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Maintain last written LSN for each page to enable prefetch on vacuum,… #245

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merged 2 commits into from
Nov 24, 2022
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Maintain last written LSN for each page to enable prefetch on vacuum,… #245

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Maintain last written LSN for each page to enable prefetch on vacuum,…
knizhnik Nov 23, 2022
ff0213d
Move PageSetLSN in heap_xlog_visible before MarkBufferDirty
knizhnik Nov 23, 2022
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11 changes: 11 additions & 0 deletions src/backend/access/heap/heapam.c
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Original file line number Diff line number Diff line change
Expand Up @@ -8945,6 +8945,17 @@ heap_xlog_visible(XLogReaderState *record)

PageSetAllVisible(page);

/*
* NEON: despite to the comment above we need to update page LSN here.
* See discussion at hackers: https://www.postgresql.org/message-id/flat/039076d4f6cdd871691686361f83cb8a6913a86a.camel%40j-davis.com#101ba42b004f9988e3d54fce26fb3462
* For Neon this assignment is critical because otherwise last written LSN tracked at compute doesn't
* match with page LSN assignee by WAL-redo and as a result, prefetched page is rejected.
*
* It is fixed in upstream in https://github.com/neondatabase/postgres/commit/7bf713dd2d0739fbcd4103971ed69c17ebe677ea
* but until it is merged we still need to carry a patch here.
*/
PageSetLSN(page, lsn);

MarkBufferDirty(buffer);
}
else if (action == BLK_RESTORED)
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26 changes: 9 additions & 17 deletions src/backend/access/transam/xlog.c
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Expand Up @@ -211,7 +211,7 @@ typedef struct LastWrittenLsnCacheEntry


/*
* Cache of last written LSN for each relation chunk (hash bucket).
* Cache of last written LSN for each relation page.
* Also to provide request LSN for smgrnblocks, smgrexists there is pseudokey=InvalidBlockId which stores LSN of last
* relation metadata update.
* Size of the cache is limited by GUC variable lastWrittenLsnCacheSize ("lsn_cache_size"),
Expand Down Expand Up @@ -606,8 +606,6 @@ static WALInsertLockPadded *WALInsertLocks = NULL;
*/
static ControlFileData *ControlFile = NULL;

#define LAST_WRITTEN_LSN_CACHE_BUCKET 1024 /* blocks = 8Mb */

/*
* Calculate the amount of space left on the page after 'endptr'. Beware
* multiple evaluation!
Expand Down Expand Up @@ -6100,7 +6098,7 @@ GetInsertRecPtr(void)
* It returns an upper bound for the last written LSN of a given page,
* either from a cached last written LSN or a global maximum last written LSN.
* If rnode is InvalidOid then we calculate maximum among all cached LSN and maxLastWrittenLsn.
* If cache is large enough ,iterting through all hash items may be rather expensive.
* If cache is large enough, iterating through all hash items may be rather expensive.
* But GetLastWrittenLSN(InvalidOid) is used only by zenith_dbsize which is not performance critical.
*/
XLogRecPtr
Expand All @@ -6119,7 +6117,7 @@ GetLastWrittenLSN(RelFileNode rnode, ForkNumber forknum, BlockNumber blkno)
BufferTag key;
key.rnode = rnode;
key.forkNum = forknum;
key.blockNum = blkno / LAST_WRITTEN_LSN_CACHE_BUCKET;
key.blockNum = blkno;
entry = hash_search(lastWrittenLsnCache, &key, HASH_FIND, NULL);
if (entry != NULL)
lsn = entry->lsn;
Expand All @@ -6143,9 +6141,9 @@ GetLastWrittenLSN(RelFileNode rnode, ForkNumber forknum, BlockNumber blkno)
/*
* SetLastWrittenLSNForBlockRange -- Set maximal LSN of written page range.
* We maintain cache of last written LSNs with limited size and LRU replacement
* policy. To reduce cache size we store max LSN not for each page, but for
* bucket (1024 blocks). This cache allows to use old LSN when
* requesting pages of unchanged or appended relations.
* policy. Keeping last written LSN for each page allows to use old LSN when
* requesting pages of unchanged or appended relations. Also it is critical for
* efficient work of prefetch in case massive update operations (like vacuum or remove).
*
* rnode.relNode can be InvalidOid, in this case maxLastWrittenLsn is updated.
* SetLastWrittenLsn with dummy rnode is used by createdb and dbase_redo functions.
Expand All @@ -6167,19 +6165,13 @@ SetLastWrittenLSNForBlockRange(XLogRecPtr lsn, RelFileNode rnode, ForkNumber for
LastWrittenLsnCacheEntry* entry;
BufferTag key;
bool found;
BlockNumber bucket;
BlockNumber start_bucket; /* inclusive */
BlockNumber end_bucket; /* exclusive */

start_bucket = from / LAST_WRITTEN_LSN_CACHE_BUCKET;
end_bucket = from == REL_METADATA_PSEUDO_BLOCKNO
? start_bucket + 1 : (from + n_blocks + LAST_WRITTEN_LSN_CACHE_BUCKET - 1) / LAST_WRITTEN_LSN_CACHE_BUCKET;
BlockNumber i;

key.rnode = rnode;
key.forkNum = forknum;
for (bucket = start_bucket; bucket < end_bucket; bucket++)
for (i = 0; i < n_blocks; i++)
{
key.blockNum = bucket;
key.blockNum = from + i;
entry = hash_search(lastWrittenLsnCache, &key, HASH_ENTER, &found);
if (found)
{
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4 changes: 2 additions & 2 deletions src/backend/utils/misc/guc.c
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Expand Up @@ -2452,11 +2452,11 @@ static struct config_int ConfigureNamesInt[] =

{
{"lsn_cache_size", PGC_POSTMASTER, UNGROUPED,
gettext_noop("Size of las written LSN cache used by Neon."),
gettext_noop("Size of last written LSN cache used by Neon."),
NULL
},
&lastWrittenLsnCacheSize,
1024, 10, 1000000, /* 1024 is enough to hold 10GB database with 8Mb bucket */
128*1024, 1024, INT_MAX,
NULL, NULL, NULL
},

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