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| author | Darrick J. Wong <darrick.wong@oracle.com> | 2016-10-17 18:26:51 -0700 |
|---|---|---|
| committer | Darrick J. Wong <darrick.wong@oracle.com> | 2016-12-20 22:46:32 -0800 |
| commit | 33bb782ac245caf8b4bc4c167ad662bf32077be5 (patch) | |
| tree | 4a2a0c4ee3a4f6f68befcf2855d4ff36c6b278e4 | |
| parent | 74added08c67ce724c99664d2eadcbb129d1b86a (diff) | |
| download | xfs-documentation-33bb782ac245caf8b4bc4c167ad662bf32077be5.tar.gz | |
docs: update refcount btree to reflect new cow staging extent policy
We now record the extents being used to stage a copy on write operation
in the refcount btree with the high bit of rc_startblock set and a
reference count of 1. Setting the high bit helps us reduce CoW GC time
at mount by moving all the records to the right side of the tree.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
| -rw-r--r-- | design/XFS_Filesystem_Structure/refcountbt.asciidoc | 19 |
1 files changed, 14 insertions, 5 deletions
diff --git a/design/XFS_Filesystem_Structure/refcountbt.asciidoc b/design/XFS_Filesystem_Structure/refcountbt.asciidoc index dbbb98e..508a9dd 100644 --- a/design/XFS_Filesystem_Structure/refcountbt.asciidoc +++ b/design/XFS_Filesystem_Structure/refcountbt.asciidoc @@ -25,7 +25,10 @@ struct xfs_refcount_rec { ---- *rc_startblock*:: -AG block number of this record. +AG block number of this record. The high bit is set for all records +referring to an extent that is being used to stage a copy on write +operation. This reduces recovery time during mount operations. The +reference count of these staging events must only be 1. *rc_blockcount*:: The length of this extent. @@ -81,12 +84,18 @@ lsn = 0x200004ec2 uuid = f1f89746-e00b-49c9-96b3-ecef0f2f14ae owner = 0 crc = 0xc3962813 (correct) -recs[1-80] = [startblock,blockcount,refcount] - 1:[65780,1,2] 2:[65781,1,3] 3:[65785,2,2] 4:[66640,1,2] - 5:[69602,4,2] 6:[72256,16,2] 7:[72871,4,2] 8:[72879,20,2] - 9:[73395,4,2] 10:[75063,4,2] 11:[79093,4,2] 12:[86344,16,2] +recs[1-80] = [startblock,blockcount,refcount,cowflag] + 1:[65780,1,2,0] 2:[65781,1,3,0] 3:[65785,2,2,0] 4:[66640,1,2,0] + 5:[69602,4,2,0] 6:[72256,16,2,0] 7:[72871,4,2,0] 8:[72879,20,2,0] + 9:[73395,4,2,0] 10:[75063,4,2,0] 11:[79093,4,2,0] 12:[86344,16,2,0] + ... + 80:[35235,10,1,1] ---- +Notice record 80. The copy on write flag is set and the reference count is +1, which indicates that the extent 35,235 - 35,244 are being used to stage a +copy on write activity. The "cowflag" field is the high bit of rc_startblock. + Record 6 in the reference count B+tree for AG 0 indicates that the AG extent starting at block 72,256 and running for 16 blocks has a reference count of 2. This means that there are two files sharing the block: |