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Explain why we do not want to perform prewarm of LFC at replica and how it is avoided now

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This commit is contained in:
Konstantin Knizhnik
2024-09-30 15:04:00 +03:00
parent 0d3503a187
commit df289738b8
1 changed files with 14 additions and 1 deletions
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15
pgxn/neon/file_cache.c
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@@ -532,6 +532,18 @@ lfc_init(void)
BackgroundWorker bgw;
memset(&bgw, 0, sizeof(bgw));
bgw.bgw_flags = BGWORKER_SHMEM_ACCESS;
/*
* Prewarming LFC at replica is problematic and doubtful.
* 1. It has not so much sense because replica is skipping all WAL records which target pages is not present
* in shared buffers and invalidate LFC in this case. And as far as size of shared buffers is very small,
* there is really no sense to try to prewarm LFC which will be invalidated in any case.
* 2. Unlike primary,it is not possible to retrieve most recent version of the page. We should follow current apply LSN.
* It significantly complicates prewarming.
*
* BgWorkerStart_RecoveryFinished means we won't ever get started on a hot_standby see
* https://www.postgresql.org/docs/10/static/bgworker.html as it's not
* documented in bgworker.c.
*/
bgw.bgw_start_time = BgWorkerStart_RecoveryFinished;
snprintf(bgw.bgw_library_name, BGW_MAXLEN, "neon");
snprintf(bgw.bgw_function_name, BGW_MAXLEN, "LfcPrewarmMain");
@@ -600,8 +612,9 @@ lfc_init_prewarm(void)
size_t i, max_entries = lfc_prewarm_limit;
uint32_t hash;
FileCacheEntry *entry;
int fd = OpenTransientFile("lfc.state", O_RDONLY | PG_BINARY);
int fd;
fd = OpenTransientFile("lfc.state", O_RDONLY | PG_BINARY);
if (fd < 0)
{
elog(LOG, "LFC: state file is missing");