View source for User notes on dedupe

== Introduction ==

This page is for end users/tester to try btrfs in-band de-duplication
(dedupe for short).<br/>

Btrfs in-band dedupe provides write time de-duplication, which will only
write one copy of duplicated data into disk.
Such behavior could save storage under specific write patter, like:
 # mkfs.btrfs /dev/vdb
 # mount /dev/vdb /mnt/btrfs
 # btrfs dedup enable /mnt/btrfs
 # xfs_io -f -c "pwrite 0 128K" -c "fsync" /mnt/btrfs/tmp
 # xfs_io -f -c "pwrite 0 128M" /mnt/btrfs/file
 # sync
 # df -h | grep /mnt/btrfs
 /dev/vdb5        10G   18M  8.0G   1% /mnt/test
                        ^^^ Much smaller than 128M

Compared to normal write:
 # mkfs.btrfs /dev/vdb
 # mount /dev/vdb /mnt/btrfs
 # xfs_io -f -c "pwrite 0 128K" -c "fsync" /mnt/btrfs/tmp
 # xfs_io -f -c "pwrite 0 128M" /mnt/btrfs/file
 # sync
 # df -h | grep /mnt/btrfs
 /dev/vdb5        10G  146M  7.9G   2% /mnt/test

Btrfs in-band dedupe is still an out-of-tree experimental feature,
but it's becoming more and more stable, and developers are currently trying
to push it for mainline.<br/>

=== Software requirement for dedupe ===
* Kernel support
*: Can be fetched from https://github.com/littleroad/linux.git dedupe_latest
* Kernel config
*: Need '''CONFIG_BTRFS_DEBUG'''
* Btrfs-progs support
*: Can be fetched from https://github.com/littleroad/btrfs-progs.git dedupe_latest

=== Write pattern requirement for dedupe ===
* Data copy-on-write
*: For any case, if data cow is disabled, it won't go through dedupe.
*: Data cow disabled cases include:
*:* nodatacow mount option
*:* per inode datacow control flag(C)
*:* write into preallocated file extent(fallocate)
*: If unsure, default btrfs setting enables data copy-on-write.
* Buffered write
*: Dedupe routine only affects buffered write.
*: Direct IO won't go through dedupe routine.
*: O_SYNC open flag will still go through dedupe though, as it's just
*: calling fsync after each write.
*: If unsure, default write is buffered write.
* No compression
*: Support for dedupe over compression is not implemented yet.
*: If unsure, compression is disabled by default.
* Extent size
*: Only extent size larger than or equal to dedupe block size will go
*: through dedupe routine.
*: Check [[#Dedupe block size]] for more info.

== Normal Usage ==
=== Enable dedupe ===
Enable dedupe on a mounted btrfs:
 # btrfs dedupe enable <mnt_point>

This will enable dedupe with 128K dedupe block size with in-memory
backend.
Memory limit will be 32K hashes.

For more info about the dedupe parameters and their impact, check
[[#Detail Notes]] section and '''btrfs-dedupe'''(5) man page.

=== Disable dedupe ===
 # btrfs dedupe disable <mnt_point>

Disabling dedupe will trash all hashes. it may take some time.

It's OK to execute '''btrfs dedupe disable''' on a btrfs whose dedupe is already
disabled, however such operation won't have any effect though.

=== Show status of dedupe===
 # btrfs dedupe status <mnt_point>

For dedupe enabled case, output would be like:
 Status:			Enabled
 Hash algorithm:		SHA-256
 Backend:		In-memory
 Dedup Blocksize:	131072
 Number of hash: 	[128/32768]
 Memory usage: 		[14.00KiB/3.50MiB]

For dedupe disabled case, output would be like:
 Status: 			Disabled

=== Reconfigure dedupe ===
Reconfigure any parameter other than memory usage or hash limit will
trash all existing dedupe hash(es).

There are 2 method to reconfigure one or more dedupe parameters.

==== Stateful reconfigure ====
Stateful reconfigure must be executed on dedupe enabled btrfs.
Stateful reconfigure will only modify the specified parameter(s).

 # btrfs dedupe enable -b 64k -l 1k /mnt/test/
 # btrfs dedupe status /mnt/test/
 Status:			Enabled
 Hash algorithm:		SHA-256
 Backend:		In-memory
 Dedup Blocksize:	65536		<< 64K dedupe blocksize
 Number of hash: 	[0/1024]	<< 1K hash number limit
 Memory usage: 		[0.00B/112.00KiB]
 # btrfs dedupe reconfigure -b 128K /mnt/test
 # btrfs dedupe status /mnt/test/
 Status:			Enabled
 Hash algorithm:		SHA-256
 Backend:		In-memory
 Dedup  Blocksize:	131072		<< 128K dedupe blocksize
 Number of hash: 	[0/1024]	<< 1K hash number limit stay
 Memory usage: 		[0.00B/112.00KiB]

==== Stateless reconfiguration ====
Stateless reconfiguration doesn't rely on the current dedupe status,
and that's why it's called stateless.

To use stateless reconfiguration, user only need to call "enable" with
"-f" option.

Stateless reconfiguration will use default value to fill any
parameter that is not specified.

 # btrfs dedupe enable -b 64k -l 1k /mnt/test/
 # btrfs dedupe status /mnt/test/
 Status:			Enabled
 Hash algorithm:		SHA-256
 Backend:		In-memory
 Dedup Blocksize:	65536		<< 64K dedupe blocksize
 Number of hash: 	[0/1024]	<< 1K hash number limit
 Memory usage: 		[0.00B/112.00KiB]
 # btrfs dedupe enable -f -b 32K /mnt/test
 # btrfs dedupe status /mnt/test/
 Status:			Enabled
 Hash algorithm:		SHA-256
 Backend:		In-memory
 Dedup Blocksize:	32768		<< 32K dedupe blocksize
 Number of hash: 	[0/32768]	<< Reset to default 32K hash limit
 Memory usage: 		[0.00B/3.50MiB]

== Detail Notes ==
=== Timing of dedupe ===
Dedupe happens when btrfs writes data into disk.
Or more specifically, at run_delalloc_range() time.

The timing of writing data into disk includes:
* Sync system call
* Fsync system call (including O_SYNC open flag)
* Memory pressure
* Commit interval of each fs

Unless one is calling fsync/sync manually, timing of dedupe is almost
unpredictable.
Considering the high CPU usage, it's recommended to use "thread_pool"
mount option to limit the maximum CPU core dedupe can use.

This timing also means normal small write under most case won't
trigger dedupe to calculate hash immediately.

=== Dedupe block size ===
Dedupe is done in dedupe block size.
This means, only continuous data whose length is larger than or equal to dedupe
block size will go through dedupe routine.

And dedupe write will extent size to be the same as dedupe block size,
which is much smaller than normal file extent size.
This behavior may cause a lot of small extents, leading to large metadata
and long mount time.

For example, under 64K dedupe block size, the following write pattern
won't go through dedupe at all:

 File A:
 0         16K          32K           48K           64K
 |<-----New data------->|<////Hole///>|<--New Data->|

And with 64K dedupe block size, the following write pattern will only
cause the first 64K go through dedupe.

 File B:
 0         16K          32K           48K           64K        80K
 |<--------------------------New Data------------------------->|

Will cause the following extent layout:
 |<--------Extent A, dedupe write------------------>|<Extent B>|

Only Extent A will go through dedupe routine, Extent B will go through
normal write routine.

=== Hash and memory limit ===
For in-memory backend, one can set either memory usage limit or number
of hash limit.

When current dedupe hash pool is larger than the limit, dedupe will
drop hashes until the memory usage/hash number drops to limit.

The hash drop follows last-recent-use(LRU) behavior.
Newly added hash or hash search hit will cause the hash to be the
newest hash of the hash pool.

   
And the memory usage of dedupe grows with the dedupe hash pool size.
Btrfs will not pre-allocating memory for dedupe at the enabling time.
So it's possible to set a very huge dedupe pool size, as long as
OOM is not triggered.

=== Global hash pool  ===
At the time of writing (wang_dedupe_20160719 branch), the dedupe hash
pool is global, which means no matter which subvolume/snapshot user is
writing data into, they all share the same dedupe hash pool.

== Known Bugs ==
Btrfs has several existing bugs that will affects both in-band and
out-of-band dedupe (reflink).

At the write time (v4.7 kernel), the following bugs are known:
=== fiemap soft lockup ===
If a file is consist of file extents pointing to the same extent,
fiemap ioctl will cause soft lockup and hangs for a long long long time.

4096 extents can already trigger it.

The bug is already fixed in v4.8 mainline.

=== send soft lockup or OOM ===
Much like fiemap soft lockup, send will hang and cause soft lockup.
And if the system doesn't have enough RAM, it will also trigger OOM.
Increase swap won't help, as the memory btrfs is allocating is not swapable.