linux-ext4 - Why doesn't disk io occur to read file system metadata despite clearing dentry and inode with drop

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Message-ID: <CAARcW+qa7aRbh+BeFWTndGLC8owsy9VPUqcJ-BYN-Yw3jQM-_w@mail.gmail.com>
Date:   Wed, 11 Sep 2019 15:27:07 +0900
From:   Daegyu Han <hdg9400@...il.com>
To:     linux-ext4@...r.kernel.org
Subject: Why doesn't disk io occur to read file system metadata despite
 clearing dentry and inode with drop_cache command?

I am confused about "echo # >/proc/sys/vm/drop_caches" and blockdev --flushbufs.
According to OSStep book written by Remzi,If the target inodes are not
cached in memory, disk IO should be occur to readthe inode, which will
make a dentry data structure on memory.
To my knowledge, echo 3 >/proc/sys/vm/drop_caches is to drop(clear)
page cahche, inodes and dentry. I have experimented with blktrace to
figure out whether disk io is really occurring to read the inode.

1. echo 3 > /proc/sys/vm/drop_caches
However, there is no disk io to read inode. I can only see the disk io
to read 16KB data block.
2. echo 3 > /proc/sys/vm/drop_caches` and `blockdev --flushbufs
/dev/nvme0n1I found block access (+8(512*8=4KB)) to read inode.

A quick look at how blockdev --flushbufs works in the kernel code
shows that it clears the superblock.
Why doesn't disk io occur to read inodes with drop_cache alone?
The kernel book called ULK says that inodes and superblocks are cached
in buffer-cache.Is this the reason for this?
I infer as follows:Is the buffer_head data structure not flushed to
disk by drop_cache alone because the storage device is still mapped in
memory?

Thank you