linux-ext4 - Question about EXT4_IOC_MOVE

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Date:	Sun, 20 Jul 2014 13:16:45 +0300
From:	Timofey Titovets <nefelim4ag@...il.com>
To:	linux-ext4@...r.kernel.org
Subject: Question about EXT4_IOC_MOVE_EXT

Good time of day, i am working for optimization of readahead in
systemd and now, i try to implement EXT4_IOC_MOVE_EXT ioctl. I have
several questions:

1. In fs every file has a file descriptor, file descriptor is a
integer number. Can we say what, by file descriptor, we can determine
order of files where he places on a disk?
As example: a - 5; b - 54; c - 31; d - 20;
This mean what files place order like:
start of disk -> a -> d ->  c -> b -> end of disk
and if i swap files by EXT4_IOC_MOVE_EXT, i can reorder it to:
start of disk -> a -> b -> c -> d -> end of disk
This is true?

2. Can i pack files in custom order on a disk? (i have order of
accessing files on a disk, and i want place him in accessing order
like e4rat project do)
If yes, how? May be we have ioctl call or e2fsprogs has realisation of this?
( e4rat as i understand by looking source, patch kernel, to get
features of files reorder)

3. In
struct move_extent {
    __s32 reserved;    /* original file descriptor */
    __u32 donor_fd;    /* donor file descriptor */
    __u64 orig_start;  /* logical start offset in block for orig */
    __u64 donor_start; /* logical start offset in block for donor */
    __u64 len;         /* block length to be moved */
    __u64 moved_len;   /* moved block length */
};
Can i ignore (not set) orig_start, donor_start, len, moved_len?
What happen if i set it by zeros?
if i move original file descriptor to file descriptor with zero size
of file, what happen? or in begger file donor then original?

-----
Best regards,
Timofey.
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