Date:	Tue, 21 Apr 2015 12:56:13 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/3] ext4: Speedup ext4 orphan inode handling

On Mon 20-04-15 10:35:01, Andreas Dilger wrote:
> On Apr 20, 2015, at 6:25 AM, Jan Kara <jack@...e.cz> wrote:
> > On Fri 17-04-15 17:53:03, Andreas Dilger wrote:
> >> On Apr 16, 2015, at 9:42 AM, Jan Kara <jack@...e.cz> wrote:
> >>> Signed-off-by: Jan Kara <jack@...e.cz>
> >>> ---
> >>> fs/ext4/ext4.h  |  52 +++++++++++--
> >>> fs/ext4/namei.c |  95 +++++++++++++++++++++--
> >>> fs/ext4/super.c | 237 ++++++++++++++++++++++++++++++++++++++++++++++++--------
> >>> 3 files changed, 341 insertions(+), 43 deletions(-)
> >>> 
> >>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> >>> index abed83485915..768a8b9ee2f9 100644
> >>> --- a/fs/ext4/ext4.h
> >>> +++ b/fs/ext4/ext4.h
> >>> @@ -208,6 +208,7 @@ struct ext4_io_submit {
> >>> #define EXT4_UNDEL_DIR_INO	 6	/* Undelete directory inode */
> >>> #define EXT4_RESIZE_INO		 7	/* Reserved group descriptors inode */
> >>> #define EXT4_JOURNAL_INO	 8	/* Journal inode */
> >>> +#define EXT4_ORPHAN_INO		 9	/* Inode with orphan entries */
> >> 
> >> Contrary to your patch description that said this was using ino #12,
> >> this conflicts with EXT2_EXCLUDE_INO for snapshots.  Why not use
> >> s_last_orphan in the superblock to reference the orphan inode?  Since
> >> this feature already requires a new EXT2_FEATURE_RO_COMPAT flag, the
> >> existing orphan inode number could be reused.  See below how this could
> >> still in the ENOSPC case.
> > 
> >  So I think you misunderstood one thing: Orphan file is statically
> > allocated (when feature gets enabled by mkfs). Kernel doesn't handle
> > resizing in any way - that way we can assume we modify exactly one block to
> > add inode to orphan file.
> 
> You are right, I totally didn't notice this was the case.  I thought the
> orphan inode size would grow as needed by the workload, and the ENOSPC
> handling would only be used if the filesystem was actually out of space.
> If we stick with the static orphan inode size, it makes sense to add a
> comment explaining this clearly.
  I'll add a comment at ENOSPC handling.

> > If we run out of space in orphan file - but
> > note that e.g. if you have 128 KB orphan file, it can contain 32768 orphan
> > entries and you never have that many orphan inodes at once under normal
> > circumstances - we fall back to old style orphan list. So if you have more
> > orphan inodes than you expected when creating orphan file, the fs still
> > handles that gracefully, it just falls back to the old unscalable code.
> 
> What I was thinking is that the orphan inode would always be linked from
> the superblock s_last_orphan if FEATURE_RO_COMPAT_ORPHAN_INODE is set:
> 
> ext4_super_block[s_last_orphan] -> orphan_inode[i_dtime] [ ->orphan list ]
> 
> Then, if no space is available to grow the orphan inode it would hook
> orphans off i_dtime from the orphan inode as needed as is done today.
  But what is the advantage of doing this? It would possibly remove the
need to increase the number of reserved inodes, that's about it. But it
would add some special cases to orphan handling instead of just being able
to use the old code as is. Furthermore the orphan file feature would no
longer be a compat one AFAIU your proposal. So IMHO increasing the number
of special inodes and using one of them is the easiest solution.
 
> > IMHO adding code to grow orphan file isn't simply worth it (but we can do
> > it if we decide so in the future). Also we have to keep code to handle old
> > style orphan list anyway in kernel so the fallback doesn't really incurr
> > any significant additional complexity.
> 
> How big is the orphan file by default?  I'd think it will be a bit tricky
> to get this right, since it depends on both the size of the filesystem
> (e.g. you don't want 128KB orphan file on a 1.44MB floppy) and the number
> of cores.  Given that there is already existing code to handle extending
> a file easily (e.g. ext4_append()) I don't think that is too hard.  Then
> the orphan inode will only grow as large as needed.
  Well, I wouldn't be worried about 1.44MB floppies :) You likely won't
format them with a journal (and thus orphan file) anyway. I'd base default
orphan file size just on the fs size. For filesystems in the 'floppy' /
'small' profile I'd disable the feature by default. There we care more about
size than about scalability. For filesystems 512 MB large I'd use 128 KB
orphan file size and growing the orphan file size linearly with fs size
upto 8 GB where orphan file size will be 2 MB - that's enough for 512 CPUs
to operate in parallel which should be enough for a few coming years
(growing orphan file size is easy if you later find out it isn't enough).

> It could also detect SMP collisions by whether the buffer heads are locked
> already, and then allocate a new block to reduce contention.  That way we
> get just the right SMP scalability for the system and workload being run.
  All this is possible but I would like to avoid overengineering it. So I
would wait until I see users who actually need this. Kernel has orphan file
under its control and if we later decide kernel should really handle
growing / shrinking it, we can add that feature without any change to the
on-disk format.

> >> What do you think about making the on-disk orphan inode numbers store
> >> 64-bit values?  That would be easy to do now, and would avoid a format
> >> change in the future if we wanted to use 64-bit inodes.
> >> 
> >> That said, if the orphan inode is deleted after orphan recovery (see
> >> more below) the only thing needed for compatibility is to store the
> >> inode number size into the orphan inode somewhere so it could be changed.
> >> Maybe i_version and/or i_generation since they are not directly user
> >> accessible.
> > 
> >  So orphan entry is cleared once inode isn't orphan anymore. So a clean
> > filesystem currently has completely zeroed out orphan file. Switching to
> > 64-bit inode numbers would be trivial then and you can just pick the format
> > of the orphan file based on the 64BIT_INODE incompat feature we'll have to
> > have in sb anyway. So I don't think we need to do anything in that regard
> > now.
> 
> But if someone wants to enable 64BIT_INODE then they need to set this flag
> on the superblock, and it would confuse the kernel to thinking that the
> orphan inode has 64-bit inode numbers, when it still only has 32-bit inodes.
  So I'm bit confused. When you set 64BIT_INODE flag, you still need to
walk over all the directory structure and convert all the directories. Also
you presumably enforce the filesystem is clean. At that point the orphan file
is full of zeros so when you mount the fs, kernel will just start looking
at those zeros as 64-bit numbers which is fine. When we have inode number
size also stored within the orphan file, we have to explicitely convert it.

> It seems safer to store the inode number size with the orphan inode.  One
> option is to put it in the low byte of the proposed per-block magic, so if
> the inode number size changes the magic will change as well.
  So I don't really mind having inode number as a part of magic but I'm
just wondering about the advantage...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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