Re: [PATCH v6 6/7] xfs: support shrinking unused space in the last AG

Hi Darrick,

On Wed, Feb 03, 2021 at 10:12:11AM -0800, Darrick J. Wong wrote:

On Wed, Feb 03, 2021 at 10:51:46PM +0800, Gao Xiang wrote:

...

quoted

quoted

quoted

+		}
+
 		if (error)
 			goto out_trans_cancel;
 	}

@@ -137,15 +157,15 @@ xfs_growfs_data_private(
 	 */
 	if (nagcount > oagcount)
 		xfs_trans_mod_sb(tp, XFS_TRANS_SB_AGCOUNT, nagcount - oagcount);
-	if (nb > mp->m_sb.sb_dblocks)
+	if (nb != mp->m_sb.sb_dblocks)
 		xfs_trans_mod_sb(tp, XFS_TRANS_SB_DBLOCKS,
 				 nb - mp->m_sb.sb_dblocks);

Maybe use delta here?

The reason is the same as above, `delta' here was changed due to 
xfs_resizefs_init_new_ags(), which is not nb - mp->m_sb.sb_dblocks
anymore. so `extend` boolean is used (rather than just use delta > 0)

Long question:

The reason why we use (nb - dblocks) is because growfs is an all or
nothing operation -- either we succeed in writing new empty AGs and
inflating the (former) last AG of the fs, or we don't do anything at
all.  We don't allow partial growing; if we did, then delta would be
relevant here.  I think we get away with not needing to run transactions
for each AG because those new AGs are inaccessible until we commit the
new agcount/dblocks, right?

In your design for the fs shrinker, do you anticipate being able to
eliminate all the eligible AGs in a single transaction?  Or do you
envision only tackling one AG at a time?  And can we be partially
successful with a shrink?  e.g. we succeed at eliminating the last AG,
but then the one before that isn't empty and so we bail out, but by that
point we did actually make the fs a little bit smaller.

Thanks for your question. I'm about to sleep, I might try to answer
your question here.

As for my current experiement / understanding, I think eliminating all
the empty AGs + shrinking the tail AG in a single transaction is possible,
that is what I'm done for now;
 1) check the rest AGs are empty (from the nagcount AG to the oagcount - 1
    AG) and mark them all inactive (AGs freezed);
 2) consume an extent from the (nagcount - 1) AG;
 3) decrease the number of agcount from oagcount to nagcount.

Both 2) and 3) can be done in the same transaction, and after 1) the state
of such empty AGs is fixed as well. So on-disk fs and runtime states are
all in atomic.

There's this comment at the bottom of xfs_growfs_data() that says that
we can return error codes if the secondary sb update fails, even if the
new size is already live.  This convinces me that it's always been the
case that callers of the growfs ioctl are supposed to re-query the fs
geometry afterwards to find out if the fs size changed, even if the
ioctl itself returns an error... which implies that partial grow/shrink
are a possibility.

I didn't realize that possibility but if my understanding is correct
the above process is described as above so no need to use incremental
shrinking by its design. But it also support incremental shrinking if
users try to use the ioctl for multiple times.

If I'm wrong, kindly point out, many thanks in advance!

Thanks,
Gao Xiang