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Date:	Sun, 23 Jun 2013 11:00:51 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	"tytso@....edu" <tytso@....edu>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"a.sangwan@...sung.com" <a.sangwan@...sung.com>,
	Namjae Jeon <linkinjeon@...il.com>,
	Namjae Jeon <namjae.jeon@...sung.com>
Subject: Re: [PATCH 0/3] ext4: introduce two new ioctls

On 2013-06-23, at 0:07, Namjae Jeon <linkinjeon@...il.com> wrote:

> From: Namjae Jeon <namjae.jeon@...sung.com>
> 
> This patch series introduces 2 new ioctls for ext4.
> 
> Truncate_block_range ioctl truncates blocks from source file.

How is this different from fallocate(FALLOC_FL_PUNCH_HOLE)?  That is already in existing kernels, and portable across multiple filesystems. 

> Transfer_block_range ioctl transfers data blocks from source file
> and append them at the end of destination file.

There is already a similar ioctl for defragmenting files. Is it possible to use that, or does it have different semantics?

> Ioctl1:        EXT4_IOC_TRUNCATE_BLOCK_RANGE:
> This ioctl truncates a range of data blocks from file.
> It is useful to remove easily and quickly the garbage data
> at the middle of file.
> 
> e.g. we have a movie file and there is long advertisement in movie file.
> user want to remove only advertisement range.

While this works in theory, there is very little chance that the movie data will align exactly to filesystem block boundaries. 

Cheers, Andreas 

> 1) Movie file (8GB), There is the adverisement of 500MB size.
> ____________________________________________________________________
> |                     |                   |                         |
> |   a) range          | b) Advertisement  |     c) range            | 
> |                     | (unneeded data)   |                         |
> |_____________________|___________________|_________________________| 
> 
> 2) Currently if user want to remove portion b), the conventional way
>    would be to copy a) and c) (7.5GB) to new file by reading data from
>    original file and writing to new file, followed up by  delete original
>    file and rename new file. It will take long time.
>    When we measure time, it takes around 3 minutes.
> 
> 3) If we use EXT4_IOC_TRUNCATE_BLOCK_RANGE, we can have garbage data removed
>    in less than a second. Also, no need to perform deletion and rename.
> _______________________________________________
> |                     |                        |
> |   a) range          |     c) range           | 
> |                     |                        |
> |_____________________|________________________| 
> 
> 
> #define EXT4_IOC_TRUNCATE_BLOCK_RANGE  _IOW('f', 18, struct truncate_range)
> struct truncate_range {
>       __u32 start_block;
>       __u32 length;
> };
> 
> example =>
> Originally the file "abc" has the below extent tree:
> debugfs:  ex abc
> Level Entries       Logical        Physical Length Flags
> 0/ 0   1/  3     0 -     4  33615 -  33619      5 
> 0/ 0   2/  3     5 -     9  33855 -  33859      5 
> 0/ 0   3/  3    10 -    14  69657 -  69661      5
> 
> ls -lh abc
> -rw-r--r--    1 root     0          60.0K Jan  1 00:01 abc
> 
> du -h abc
> 60.0K    abc
> 
> e4_truncate_block_range abc 2 10
> Return:
> : Success
> 
> After executing truncate_block_range ioctl, the extent tree:
> ex abc
> Level Entries       Logical        Physical Length Flags
> 0/ 0   1/  2     0 -     1  33615 -  33616      2 
> 0/ 0   2/  2     2 -     4  69659 -  69661      3 
> 
> ls -lh abc
> -rw-r--r--    1 root     0          20.0K Jan  1 00:08 abc
> 
> du -h abc
> 20.0K    abc
> 
> This ioctl works in 2 parts:
> 1) remove _only_ data blocks that resides within specified range.
> If the entire range is a hole than nothing is removed.
> 
> 2) update file's logical block offsets ranging from block number
> "start_block + length" to last logical block of file such that
> lblk_number = lblk_number - length;
> This is done by updating starting block of all the extents that
> resides within the range.
> 
> If "start_block + length" is already equal to the last block of file
> than no block is updated. This case is similar to convential truncate.
> 
> In the above example:
> The data blocks ranging from [2 - 11] have been removed
> and the logical offsets of the file beyond block number 12 till last block 
> of file are updated by subtracting length from each of logical numbers.
> This gives a contiguous logical space to the file.
> Also, the logical size and disksize of the file are updated accordingly.
> 
> Ioctl2:        EXT4_IOC_TRANSFER_BLOCK_RANGE:
> This ioctl transfers a range of data blocks from source file and append
> them at the end of the destination file.
> This is not actual data transfer but only metadata is moved.
> 
> ____________________________________________________________________
> |                     |                   |                         |
> |   a) range          | b) range          |     c) range            |
> |                     |                   |                         |
> |_____________________|___________________|_________________________|
> 
> If user does not want b) in the orig file but wants to make a new file
> comprising only b) OR wants b) at the end of an already existing file,
> the conventional way of doing it would be to:
> 1) Copy b) to new file
> 2) Copy c) to temp file
> 3) Truncate orig file to a)
> 4) Copy c) from temp file to the end of orig file.
> 5) Delete temp file.
> 
> After this operations =>
> orig_file:
> __________________________________________
> |                     |                   |
> |   a) range          | c) range          |
> |                     |                   |
> |_____________________|___________________|
> 
> new_file:
> _______________________
> |                     |
> |   b) range          |
> |                     |
> |_____________________|
> 
> Again, this operation would take a long time (depending on the sizes of range)
> if done using conventional way while using transfer_block_range ioctl reduces
> the time within a second.
> 
> #define EXT4_IOC_TRANSFER_BLOCK_RANGE  _IOW('f', 19, struct transfer_range)
> struct transfer_range {
>    __u32 dest_fd;
>    __u32 start_block;
>    __u32 length;
> };
> 
> example=>
> debugfs: ex source
> Level Entries       Logical        Physical Length Flags
> 0/ 1   1/  1     0 -    24  32809              25
> 1/ 1   1/  5     0 -     4   4071 -   4075      5 
> 1/ 1   2/  5     5 -     9   4081 -   4085      5 
> 1/ 1   3/  5    10 -    14   4091 -   4095      5 
> 1/ 1   4/  5    15 -    19   4101 -   4105      5 
> 1/ 1   5/  5    20 -    24   4151 -   4155      5 
> 
> debugfs:  ex dest
> Level Entries       Logical        Physical Length Flags
> 0/ 0   1/  3     0 -     4  32825 -  32829      5 
> 0/ 0   2/  3     5 -     9  33545 -  33549      5 
> 0/ 0   3/  3    10 -    14  33615 -  33619      5
> 
> ls -lh source 
> -rw-r--r--    1 root     0         100.0K Jan  1 00:01 source
> ls -lh dest 
> -rw-r--r--    1 root     0          60.0K Jan  1 00:01 dest
> 
> du -h source 
> 104.0K    source
> du -h dest 
> 60.0K    dest
> 
> e4_transfer_block_range source dest 2 10
> Return:
> : Success
> 
> debugfs:  ex source
> Level Entries       Logical        Physical Length Flags
> 0/ 1   1/  1     0 -    24  32809              25
> 1/ 1   1/  4     0 -     1   4071 -   4072      2 
> 1/ 1   2/  4    12 -    14   4093 -   4095      3 
> 1/ 1   3/  4    15 -    19   4101 -   4105      5 
> 1/ 1   4/  4    20 -    24   4151 -   4155      5 
> debugfs:  ex dest
> Level Entries       Logical        Physical Length Flags
> 0/ 1   1/  1     0 -    24  32835              25
> 1/ 1   1/  6     0 -     4  32825 -  32829      5 
> 1/ 1   2/  6     5 -     9  33545 -  33549      5 
> 1/ 1   3/  6    10 -    14  33615 -  33619      5 
> 1/ 1   4/  6    15 -    17   4073 -   4075      3 
> 1/ 1   5/  6    18 -    22   4081 -   4085      5 
> 1/ 1   6/  6    23 -    24   4091 -   4092      2 
> 
> ls -lh source
> -rw-r--r--    1 root     0         100.0K Jan  1 00:04 source
> ls -lh dest 
> -rw-r--r--    1 root     0         100.0K Jan  1 00:04 dest
> 
> du -h source 
> 64.0K    source
> du -h dest
> 104.0K    dest
> 
> The data blocks lying between [start_block to start_block + length) are appended
> contiguously at the end of destination file.
> The block transfer leaves a hole in the source file.
> If any hole is encountered in the range, it is ommited.
> 
> This ioctl does not change the logical size of the source file hence
> leaves a hole in place of transfered range.
> If user want contiguous logical space for source file,
> it can truncate the hole by calling truncate_range_ioctl for source file.
> 
> Example for above "source" file:
> e4_truncate_block_range source 2 10
> Return:
> : Success
> debugfs:  ex source
> Level Entries       Logical        Physical Length Flags
> 0/ 1   1/  1     0 -    14  32809              15
> 1/ 1   1/  4     0 -     1   4071 -   4072      2 
> 1/ 1   2/  4     2 -     4   4093 -   4095      3 
> 1/ 1   3/  4     5 -     9   4101 -   4105      5 
> 1/ 1   4/  4    10 -    14   4151 -   4155      5 
> 
> Namjae Jeon (3):
>  ext4: Add EXT4_IOC_TRUNCATE_BLOCK_RANGE ioctl
>  ext4: make mext_next_extent non static and move get_ext_path
>  ext4: Add EXT4_IOC_TRANSFER_BLOCK_RANGE ioctl
> 
> -- 
> 1.7.9.5
> 
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