From 92821e2ba4ae26887223326fb0b95cdab963b768 Mon Sep 17 00:00:00 2001 From: David Chinner Date: Thu, 24 May 2007 15:26:31 +1000 Subject: [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 Signed-off-by: Christoph Hellwig Signed-off-by: Tim Shimmin --- fs/xfs/xfs_sb.h | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) (limited to 'fs/xfs/xfs_sb.h') diff --git a/fs/xfs/xfs_sb.h b/fs/xfs/xfs_sb.h index 467854b45c8f..ef42537a607a 100644 --- a/fs/xfs/xfs_sb.h +++ b/fs/xfs/xfs_sb.h @@ -74,12 +74,13 @@ struct xfs_mount; */ #define XFS_SB_VERSION2_REALFBITS 0x00ffffff /* Mask: features */ #define XFS_SB_VERSION2_RESERVED1BIT 0x00000001 -#define XFS_SB_VERSION2_RESERVED2BIT 0x00000002 +#define XFS_SB_VERSION2_LAZYSBCOUNTBIT 0x00000002 /* Superblk counters */ #define XFS_SB_VERSION2_RESERVED4BIT 0x00000004 #define XFS_SB_VERSION2_ATTR2BIT 0x00000008 /* Inline attr rework */ #define XFS_SB_VERSION2_OKREALFBITS \ - (XFS_SB_VERSION2_ATTR2BIT) + (XFS_SB_VERSION2_LAZYSBCOUNTBIT | \ + XFS_SB_VERSION2_ATTR2BIT) #define XFS_SB_VERSION2_OKSASHFBITS \ (0) #define XFS_SB_VERSION2_OKREALBITS \ @@ -181,6 +182,9 @@ typedef enum { #define XFS_SB_SHARED_VN XFS_SB_MVAL(SHARED_VN) #define XFS_SB_UNIT XFS_SB_MVAL(UNIT) #define XFS_SB_WIDTH XFS_SB_MVAL(WIDTH) +#define XFS_SB_ICOUNT XFS_SB_MVAL(ICOUNT) +#define XFS_SB_IFREE XFS_SB_MVAL(IFREE) +#define XFS_SB_FDBLOCKS XFS_SB_MVAL(FDBLOCKS) #define XFS_SB_FEATURES2 XFS_SB_MVAL(FEATURES2) #define XFS_SB_NUM_BITS ((int)XFS_SBS_FIELDCOUNT) #define XFS_SB_ALL_BITS ((1LL << XFS_SB_NUM_BITS) - 1) @@ -188,7 +192,7 @@ typedef enum { (XFS_SB_UUID | XFS_SB_ROOTINO | XFS_SB_RBMINO | XFS_SB_RSUMINO | \ XFS_SB_VERSIONNUM | XFS_SB_UQUOTINO | XFS_SB_GQUOTINO | \ XFS_SB_QFLAGS | XFS_SB_SHARED_VN | XFS_SB_UNIT | XFS_SB_WIDTH | \ - XFS_SB_FEATURES2) + XFS_SB_ICOUNT | XFS_SB_IFREE | XFS_SB_FDBLOCKS | XFS_SB_FEATURES2) /* @@ -414,6 +418,12 @@ static inline int xfs_sb_version_hasmorebits(xfs_sb_t *sbp) * ((sbp)->sb_features2 & XFS_SB_VERSION2_FUNBIT) */ +static inline int xfs_sb_version_haslazysbcount(xfs_sb_t *sbp) +{ + return (XFS_SB_VERSION_HASMOREBITS(sbp) && \ + ((sbp)->sb_features2 & XFS_SB_VERSION2_LAZYSBCOUNTBIT)); +} + #define XFS_SB_VERSION_HASATTR2(sbp) xfs_sb_version_hasattr2(sbp) static inline int xfs_sb_version_hasattr2(xfs_sb_t *sbp) { -- cgit v1.2.3