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From "Roman Kondakov (JIRA)" <j...@apache.org>
Subject [jira] [Created] (IGNITE-9592) MVCC: Use linked lists to store multiple versions.
Date Thu, 13 Sep 2018 21:59:00 GMT
Roman Kondakov created IGNITE-9592:
--------------------------------------

             Summary: MVCC: Use linked lists to store multiple versions.
                 Key: IGNITE-9592
                 URL: https://issues.apache.org/jira/browse/IGNITE-9592
             Project: Ignite
          Issue Type: Improvement
          Components: mvcc
            Reporter: Roman Kondakov


Currently we store all versions of each row in primary index. It is not very efficient for
several reasons:
 * We have to insert and remove tree item each time a new version is created or an old version
deleted. This leads to a considerable tree operations number increasing as well as tree splits
and merges operations.
 * Also this leads to a contention on leaf page write lock - each update operation has
to obtain exclusive access to insert a new version of row. During this update no body on that
leaf can not be able to update or even read data of neighbour keys.
 * Primary key tree consumes more space if it stores each version.
 * Other vendors do not store each version in primary indexes (as far as I know).

Possible solution - store only key and link to the newest version in the primary index. Instead
of this {{CacheDataTree}} item
{{|           key part          |           |        
|}}
{{|-----------------------------|  lockVer  |   link  |}}
{{| cache_id | hash |  mvccVer  |           |         |}}
 we'll have:
{{|   key part      |           |  link to the   |}}
{{|-----------------|  lockVer  |     newest     |}}
{{| cache_id | hash |           |    version     |}}
Note, we do not have mvccVer in tree item. Link to the newer version leads to the most recent
inserted data item. To find older versions, each DatRow is provided with "prevLink" - the
link to previous version of row. DataRow layout can be changed from

| header | xid_max | xid_min | KV bytes |

to the next one:

| header | xid_max | xid_min | *PREV_LINK* | KV bytes |

Where *PREV_LINK* field points to the previous version of the row.  Traversing this prevRow
links we can iterate over all available versions without affecting primary key tree.

When the new version is created we just insert the new row in datastore, then do CAS on the
link to the newest version in primary key tree in order it points to the new row. PrevLink
in the new row should point on the previous one. That is all. We've just inserted a new version
just with a long field CAS in the CacheDataTree. Without obtaining write lock and other headache.

Secondary indexes are handled in the same manner as before.



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