Linux 块设备的 block 大小的多少？

[nullboy]

性能监控的时候使用vmstat，查看 man

bo: Blocks sent to a block device (blocks/s).

下图是分别运行vmstat 1 和iostat 1截图

盲猜 block 大小为 1KB，

但是通过fdisk -l

root@debian:~# fdisk -l
Disk /dev/sda: 80 GiB, 85899345920 bytes, 167772160 sectors
Disk model: VMware Virtual S
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x5e118ce6

看到I/O size (minimum/optimal): 512 bytes / 512 bytes

执行dumpe2fs -h /dev/sda1

root@debian:~# dumpe2fs -h /dev/sda1
dumpe2fs 1.44.5 (15-Dec-2018)
Filesystem volume name:   <none>
Last mounted on:          /
Filesystem UUID:          7305e4f3-57d3-49f9-8348-8416ad736173
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent 64bit flex_bg sparse_super large_file huge_file dir_nlink extra_isize metadata_csum
Filesystem flags:         signed_directory_hash 
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              4964352
Block count:              19847168
Reserved block count:     992358
Free blocks:              15643755
Free inodes:              4807531
First block:              0
Block size:               4096
Fragment size:            4096
Group descriptor size:    64
Reserved GDT blocks:      1024
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
Flex block group size:    16
Filesystem created:       Tue Sep 24 15:52:12 2019
Last mount time:          Wed Jun 24 08:13:34 2020
Last write time:          Wed Jun 24 08:13:32 2020
Mount count:              27
Maximum mount count:      -1
Last checked:             Tue Sep 24 15:52:12 2019
Check interval:           0 (<none>)
Lifetime writes:          282 GB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:	          256
Required extra isize:     32
Desired extra isize:      32
Journal inode:            8
First orphan inode:       4195685
Default directory hash:   half_md4
Directory Hash Seed:      aaa256b0-0045-4830-aed1-533d6f7f0a28
Journal backup:           inode blocks
Checksum type:            crc32c
Checksum:                 0xba8682f2
Journal features:         journal_incompat_revoke journal_64bit journal_checksum_v3
Journal size:             512M
Journal length:           131072
Journal sequence:         0x00c303f9
Journal start:            103755
Journal checksum type:    crc32c
Journal checksum:         0x9e666967

看到

Block size:               4096
Reserved GDT blocks:      1024

那么问题来了，block 的大小到底的多少？

[freenull]

我记得一直是 4K

[loading]

我只知道文件系统的 block 是可以指定的。

[nullboy]

177 次点击 2 回答，呼叫大佬.......

[feather12315]

通常 physical block 为 512，
格式化默认的 logic block 为 4K

[nullboy]

@feather12315 那为啥 vmstat bi bo 对应的是 1KB 呢？

[feather12315]

@nullboy
1. vmstat 存在参数-S 用于指定 block size （这个是可调节的）。同样，du 、dh 也是
2. 磁盘与光盘的最小存储单位是扇区 (sector)，操作系统每次 I/O 的首选长度 (block size) 称为块 (block)，文件系统上最小的分配单位 (fragment size) 叫做 fragment

[linvon]

磁盘里面的和文件系统里面的两个基本单位都叫 block size，但是实际上是两个东西，一个是硬盘扇区的大小，应该叫做 sector size ；一个是文件系统的 block 的大小。磁盘扇区一般都是 512bytes 。

[vuuv]

我记得是 1K 。
刚才 man vmstat 确认了。在 note 章节：
```
All linux blocks are currently 1024 bytes. Old
kernels may report blocks as 512 bytes, 2048 bytes,
or 4096 bytes.
```