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btrfs-progs: docs: clarify why mkfs selects single for SSDs 

The section raised some user questions on IRC.

Signed-off-by: David Sterba <dsterba@suse.com>
master
David Sterba 2016-05-06 17:56:51 +02:00
parent 5079ae684a
commit 21dc9050b5
1 changed files with 18 additions and 5 deletions
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23
Documentation/mkfs.btrfs.asciidoc
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@ -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.