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wip prototype

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This commit is contained in:
John Spray
2025-02-17 22:33:29 +01:00
parent a8f59f851d
commit 77b1fd40b5
2 changed files with 37 additions and 3 deletions
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12
pageserver/src/tenant/layer_map.rs
View File

@@ -571,7 +571,7 @@ impl LayerMap {
}
/// Get a ref counted pointer for the first in memory layer that matches the provided predicate.
pub fn find_in_memory_layer<Pred>(&self, mut pred: Pred) -> Option<Arc<InMemoryLayer>>
pub(crate) fn find_in_memory_layer<Pred>(&self, mut pred: Pred) -> Option<Arc<InMemoryLayer>>
where
Pred: FnMut(&Arc<InMemoryLayer>) -> bool,
{
@@ -900,6 +900,16 @@ impl LayerMap {
Ok(())
}
pub(crate) fn get_newest_image_after(&self, lsn: Lsn) -> Option<Arc<PersistentLayerDesc>> {
// TODO: an efficient equivalent, this is a crude placeholder
for layer in self.iter_historic_layers() {
if !layer.is_delta() && layer.image_layer_lsn() >= lsn {
return Some(layer);
}
}
None
}
/// `read_points` represent the tip of a timeline and any branch points, i.e. the places
/// where we expect to serve reads.
///

28
pageserver/src/tenant/timeline.rs
View File

@@ -3775,6 +3775,8 @@ impl Timeline {
let mut completed_keyspace = KeySpace::default();
let mut image_covered_keyspace = KeySpaceRandomAccum::new();
let mut in_memory_layers_considered = Vec::new();
// Prevent GC from progressing while visiting the current timeline.
// If we are GC-ing because a new image layer was added while traversing
// the timeline, then it will remove layers that are required for fulfilling
@@ -3810,12 +3812,34 @@ impl Timeline {
let in_memory_layer = layers.find_in_memory_layer(|l| {
let start_lsn = l.get_lsn_range().start;
cont_lsn > start_lsn
!in_memory_layers_considered.contains(&start_lsn) && cont_lsn > start_lsn
});
match in_memory_layer {
Some(l) => {
let lsn_range = l.get_lsn_range().start..cont_lsn;
in_memory_layers_considered.push(l.get_lsn_range().start);
// Search for image layers that overlap with the in-memory layer: this is rare but permitted, and
// we must bound the `lsn_range` of this layer to avoid skipping past the image layer.
// TODO: a narrower search that only hits on image layers matching `unmapped_keyspace`
let lsn_range = if let Some(image) =
layers.get_newest_image_after(l.get_lsn_range().start)
{
// Note that this does not guarantee serving a read from an image layer, just that we will
// not skip considering thge image layer in our Fringe. We can still end up doing walredo work
// in spite of the presence of an image layer, if the inmemory layers we visit contain enough
// information to fully construct a page. For example:
// - ephemeral layer contains I1, D1, D2, <LSN X>
// - image layer at LSN X contains image equal to I2
// - we will end up doing a walredo of I1+D1+D2, rather than reading from the image layer
//
// This is not a problem for correctness, and is rare enough that the wasted time doing walredo
// doesn't matter.
image.get_lsn_range().start + 1..cont_lsn
} else {
l.get_lsn_range().start..cont_lsn
};
fringe.update(
ReadableLayer::InMemoryLayer(l),
unmapped_keyspace.clone(),