[XFS] Lazy Superblock Counters

When we have a couple of hundred transactions on the fly at once, they all
typically modify the on disk superblock in some way.
create/unclink/mkdir/rmdir modify inode counts, allocation/freeing modify
free block counts.

When these counts are modified in a transaction, they must eventually lock
the superblock buffer and apply the mods. The buffer then remains locked
until the transaction is committed into the incore log buffer. The result
of this is that with enough transactions on the fly the incore superblock
buffer becomes a bottleneck.

The result of contention on the incore superblock buffer is that
transaction rates fall - the more pressure that is put on the superblock
buffer, the slower things go.

The key to removing the contention is to not require the superblock fields
in question to be locked. We do that by not marking the superblock dirty
in the transaction. IOWs, we modify the incore superblock but do not
modify the cached superblock buffer. In short, we do not log superblock
modifications to critical fields in the superblock on every transaction.
In fact we only do it just before we write the superblock to disk every
sync period or just before unmount.

This creates an interesting problem - if we don't log or write out the
fields in every transaction, then how do the values get recovered after a
crash? the answer is simple - we keep enough duplicate, logged information
in other structures that we can reconstruct the correct count after log
recovery has been performed.

It is the AGF and AGI structures that contain the duplicate information;
after recovery, we walk every AGI and AGF and sum their individual
counters to get the correct value, and we do a transaction into the log to
correct them. An optimisation of this is that if we have a clean unmount
record, we know the value in the superblock is correct, so we can avoid
the summation walk under normal conditions and so mount/recovery times do
not change under normal operation.

One wrinkle that was discovered during development was that the blocks
used in the freespace btrees are never accounted for in the AGF counters.
This was once a valid optimisation to make; when the filesystem is full,
the free space btrees are empty and consume no space. Hence when it
matters, the "accounting" is correct. But that means the when we do the
AGF summations, we would not have a correct count and xfs_check would
complain. Hence a new counter was added to track the number of blocks used
by the free space btrees. This is an *on-disk format change*.

As a result of this, lazy superblock counters are a mkfs option and at the
moment on linux there is no way to convert an old filesystem. This is
possible - xfs_db can be used to twiddle the right bits and then
xfs_repair will do the format conversion for you. Similarly, you can
convert backwards as well. At some point we'll add functionality to
xfs_admin to do the bit twiddling easily....

SGI-PV: 964999
SGI-Modid: xfs-linux-melb:xfs-kern:28652a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/fs/xfs/xfs_ag.h b/fs/xfs/xfs_ag.h
index 9ece7f87ec5b..b1dd0029c60e 100644
--- a/fs/xfs/xfs_ag.h
+++ b/fs/xfs/xfs_ag.h
@@ -68,6 +68,7 @@ typedef struct xfs_agf {
         __be32          agf_flcount;    /* count of blocks in freelist */
         __be32          agf_freeblks;   /* total free blocks */
         __be32          agf_longest;    /* longest free space */
+        __be32          agf_btreeblks;  /* # of blocks held in AGF btrees */
 } xfs_agf_t;
 
 #define XFS_AGF_MAGICNUM        0x00000001
@@ -81,7 +82,8 @@ typedef struct xfs_agf {
 #define XFS_AGF_FLCOUNT         0x00000100
 #define XFS_AGF_FREEBLKS        0x00000200
 #define XFS_AGF_LONGEST         0x00000400
-#define XFS_AGF_NUM_BITS        11
+#define XFS_AGF_BTREEBLKS       0x00000800
+#define XFS_AGF_NUM_BITS        12
 #define XFS_AGF_ALL_BITS        ((1 << XFS_AGF_NUM_BITS) - 1)
 
 /* disk block (xfs_daddr_t) in the AG */
@@ -186,11 +188,13 @@ typedef struct xfs_perag
         __uint32_t      pagf_flcount;   /* count of blocks in freelist */
         xfs_extlen_t    pagf_freeblks;  /* total free blocks */
         xfs_extlen_t    pagf_longest;   /* longest free space */
+        __uint32_t      pagf_btreeblks; /* # of blocks held in AGF btrees */
         xfs_agino_t     pagi_freecount; /* number of free inodes */
+        xfs_agino_t     pagi_count;     /* number of allocated inodes */
+        int             pagb_count;     /* pagb slots in use */
 #ifdef __KERNEL__
         lock_t          pagb_lock;      /* lock for pagb_list */
 #endif
-        int             pagb_count;     /* pagb slots in use */
         xfs_perag_busy_t *pagb_list;    /* unstable blocks */
 } xfs_perag_t;