linux-ext4 - Re: Question about ext4 extents and file fragmentation

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Message-ID: <d32355a6-19b3-7861-9323-a761ea02d9f5@gmail.com>
Date:   Thu, 21 Mar 2019 16:59:34 +0100
From:   Mikhail Morfikov <mmorfikov@...il.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: Question about ext4 extents and file fragmentation

On 21/03/2019 16:05, Theodore Ts'o wrote:
> It *could* be read sequentially from the beginning of the file to the
> end.  There are many things that might cause that not to happen, that
> have nothing to do with how we store the logical to physicla map.

And this is what I wanted to know, because some people tell that if you 
store a file in a filesystem, it can't be read sequentially as a whole 
because of the filesystem layer (compared to "dd if=/dev/sda ..."). So 
the filesystem layer doesn't really matter and doesn't really add any 
additional delays compared to the raw read of a device when we deal with 
data that is stored in contiguous blocks. I know that many things can 
prevent the sequential read from happening, but I just wanted it to be 
clarified. 

Thank you for the answer, I really appreciate it.

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