linux-ext4 - Question about ext4 extents and file fragmentation

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Date:   Wed, 20 Mar 2019 23:44:19 +0100
From:   Mikhail Morfikov <mmorfikov@...il.com>
To:     linux-ext4@...r.kernel.org
Subject: Question about ext4 extents and file fragmentation

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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