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Do not set last written LSN for target page prefetch request

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Konstantin Knizhnik
2022-12-15 10:37:44 +02:00
parent 407d6b120b
commit e1e533243b
1 changed files with 27 additions and 27 deletions
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54
pgxn/neon/pagestore_smgr.c
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@@ -675,39 +675,39 @@ prefetch_do_request(PrefetchRequest *slot, bool *force_latest, XLogRecPtr *force
request.req.lsn = lsn;
prefetch_lsn = Max(prefetch_lsn, lsn);
slot->effective_request_lsn = prefetch_lsn;
/*
* Remember request LSN in the last-written LSN cache to avoid false
* prefetch invalidations.
*
* Imagine what would happen without this, when you perform a large
* sequential scan with UPDATE. The sequential scan issues a prefetch
* request for each page in order, and every page is also dirtied. On
* each page, the oldest page in the last-written LSN cache is evicted,
* which advances the global last-written LSN. The pages being scanned are
* not in the last-written cache, so each prefetch request will use the
* global last-written LSN in the request and memorize that in the
* slot. However, when we receive the response to the prefetch request,
* the global last-written LSN has already moved forwards, and the
* cross-check we make that the last-written LSN matches will fail, and we
* discard the prefetched response unnecessary.
*
* Inserting the LSN we use in the prefetch request to the last-written LSN
* cache avoids that problem. With that, we will use the cached value in
* the cross-check, instead of the more recent global last-written LSN value.
*/
SetLastWrittenLSNForBlock(
request.req.lsn,
slot->buftag.rnode,
slot->buftag.forkNum,
slot->buftag.blockNum
);
}
Assert(slot->response == NULL);
Assert(slot->my_ring_index == MyPState->ring_unused);
page_server->send((NeonRequest *) &request);
/*
* Remember request LSN in the last-written LSN cache to avoid false
* prefetch invalidations.
*
* Imagine what would happen without this, when you perform a large
* sequential scan with UPDATE. The sequential scan issues a prefetch
* request for each page in order, and every page is also dirtied. On
* each page, the oldest page in the last-written LSN cache is evicted,
* which advances the global last-written LSN. The pages being scanned are
* not in the last-written cache, so each prefetch request will use the
* global last-written LSN in the request and memorize that in the
* slot. However, when we receive the response to the prefetch request,
* the global last-written LSN has already moved forwards, and the
* cross-check we make that the last-written LSN matches will fail, and we
* discard the prefetched response unnecessary.
*
* Inserting the LSN we use in the prefetch request to the last-written LSN
* cache avoids that problem. With that, we will use the cached value in
* the cross-check, instead of the more recent global last-written LSN value.
*/
SetLastWrittenLSNForBlock(
request.req.lsn,
slot->buftag.rnode,
slot->buftag.forkNum,
slot->buftag.blockNum
);
/* update prefetch state */
MyPState->n_requests_inflight += 1;
MyPState->n_unused -= 1;