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Diffstat (limited to 'src/backend/storage/freespace/README')
| -rw-r--r-- | src/backend/storage/freespace/README | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/src/backend/storage/freespace/README b/src/backend/storage/freespace/README index e7ff23b..dc2a63a 100644 --- a/src/backend/storage/freespace/README +++ b/src/backend/storage/freespace/README @@ -169,9 +169,7 @@ Recovery -------- The FSM is not explicitly WAL-logged. Instead, we rely on a bunch of -self-correcting measures to repair possible corruption. As a result when -we write to the FSM we treat that as a hint and thus use MarkBufferDirtyHint() -rather than MarkBufferDirty(). +self-correcting measures to repair possible corruption. First of all, whenever a value is set on an FSM page, the root node of the page is compared against the new value after bubbling up the change is @@ -188,6 +186,18 @@ goes through fsm_set_avail(), so that the upper nodes on those pages are immediately updated. Periodically, VACUUM calls FreeSpaceMapVacuum[Range] to propagate the new free-space info into the upper pages of the FSM tree. +As a result when we write to the FSM we treat that as a hint and thus use +MarkBufferDirtyHint() rather than MarkBufferDirty(). Every read here uses +RBM_ZERO_ON_ERROR to bypass checksum mismatches and other verification +failures. We'd operate correctly without the full page images that +MarkBufferDirtyHint() provides, but they do decrease the chance of losing slot +knowledge to RBM_ZERO_ON_ERROR. + +Relation extension is not WAL-logged. Hence, after WAL replay, an on-disk FSM +slot may indicate free space in PageIsNew() blocks that never reached disk. +We detect this case by comparing against the actual relation size, and we mark +the block as full in that case. + TODO ---- |