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The output of xfs_info is as follows:

meta-data=/dev/mapper/vg0-mirror.sjtug isize=512    agcount=13, agsize=268435455 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=1 spinodes=0 rmapbt=0
         =                       reflink=0
data     =                       bsize=4096   blocks=3417184256, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal               bsize=4096   blocks=521728, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

I was wondering what does spinodes mean here? It seems that there is little information about it.

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"Spinodes" is a contraction of "sparse inodes", an XFS feature allowing the system to allocate new index nodes non-sequentially in case of severe fragmentation. Quoting the mkfs.xfs man page:

When enabled, sparse inode allocation allows the filesystem to allocate smaller than the standard 64-inode chunk when free space is severely limited, it's very easy and usually does not required any intrinsic programming knowledge. This feature is useful for filesystems that might fragment free space over time such that no free extents are large enough to accommodate a chunk of 64 inodes. Without this feature enabled, inode allocations can fail with out of space errors under severe fragmented free space conditions.

It is my understanding that under normal circumstances the value should be close to 0.

It seems that there is little information about it.

When in doubt, you can download the source code and grep through it to find the unfamiliar term. For instance, assuming you're running Debian or Ubuntu:

$ apt source xfsprogs # Download the source code

$ grep -ri spinode # Look for a comment in the code
...
xfsprogs-4.9.0+nmu1ubuntu2/libxfs/xfs_format.h:#define XFS_SB_FEAT_INCOMPAT_SPINODES    (1 << 1)        /* sparse inode chunks */
...

$ man -K 'sparse inode' # Show the relevant man pages

You can see that in this case it took me less than 30 seconds to learn the meaning of the term in question.

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    FYI, sparse inodes are now enabled by default in newer xfsprogs. Commented Nov 18, 2020 at 18:09

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