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explain why there's no need to check standby_horizon lease deadline for getpage requests

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Christian Schwarz
2025-07-25 09:30:27 +00:00
parent b47d3900b9
commit 23d1029afd
1 changed files with 10 additions and 3 deletions
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13
pageserver/src/page_service.rs
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@@ -2144,10 +2144,17 @@ impl PageServerHandler {
));
}
// Clients should only read from recent LSNs on their timeline, or from locations holding an LSN lease.
// Reject LSNs for which we may have garbage collected some of the required reconstruct data,
// or may do that garbage collection at any point concurrent to this request's execution.
//
// We may have older data available, but we make a best effort to detect this case and return an error,
// to distinguish a misbehaving client (asking for old LSN) from a storage issue (data missing at a legitimate LSN).
// The only LSNs that are valid to read are:
// - While we hold `latest_gc_cutoff_lsn`, the *range* between its value
// and last_record_lsn. (latest_gc_cutoff_lsn is a guard on Timeline::applied_gc_cutoff_lsn).
// - An LSN leased via the lsn lease mechanism. Leased LSNs are respected by
// GC but don't prevent applied_gc_cutoff_lsn from advancing.
//
// NB: Unlike LSN leases, the standby_horizon leases used by RO replicas _do_ prevent the
// applied_gc_cutoff_lsn from advancing, so, there's no special treatment for them here.
if request_lsn < **latest_gc_cutoff_lsn && !timeline.is_gc_blocked_by_lsn_lease_deadline() {
let gc_info = &timeline.gc_info.read().unwrap();
if !gc_info.lsn_covered_by_lease(request_lsn) {