linux-ext4 - Re: [PATCH] ext4: xattr-in-inode support

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Date:   Fri, 21 Apr 2017 10:54:23 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        James Simmons <jsimmons@...radead.org>, tahsin@...gle.com,
        nauman@...gle.com, Theodore Tso <tytso@...gle.com>
Subject: Re: [PATCH] ext4: xattr-in-inode support

On Fri, Apr 21, 2017 at 12:22 AM, Andreas Dilger <adilger@...ger.ca> wrote:
> On Apr 20, 2017, at 1:58 AM, Jan Kara <jack@...e.cz> wrote:
>>
[...]
>> One idea I had in mind was that one way of supporting larger xattrs would
>> be to support something like xattr fork - i.e., in the xattr space of the
>> inode we would have root of an extent tree describing xattr space of the
>> inode. Then inside the space described by the extent tree would be stored
>> xattrs - possibly in the same format as they are currently stored in a
>> block (we would just redefine that e_value_block+e_value_offs describe the
>> offset of xattr value inside the xattr space).

BTW, 'xattr fork' is the xfs way AFAIK and btrfs has 'xattr inodes'.

>
> Yes, this is what I was trying to get at with my previous email as well.
> There isn't much difference between allocating a bunch of blocks directly
> as the xattr space vs. an inode that is allocating those blocks.  The main
> difference from the current xattr inode implementation is that this packs
> multiple xattrs into a single inode, while the current code only stores a
> single value starting at offset=0, without any header.
>

That's not the only difference, is it?
Current ea-in-inode code can allocate many xattr inodes per regular inode.
'xattr fork' is equivalent to allocating a single 'xattr inode' per
regular inode.

I wonder if one-xattr-inode and one-EA-per-block can work out:
- EA block cannot have more than 1 EA
- EA can span more than 1 EA block (i.e. compound EA block)
- Refcounting is in the EA block as it is now, but may refcount a compund block
- inode A may have a reference to xattr-inode Ax, which is the host for writing
  new unshared EAs for inode A
- EA of inode B may have a shared EA with e_value_block pointing at block
  of xattr-inode Ax
- When refcount of any EA (compound) block drops to zero, punch holes in
  the xattr-inode hosting these blocks
- When inode A is deleted, it drops refcount on all the EA blocks its EAs
  are referencing
- If xattr-inode Ax has remaining EA block when inode A is going away,
  it transitions into a 'shared xattr-inode' and lives on the orphan list, or
  another dedicated list, until its own blocks count drops to zero.

I probably missed some details, maybe important ones as well,
but if I haven't, then this could reuse some of the existing EA dedup
code and cap the inodes overhead significantly (*).

(*) shared xattr-inodes may be compacted by handing their blocks over to
     a dedicated SHARED_XATTR_INO or to any random xattr-inode victim
     for that matter (i.e. of root inode).

Amir.