diff options
| -rw-r--r-- | Documentation/mkfs.btrfs.asciidoc | 32 | ||||
| -rw-r--r-- | utils.c | 2 |
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). ++ +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 @@ There are the following block group types available: 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 ------------ @@ -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; } |