some comments on block/sector size ( ch 14 )

[gf-mse]

to https://github.com/shichao-an/notes/blob/master/docs/lkd/ch14.md :
I'm not sure whether that fits as book comments, but since it was probably the best introduction in the matter that I could quickly find online, I want to share my two cents, that one may or may not find useful:

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Suppose we need to find out what's the optimal buffer size for reading from a device. Or have special needs -- e.g. want to tune a ZFS filesystem recordsize for a custom on-disk database -- may be even with enabled deduplication, what will make our needs really special.

So we wonder how the system reads and writes data from/to the device. These i/o buffers would be directly related to sector or block sizes.

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As for device block sizes -- they, of course, first come from the device itself ; look at

/sys/block/sda/queue :

    hw_sector_size
    logical_block_size
    physical_block_size

That would be the same as smartctl or hdparm would tell us:

smartctl -a /dev/sda    # grep -i block
hdparm -I /dev/sda     # grep -i block

"Physical" and "logical" here would differ e.g. for Advanced Format disks ( and we'll be more interested in physical_block_size, if it's different ).

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Next, there's something reported by man 2 stat ( struct stat -> st_blksize ) :

stat /etc/passwd ... -> ... IO Block: 4096

This essentially comes from the filesystem. ( In short, fs/stat.c shows that st_blksize comes from the inode, the inode gets it from the superblock, and the superblock normally has it as a power of two capped at PAGE_SIZE, i.e. the memory page size. )

So basically this boils down to a memory page size. Good to know.

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PS. This is for example how cat determines the optimal buffer size :

• https://unix.stackexchange.com/questions/17295/when-is-dd-suitable-for-copying-data-or-when-are-read-and-write-partial
• https://unix.stackexchange.com/questions/245499/how-does-cat-know-the-optimum-block-size-to-use

-- and it does not perform that bad at all:

• https://unix.stackexchange.com/questions/9432/is-there-a-way-to-determine-the-optimal-value-for-the-bs-parameter-to-dd/9492#9492