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What is the significance of the file system block size? If my filesystem block size is set at, say 8K, does that mean that all read/write I/O will happen at size 8K? So if my application wants to read say 16 bytes at offset 4097 then a 4K block starting from offset 4096 will be read?
How do writes work in this case? Suppose I want to write say 64 bytes.
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To choose the best logical block size for your system, consider both the performance you want and the available space. For most UFS systems, an 8-Kbyte file system provides the best performance, offering a good balance between disk performance and the use of space in primary memory and on disk.
If the size is too small, we must do more I/O to read the same information, which can slow down the system. If the size is too large, then we get internal fragmentation in each block, thus wasting disk space. The average size of the system's files also affects this problem.
The size of a subblock and the number of subblocks in a block are determined by the block size. IBM Spectrum Scale™ supports the following file system block sizes: 64 KiB, 128 KiB, 256 KiB, 512 KiB, 1 MiB, 2 MiB, 4 MiB, 8 MiB and 16 MiB (for IBM Spectrum Scale RAID only).
The default block size is 1024 bytes for file systems smaller than 1 TB, and 8192 bytes for file systems 1 TB or larger. Choose a block size based on the type of application being run. For example, if there are many small files, a 1 KB block size may save space.
You are right. The block size is the unit of work for the file system. Every read and write is done in full multiples of the block size.
The block size is also the smallest size on disk a file can have. If you have a 16 byte Block size,then a file with 16 bytes size occupies a full block on disk.
The book "Practical file system design" states:
Block: The smallest unit writable by a disk or file system. Everything a file system does is composed of operations done on blocks. A file system block is always the same size as or larger (in integer multiples) than the disk block size.
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