Zach Brown 2ac239a4cb Add deletion items ...

So far we were only able to add items to the segments.  To support
deletion we have to insert deletion items and then remove them and the
item they reference when their segments are compacted.

As callers attempt to delete items from the item cache we replace the
existing item with a deletion marker with the key but no value.  Now
that there are deletion items in the cache we have to teach the other
item cache operations to skip them.  There's some noise in the patch
from moving functions around so that item insertion can free a deletion
item it finds.

The deletion items are written out to the segment as usual except now
the in-segment item struct has a flag to mark a deletion item and the
deletion item is removed from the cache once its written to the segment.

Item reading knows to skip deletion items and not add them back into
the cache.

Compaction proceeds as usual for most of the levels with the deletion
item clobbering any older higher level items with the same key.
Eventually the deletion item itself is removed by skipping over it when
compacting to the largest final level.  We support this by adding a
little call that describes the max level of the tree at the time the
compaction starts so that compaction can tell when it should skip
copying the deletion item to the final lower level.

All of this is for deletion of items with a precise key.  In the future
we'll expand the deletion items so that they can reference a contiguous
range of keys.

Signed-off-by: Zach Brown <zab@versity.com>

2017-04-18 13:44:54 -07:00

kmod

Add deletion items

2017-04-18 13:44:54 -07:00

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