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The btrfs file system mount seems to automagically load the btrfs module which is good.
On this system there is no btrfs RAID configured. That is why I am wondering why also these dependencies get automagically loaded:
xor 24576 1 btrfs
raid6_pq 114688 1 btrfsAnd dmesg shows that both a raid6 and xor algorighm are being determined during system startup:
[ 0.526784] raid6: sse2x1 gen() 971 MB/s
[ 0.583377] raid6: sse2x1 xor() 1481 MB/s
[ 0.640022] raid6: sse2x2 gen() 1623 MB/s
[ 0.696685] raid6: sse2x2 xor() 1760 MB/s
[ 0.753352] raid6: sse2x4 gen() 2555 MB/s
[ 0.810024] raid6: sse2x4 xor() 1738 MB/s
[ 0.810027] raid6: using algorithm sse2x4 gen() 2555 MB/s
[ 0.810028] raid6: .... xor() 1738 MB/s, rmw enabled
[ 0.810031] raid6: using ssse3x2 recovery algorithm
[ 0.810434] xor: measuring software checksum speed
[ 0.843360] prefetch64-sse: 4558.800 MB/sec
[ 0.876688] generic_sse: 4140.000 MB/sec
[ 0.876691] xor: using function: prefetch64-sse (4558.800 MB/sec)
[ 0.898101] Btrfs loaded, crc32c=crc32c-intelAs long as RAID5 and RAID6 are still highly experimental having potential parity write gaps, shouldn't there be a method (like f.e. kernel flags or module blocking) to skip the raid6 and xor module loading?
In case skipping raid6_pq is possible, how is that best done?
Last edited by probackup-nl (2017-11-15 23:58:34)
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Not possible see https://git.kernel.org/pub/scm/linux/ke … h=v4.13.13
btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
file-item.o inode-item.o inode-map.o disk-io.o \
transaction.o inode.o file.o tree-defrag.o \
extent_map.o sysfs.o struct-funcs.o xattr.o ordered-data.o \
extent_io.o volumes.o async-thread.o ioctl.o locking.o orphan.o \
export.o tree-log.o free-space-cache.o zlib.o lzo.o \
compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
uuid-tree.o props.o hash.o free-space-tree.oThe important entry being raid56.o being unconditionally need to make btrfs-y the target for the btrfs module.
/fs/btrfs/raid56.c contains https://git.kernel.org/pub/scm/linux/ke … 13.13#n518 which uses xor_blocks
http://elixir.free-electrons.com/linux/ … /xor.c#L35 from the xor module
it also contains https://git.kernel.org/pub/scm/linux/ke … 3.13#n1899 which uses raid6_2data_recov
http://elixir.free-electrons.com/linux/ … lgos.c#L97 from the raid6_pq module
This is also why https://git.kernel.org/pub/scm/linux/ke … h=v4.13.13 enabling btrfs support in the kernel
config BTRFS_FS
tristate "Btrfs filesystem support"
select CRYPTO
select CRYPTO_CRC32C
select ZLIB_INFLATE
select ZLIB_DEFLATE
select LZO_COMPRESS
select LZO_DECOMPRESS
select RAID6_PQ
select XOR_BLOCKS
select SRCUselects those other options to ensure the required modules are built.
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In Ubuntu since 4.15.0 it seems possible to reduce the boot latency by passing a kernel param like CONFIG_RAID6_PQ_DEFAULT_ALG="avx2x4"
https://bugs.launchpad.net/ubuntu/+sour … ug/1812728
I have no clue if this patch went upstream.
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