diff options
Diffstat (limited to 'Documentation/mkfs.btrfs.asciidoc')
| -rw-r--r-- | Documentation/mkfs.btrfs.asciidoc | 54 |
1 files changed, 42 insertions, 12 deletions
diff --git a/Documentation/mkfs.btrfs.asciidoc b/Documentation/mkfs.btrfs.asciidoc index 12d8840..0b145c7 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 @@ -248,17 +249,46 @@ PROFILES There are the following block group types available: -[ width="60%",options="header" ] +[ cols="^,^,^,^,^",width="60%" ] |============================================================= -| Profile | Redundancy | Striping | Min/max devices -| single | 1 copy | n/a | 1/any -| DUP | 2 copies / 1 device | n/a | 1/1 -| RAID0 | n/a | 1 to N | 2/any -| RAID10 | 2 copies | 1 to N | 4/any -| RAID5 | 2 copies | 3 to N - 1 | 2/any -| RAID6 | 3 copies | 3 to N - 2 | 3/any +.2+^.<h| Profile 3+^.^h| Redundancy .2+^.<h| Min/max devices + ^.^h| Copies ^.^h| Parity ^.<h| Striping +| single | 1 | | | 1/any +| DUP | 2 / 1 device | | | 1/1 ^(see note)^ +| RAID0 | | | 1 to N | 2/any +| RAID1 | 2 | | | 2/any +| RAID10 | 2 | | 1 to N | 4/any +| RAID5 | 1 | 1 | 2 to N - 1 | 2/any +| RAID6 | 1 | 2 | 3 to N - 2 | 3/any |============================================================= +'Note:' DUP may exist on more than 1 device if it starts on a single device and +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 ------------ |