When we have a big file on an ext4 partition, and filefrag shows the following: filefrag -ve /bigfile Filesystem type is: ef53 File size of /bigfile is 1439201280 (351368 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 32767: 34816.. 67583: 32768: 1: 32768.. 63487: 67584.. 98303: 30720: 2: 63488.. 96255: 100352.. 133119: 32768: 98304: 3: 96256.. 126975: 133120.. 163839: 30720: 4: 126976.. 159743: 165888.. 198655: 32768: 163840: 5: 159744.. 190463: 198656.. 229375: 30720: 6: 190464.. 223231: 231424.. 264191: 32768: 229376: 7: 223232.. 253951: 264192.. 294911: 30720: 8: 253952.. 286719: 296960.. 329727: 32768: 294912: 9: 286720.. 319487: 329728.. 362495: 32768: 10: 319488.. 351367: 362496.. 394375: 31880: last,eof /bigfile: 5 extents found 1. How many fragments does this file really have? 11 or 5? 2. Should the extents 0 and 1 be treated as one fragment or two separate ones? I know they could be one from the human perspective, but is it really one for ext4 filesystem? 3. What does actually happen during the read in the case of some HDD and its magnetic heads? If the head finishes reading the whole extent (ext 0), will it be able to read the data of the next extent (ext 1) without any delays like in the case of raw read (for instance dd if=/dev/sda ...), or will it be delayed because of the filesystem layer, and the head will have to spend some time to be positioned again in order to read the next extent?
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