Re: Increase in XFS journal flushes with (direct_write;fdatasync)+

[Pankaj Raghav <pankaj.raghav@linux.dev>]

>> The comment needs to explain the why and not the how.   AFAICS the
>> why is that lazytime is not propagated to the disk at this mount,
>> so incrementing i_version should not happen, but I'm adding Jeff
>> for insights.
>>
>>
> 
> That looks correct to me. I think the logic here is:
> 
> If we're going to disk anyway, then we might as well force an i_version
> update. In the case where we're not (dirty == I_DIRTY_TIME), then we
> only want to do an i_version update if someone has queried it. If an
> i_version update does occur, then we need to go to disk by setting
> I_DIRTY_SYNC.

Does this reflect the why:

diff --git a/fs/inode.c b/fs/inode.c
index 6a3cbc7dcd28..62c579a0cf7d 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2124,7 +2124,13 @@ static int inode_update_cmtime(struct inode *inode, unsigned int flags)
                            inode_iversion_need_inc(inode))
                                return -EAGAIN;
                } else {
-                       if (inode_maybe_inc_iversion(inode, !!dirty))
+                       /*
+                        * Don't force iversion increment for pure lazytime
+                        * updates (I_DIRTY_TIME only), let I_VERSION_QUERIED
+                        * dictate whether the increment is needed.
+                        */
+                       if (inode_maybe_inc_iversion(inode,
+                                                    dirty != I_DIRTY_TIME))
                                dirty |= I_DIRTY_SYNC;
                }
        }


--
Pankaj