse.com> --- To wider audience: feel free to suggest improvements to the manual page text if you think it's not clear or too technical etc. Documentation/mkfs.btrfs.asciidoc | 32 ++++++++++++++++++++++++++++---- utils.c | 5 +++-- 2 files changed, 31 insertions(+), 6 deletions(-)