quoted

Date: Sat, 31 May 2014 16:40:29 +0900
From: Namjae Jeon <redacted>
To: 'Lukáš Czerner' <redacted>
Cc: 'Dave Chinner' <david@fromorbit.com>, 'Theodore Ts'o' <tytso@mit.edu>,
    'linux-ext4' [off-list ref], xfs@oss.sgi.com,
    linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
    'Ashish Sangwan' [off-list ref]
Subject: RE: [PATCH v2 0/10] fs: Introduce FALLOC_FL_INSERT_RANGE for
    fallocate

quoted

quoted

Date: Thu, 08 May 2014 19:23:19 +0900
From: Namjae Jeon <redacted>
To: Dave Chinner <david@fromorbit.com>, Theodore Ts'o <tytso@mit.edu>
Cc: linux-ext4 <redacted>, xfs@oss.sgi.com,
    linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
    Ashish Sangwan [off-list ref]
Subject: [PATCH v2 0/10] fs: Introduce FALLOC_FL_INSERT_RANGE for fallocate

In continuation of the work of making the process of non linear editing of
media files faster, we introduce here the new flag FALLOC_FL_INSERT_RANGE
for fallocate.

This flag will work opposite to the newly added FALLOC_FL_COLLAPSE_RANGE flag.
As such, specifying FALLOC_FL_INSERT_RANGE flag will insert zeroed-out space
in between the file within the range specified by offset and len. User can
write new data in this space. e.g. ads.
Like collapse range, currently we have the limitation that offset and len should
be block size aligned for both XFS and Ext4.

The semantics of the flag are :
1) It allocates new zeroed out on disk space of len bytes starting
   at offset byte without overwriting any existing data. All the data blocks
   from offset to EOF are shifted towards right to make space for inserting
   new blocks

Hi,

this sounds a little bit weird to me. I understand the reason for
this, but this is effectively two operations masking as one. We
shift the existing data and then we allocate unwritten extents for
the hole we've created.

What would make more sense to me is to implement only the first
operation - the shift. And then let the user to allocate unwritten
extents for the hole using simple fallocate.

The reason is that if you succeed the first part and then fail the
second due to ENOSPC or any other reason the file will end up in
undefined state unnecessarily. Yes in your current implementation
it seems that you'll always end up with the hole in the file and the
rest is properly shifted, but that may vary from file system to file
system. Some might choose to roll back the shift, some might not.

If FALLOC_FL_INSERT_RANGE (or any name you wish to choose) would
just simply shift the extents then you'll get rid of this and the
only thing that user needs to do (if he chooses to) is to use
fallocate for the hole created by the shift. If it fails, then
well, he can try again without any consequences. As a bonus you get
the possibility to leave the hole in the file which might be useful
as well.

With current behaviour this might get very confusing very quickly.

What do you and others think ?

Hi Lukas.
Insert range inherently means inserting a real range of space into
the file to provide guaranteed space with user in the inserted area
so that further writes should not fail with an -ENOSPC at least.
If insert range cannot gurantees the above semantics, It should
return error to user space.

So what will happen when there is not enough space when "inserting a
range" ? And how should user proceed from there ?

If insert range fails with an ENOSPC error, user could use collapse
range on the same range to remove the hole.
And after freeing more space, he can again try inserting range.
Ofcourse, this type of guidance should be properly documented in
manpage. When updating fallocate(2) manpage, I will keep  in mind to
describe ENOSPC handling.

quoted

If insert range has been performed on a file, It will reserve space
that write never fail in the inserted area,
In case of full partition or small available size than the range
user want, It seems odd just only left inserting a hole in the middle
of file and return success to user when no one can really write to
this hole.

There is a fallocate for allocation, so as I already said you can
shift the extents to make a hole in the file and then use fallocate
to allocate space for it and you'll get the same result. You are
basically doing that now as well, but when the allocation fails the
whole "insert range" ioct fails, however the extents are already
shifter and there is already a holi in the file so freeing some
space and running this ioctl again will not help you at all. While
if you fail a fallocate, you can free some space and run it again
without any problems. The result will be as expected.

What I am arguing about is basically that your insert range ioct is
masking two operations as one. Why not to make it transparent and
split it into "shift extents" and fallocate ? Then there is a
question about the name because it's no longer "insert range" but
rather "insert hole" which I think is better and arguably more
useful semantic.

The same thoeory can be argued against collapse range semantics too.
One can argue that collapse range should not remove any data blocks
from the specified range as that can be done by punch hole, so user
should first perform punch hole and then call collapse range to
eliminate the hole.
There are 2 reasons I make insert range to allocate space.
One is to keep insert range behavior as exactly opposite of
the collapse range and it is named as such so that it seems obvious
that it is related with collapse range.
Other one is that,  there will always be need of allocating space for
data  after making hole.  So doing this within insert range is saving
user from making 1 extra sys call.
That said, I agree with you that it is arguable and I am not biased
to this behavior.
Probably, need some more thoughts from other fs people.

Thanks for your opinion!

Thanks!
-Lukas

quoted

Thanks!

quoted

Thanks!
-LUkas

quoted

2) It should be used exclusively. No other fallocate flag in combination.
3) Offset and length supplied to fallocate should be fs block size aligned
   in case of xfs and ext4.
4) Insert range does not work for the case when offset is overlapping/beyond
   i_size. If the user wants to allocate space at the end of file they are
   advised to use either ftruncate(2) or fallocate(2) with mode 0.
5) It increses the size of file by len bytes.


Namjae Jeon (10):
 fs: Add support FALLOC_FL_INSERT_RANGE for fallocate
 xfs: Add support FALLOC_FL_INSERT_RANGE for fallocate
 ext4: Add support FALLOC_FL_INSERT_RANGE for fallocate
 xfsprogs: xfs_io: add finsert command for insert range via fallocate
 xfstests: generic/027: Standard insert range tests
 xfstests: generic/028: Delayed allocation insert range
 xfstests: generic/029: Multi insert range tests
 xfstests: generic/030: Delayed allocation multi insert
 xfstests: fsstress: Add fallocate insert range operation
 xfstests: fsx: Add fallocate insert range operation

_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs