ext3 block size question

Hello,

I was measuring the performance of ext3 file system. As the block size increases,read and write performance improves. why there is a better performance improvement for large block sizes??.

Thanks
Jayshankar

So it's my understanding that by increasing the block size, you guarantee the contiguity of files. For instance, GPFS uses a 256K block to achieve high performance for parallel and cluster computing applications. It essentially reduces the number of seeks required to read a large file. (Seek time being one of the largest components of IO latency.)

There's a drawback, however, in that large blocks incur internal fragmentation, or the unused storage space contained in a block. Consider the 1K file being backed by 256K block. Also, there's poor integration with the page cache and virtual memory by virtue of a mismatch in page/block sizes. Large blocks have to be pined in memory or the memory system has to perform read-modify-write.

This is why many file systems opt for a block size that's small, usually the same size or smaller than the page size. This puts a lot more importance on allocation techniques to insure block contiguity.

Zachary

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