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Diffstat (limited to 'Documentation/mkfs.btrfs.asciidoc')
| -rw-r--r-- | Documentation/mkfs.btrfs.asciidoc | 23 |
1 files changed, 18 insertions, 5 deletions
diff --git a/Documentation/mkfs.btrfs.asciidoc b/Documentation/mkfs.btrfs.asciidoc index e4321de9..5d79e24f 100644 --- a/Documentation/mkfs.btrfs.asciidoc +++ b/Documentation/mkfs.btrfs.asciidoc @@ -275,14 +275,27 @@ 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 redundancy and just -wastes space. +wastes filesystem space without the expected level of redundancy. 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. +details are not usually disclosed by vendors. For example we could expect that +not all blocks get deduplicated. This will provide a non-zero probability of +recovery compared to a zero chance if the single profile is used. The user +should make the tradeoff decision. The deduplication in SSDs is thought to be +widely available so the reason behind the mkfs default is to not give a false +sense of redundancy. + +As another example, the widely used USB flash or SD cards use a translation +layer between the logical and physical view of the device. The data lifetime +may be affected by frequent plugging. The memory cells could get damaged, +hopefully not destroying both copies of particular data in case of DUP. + +The wear levelling techniques can also lead to reduced redundancy, even if the +device does not do any deduplication. The controllers may put data written in +a short timespan into the same physical storage unit (cell, block etc). In case +this unit dies, both copies are lost. BTRFS does not add any artificial delay +between metadata writes. The traditional rotational hard drives usually fail at the sector level. |