Date:   Mon, 5 Aug 2019 17:37:49 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, chao@...nel.org
Subject: Re: [PATCH v2] f2fs: separate NOCoW and pinfile semantics

On 08/02, Chao Yu wrote:
> On 2019/8/2 6:27, Jaegeuk Kim wrote:
> > On 08/01, Chao Yu wrote:
> >> On 2019/8/1 12:14, Jaegeuk Kim wrote:
> >>> On 07/31, Chao Yu wrote:
> >>>> On 2019/7/31 2:02, Jaegeuk Kim wrote:
> >>>>> On 07/29, Chao Yu wrote:
> >>>>>> On 2019/7/29 13:57, Jaegeuk Kim wrote:
> >>>>>>> On 07/23, Chao Yu wrote:
> >>>>>>>> On 2019/7/23 10:36, Jaegeuk Kim wrote:
> >>>>>>>>> On 07/19, Chao Yu wrote:
> >>>>>>>>>> Pinning a file is heavy, because skipping pinned files make GC
> >>>>>>>>>> running with heavy load or no effect.
> >>>>>>>>>
> >>>>>>>>> Pinned file is a part of NOCOW files, so I don't think we can simply drop it
> >>>>>>>>> for backward compatibility.
> >>>>>>>>
> >>>>>>>> Yes,
> >>>>>>>>
> >>>>>>>> But what I concerned is that pin file is too heavy, so in order to satisfy below
> >>>>>>>> demand, how about introducing pin_file_2 flag to triggering IPU only during
> >>>>>>>> flush/writeback.
> >>>>>>>
> >>>>>>> That can be done by cold files?
> >>>>>>
> >>>>>> Then it may inherit property of cold type file, e.g. a) goes into cold area; b)
> >>>>>> update with very low frequency.
> >>>>>>
> >>>>>> Actually pin_file_2 could be used by db-wal/log file, which are updated
> >>>>>> frequently, and should go to hot/warm area, it does not match above two property.
> >>>>>
> >>>>> How about considering another name like "IPU-only mode"?
> >>>>>
> >>>>>               fallocate         write    Flag         GC
> >>>>> Pin_file:     preallocate       IPU      FS_NOCOW_FL  Not allowed
> >>>>> IPU_file:     Not preallocate   IPU      N/A          Default by temperature
> >>>>
> >>>> One question, do we need preallocate physical block address for IPU_file as
> >>>> Pin_file? since it can enhance db file's sequential read performance, not sure,
> >>>> db can handle random data in preallocated blocks.
> >>>
> >>> db file will do atomic writes, which can not be used with this. -wal may be able
> >>
> >> Now WAL mode were set by default in Android, so most of db file are -wal type now.
> > 
> > Will be back again tho.
> 
> R?

Q.

> 
> > 
> >>
> >>> to preallocate blocks, but it can eat disk space unnecessarily.
> >>
> >> I meant .db-wal file rather than .db.
> >>
> >> Yes, that's ext4 style, that would bring better performance due to less holes in
> >> block distribution.
> >>
> >> I don't think we need to worry about space issue for db-wal file. I tracked
> >> .db-wal file's update before:
> >> - there are very frequently truncation and deletion, that means the preallocated
> >> blocks won't exist for long time.
> >> - and also there are very frequently append writes, I suspect there almost very
> >> few preallocate block are not written.
> >> - total db-wal file number is less.
> > 
> > Sometimes it can be large enough for system.
> 
> For this, it's trade off:
> - lose a few disk space at the very begin of db-wal lifecycle Or
> - face fragment and read performance degradation.
> 
> > If it's from user apps and short lived, why do we need preallocation?
> 
> It triggers sequential read on db-wal file during checkpoint, though it's short
> lived, still it can affect performance.
> 
> What do you think of doing some performance test on WAL file to decide the
> preallocation policy?

Good idea. Can we?

> 
> Thanks,
> 
> > 
> >>
> >>>
> >>>>
> >>>> Other behaviors looks good to me. :)
> >>>>
> >>>> I plan to use last bit in inode.i_inline to store this flag.
> >>>
> >>> Why not using i_flag like FS_NOCOW_FL?
> >>
> >> Oops, as you listed in last email, I can see you don't want to break
> >> FS_NOCOW_FL's semantics for backward compatibility.
> >>
> >> 			Flag
> >> IPU_file		N/A			
> >>
> >> If we plan to use FS_NOCOW_FL, that's what this patch has already did, you can
> >> merge it directly... :P
> >>
> >>>
> >>>>
> >>>>> Cold_file:    Not preallocate   IPU      N/A          Move in cold area
> >>>>> Hot_file:     Not preallocate   IPU/OPU  N/A          Move in hot area
> >>>>
> >>>> Should hot file be gced to hot area? That would mix new hot data with old 'hot'
> >>>> data which actually become cold.
> >>>
> >>> But, user explicitly specified this is hot.
> >>
> >> With current implementation, GC will migrate data from hot/warm/cold area to
> >> cold area.
> >>
> >> Thanks,
> >>
> >>>
> >>>>
> >>>> Thanks,
> >>>>
> >>>>>
> >>>>>>
> >>>>>> Thank,
> >>>>>>
> >>>>>>>
> >>>>>>>>
> >>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> So that this patch propose to separate nocow and pinfile semantics:
> >>>>>>>>>> - NOCoW flag can only be set on regular file.
> >>>>>>>>>> - NOCoW file will only trigger IPU at common writeback/flush.
> >>>>>>>>>> - NOCow file will do OPU during GC.
> >>>>>>>>>>
> >>>>>>>>>> For the demand of 1) avoid fragment of file's physical block and
> >>>>>>>>>> 2) userspace don't care about file's specific physical address,
> >>>>>>>>>> tagging file as NOCoW will be cheaper than pinned one.
> >>>>>>>>
> >>>>>>>> ^^^
> >>>>>>>>
> >>>>>>>> Thanks,
> >>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >>>>>>>>>> ---
> >>>>>>>>>> v2:
> >>>>>>>>>> - rebase code to fix compile error.
> >>>>>>>>>>  fs/f2fs/data.c |  3 ++-
> >>>>>>>>>>  fs/f2fs/f2fs.h |  1 +
> >>>>>>>>>>  fs/f2fs/file.c | 22 +++++++++++++++++++---
> >>>>>>>>>>  3 files changed, 22 insertions(+), 4 deletions(-)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> >>>>>>>>>> index a2a28bb269bf..15fb8954c363 100644
> >>>>>>>>>> --- a/fs/f2fs/data.c
> >>>>>>>>>> +++ b/fs/f2fs/data.c
> >>>>>>>>>> @@ -1884,7 +1884,8 @@ static inline bool check_inplace_update_policy(struct inode *inode,
> >>>>>>>>>>  
> >>>>>>>>>>  bool f2fs_should_update_inplace(struct inode *inode, struct f2fs_io_info *fio)
> >>>>>>>>>>  {
> >>>>>>>>>> -	if (f2fs_is_pinned_file(inode))
> >>>>>>>>>> +	if (f2fs_is_pinned_file(inode) ||
> >>>>>>>>>> +			F2FS_I(inode)->i_flags & F2FS_NOCOW_FL)
> >>>>>>>>>>  		return true;
> >>>>>>>>>>  
> >>>>>>>>>>  	/* if this is cold file, we should overwrite to avoid fragmentation */
> >>>>>>>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> >>>>>>>>>> index 596ab3e1dd7b..f6c5a3d2e659 100644
> >>>>>>>>>> --- a/fs/f2fs/f2fs.h
> >>>>>>>>>> +++ b/fs/f2fs/f2fs.h
> >>>>>>>>>> @@ -2374,6 +2374,7 @@ static inline void f2fs_change_bit(unsigned int nr, char *addr)
> >>>>>>>>>>  #define F2FS_NOATIME_FL			0x00000080 /* do not update atime */
> >>>>>>>>>>  #define F2FS_INDEX_FL			0x00001000 /* hash-indexed directory */
> >>>>>>>>>>  #define F2FS_DIRSYNC_FL			0x00010000 /* dirsync behaviour (directories only) */
> >>>>>>>>>> +#define F2FS_NOCOW_FL			0x00800000 /* Do not cow file */
> >>>>>>>>>>  #define F2FS_PROJINHERIT_FL		0x20000000 /* Create with parents projid */
> >>>>>>>>>>  
> >>>>>>>>>>  /* Flags that should be inherited by new inodes from their parent. */
> >>>>>>>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> >>>>>>>>>> index 7ca545874060..ae0fec54cac6 100644
> >>>>>>>>>> --- a/fs/f2fs/file.c
> >>>>>>>>>> +++ b/fs/f2fs/file.c
> >>>>>>>>>> @@ -1692,6 +1692,7 @@ static const struct {
> >>>>>>>>>>  	{ F2FS_NOATIME_FL,	FS_NOATIME_FL },
> >>>>>>>>>>  	{ F2FS_INDEX_FL,	FS_INDEX_FL },
> >>>>>>>>>>  	{ F2FS_DIRSYNC_FL,	FS_DIRSYNC_FL },
> >>>>>>>>>> +	{ F2FS_NOCOW_FL,	FS_NOCOW_FL },
> >>>>>>>>>>  	{ F2FS_PROJINHERIT_FL,	FS_PROJINHERIT_FL },
> >>>>>>>>>>  };
> >>>>>>>>>>  
> >>>>>>>>>> @@ -1715,7 +1716,8 @@ static const struct {
> >>>>>>>>>>  		FS_NODUMP_FL |		\
> >>>>>>>>>>  		FS_NOATIME_FL |		\
> >>>>>>>>>>  		FS_DIRSYNC_FL |		\
> >>>>>>>>>> -		FS_PROJINHERIT_FL)
> >>>>>>>>>> +		FS_PROJINHERIT_FL |	\
> >>>>>>>>>> +		FS_NOCOW_FL)
> >>>>>>>>>>  
> >>>>>>>>>>  /* Convert f2fs on-disk i_flags to FS_IOC_{GET,SET}FLAGS flags */
> >>>>>>>>>>  static inline u32 f2fs_iflags_to_fsflags(u32 iflags)
> >>>>>>>>>> @@ -1753,8 +1755,6 @@ static int f2fs_ioc_getflags(struct file *filp, unsigned long arg)
> >>>>>>>>>>  		fsflags |= FS_ENCRYPT_FL;
> >>>>>>>>>>  	if (f2fs_has_inline_data(inode) || f2fs_has_inline_dentry(inode))
> >>>>>>>>>>  		fsflags |= FS_INLINE_DATA_FL;
> >>>>>>>>>> -	if (is_inode_flag_set(inode, FI_PIN_FILE))
> >>>>>>>>>> -		fsflags |= FS_NOCOW_FL;
> >>>>>>>>>>  
> >>>>>>>>>>  	fsflags &= F2FS_GETTABLE_FS_FL;
> >>>>>>>>>>  
> >>>>>>>>>> @@ -1794,6 +1794,22 @@ static int f2fs_ioc_setflags(struct file *filp, unsigned long arg)
> >>>>>>>>>>  	if (ret)
> >>>>>>>>>>  		goto out;
> >>>>>>>>>>  
> >>>>>>>>>> +	if ((fsflags ^ old_fsflags) & FS_NOCOW_FL) {
> >>>>>>>>>> +		if (!S_ISREG(inode->i_mode)) {
> >>>>>>>>>> +			ret = -EINVAL;
> >>>>>>>>>> +			goto out;
> >>>>>>>>>> +		}
> >>>>>>>>>> +
> >>>>>>>>>> +		if (f2fs_should_update_outplace(inode, NULL)) {
> >>>>>>>>>> +			ret = -EINVAL;
> >>>>>>>>>> +			goto out;
> >>>>>>>>>> +		}
> >>>>>>>>>> +
> >>>>>>>>>> +		ret = f2fs_convert_inline_inode(inode);
> >>>>>>>>>> +		if (ret)
> >>>>>>>>>> +			goto out;
> >>>>>>>>>> +	}
> >>>>>>>>>> +
> >>>>>>>>>>  	ret = f2fs_setflags_common(inode, iflags,
> >>>>>>>>>>  			f2fs_fsflags_to_iflags(F2FS_SETTABLE_FS_FL));
> >>>>>>>>>>  out:
> >>>>>>>>>> -- 
> >>>>>>>>>> 2.18.0.rc1
> >>>>>>>>> .
> >>>>>>>>>
> >>>>>>> .
> >>>>>>>
> >>>>> .
> >>>>>
> >>> .
> >>>
> > .
> > 

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