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@xxxxxxxxxx>
---
.../XFS_Filesystem_Structure/refcountbt.asciidoc | 19 ++++++++++++++-----
1 file 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:
--
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