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https://issues.apache.org/jira/browse/IGNITE-6930?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16951163#comment-16951163
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Ivan Rakov commented on IGNITE-6930:
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[~alex_pl] Thanks, please proceed to merge.
> Optionally to do not write free list updates to WAL
> ---------------------------------------------------
>
> Key: IGNITE-6930
> URL: https://issues.apache.org/jira/browse/IGNITE-6930
> Project: Ignite
> Issue Type: Task
> Components: cache
> Reporter: Vladimir Ozerov
> Assignee: Aleksey Plekhanov
> Priority: Major
> Labels: IEP-8, performance
> Fix For: 2.8
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> When cache entry is created, we need to write update the free list. When
> entry is updated, we need to update free list(s) several times. Currently
> free list is persistent structure, so every update to it must be logged to be
> able to recover after crash. This may incur significant overhead, especially
> for small entries.
> E.g. this is how WAL for a single update looks like. "D" - updates with real
> data, "F" - free-list management:
> {code}
> 1. [D] DataRecord [writeEntries=[UnwrapDataEntry[k = key, v = [ BinaryObject
> [idHash=2053299190, hash=1986931360, typeId=-1580729813]], super = [DataEntry
> [cacheId=94416770, op=UPDATE, writeVer=GridCacheVersion [topVer=122147562,
> order=1510667560607, nodeOrder=1], partId=0, partCnt=4]]]], super=WALRecord
> [size=0, chainSize=0, pos=null, type=DATA_RECORD]]
> 2. [F] PagesListRemovePageRecord [rmvdPageId=0001000000000005,
> pageId=0001000000000006, grpId=94416770, super=PageDeltaRecord
> [grpId=94416770, pageId=0001000000000006, super=WALRecord [size=37,
> chainSize=0, pos=null, type=PAGES_LIST_REMOVE_PAGE]]]
> 3. [D] DataPageInsertRecord [super=PageDeltaRecord [grpId=94416770,
> pageId=0001000000000005, super=WALRecord [size=129, chainSize=0, pos=null,
> type=DATA_PAGE_INSERT_RECORD]]]
> 4. [F] PagesListAddPageRecord [dataPageId=0001000000000005,
> super=PageDeltaRecord [grpId=94416770, pageId=0001000000000008,
> super=WALRecord [size=37, chainSize=0, pos=null, type=PAGES_LIST_ADD_PAGE]]]
> 5. [F] DataPageSetFreeListPageRecord [freeListPage=281474976710664,
> super=PageDeltaRecord [grpId=94416770, pageId=0001000000000005,
> super=WALRecord [size=37, chainSize=0, pos=null,
> type=DATA_PAGE_SET_FREE_LIST_PAGE]]]
> 6. [D] ReplaceRecord [io=DataLeafIO[ver=1], idx=0, super=PageDeltaRecord
> [grpId=94416770, pageId=0001000000000004, super=WALRecord [size=47,
> chainSize=0, pos=null, type=BTREE_PAGE_REPLACE]]]
> 7. [F] DataPageRemoveRecord [itemId=0, super=PageDeltaRecord
> [grpId=94416770, pageId=0001000000000005, super=WALRecord [size=30,
> chainSize=0, pos=null, type=DATA_PAGE_REMOVE_RECORD]]]
> 8. [F] PagesListRemovePageRecord [rmvdPageId=0001000000000005,
> pageId=0001000000000008, grpId=94416770, super=PageDeltaRecord
> [grpId=94416770, pageId=0001000000000008, super=WALRecord [size=37,
> chainSize=0, pos=null, type=PAGES_LIST_REMOVE_PAGE]]]
> 9. [F] DataPageSetFreeListPageRecord [freeListPage=0, super=PageDeltaRecord
> [grpId=94416770, pageId=0001000000000005, super=WALRecord [size=37,
> chainSize=0, pos=null, type=DATA_PAGE_SET_FREE_LIST_PAGE]]]
> 10. [F] PagesListAddPageRecord [dataPageId=0001000000000005,
> super=PageDeltaRecord [grpId=94416770, pageId=0001000000000006,
> super=WALRecord [size=37, chainSize=0, pos=null, type=PAGES_LIST_ADD_PAGE]]]
> 11. [F] DataPageSetFreeListPageRecord [freeListPage=281474976710662,
> super=PageDeltaRecord [grpId=94416770, pageId=0001000000000005,
> super=WALRecord [size=37, chainSize=0, pos=null,
> type=DATA_PAGE_SET_FREE_LIST_PAGE]]]
> {code}
> If you sum all space required for operation (size in p.3 is shown incorrectly
> here), you will see that data update required ~300 bytes, so do free list
> update!
> *Proposed solution*
> 1) Optionally do not write free list updates to WAL
> 2) In case of node restart we start with empty free lists, so data inserts
> will have to allocate new pages
> 3) When old data page is read, add it to the free list
> 4) Start a background thread which will iterate over all old data pages and
> re-create the free list, so that eventually all data pages are tracked.
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