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Improve search docstring

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This commit is contained in:
Bojan Serafimov
2023-01-11 12:52:15 -05:00
parent 7c6af6d729
commit 2951be5386
1 changed files with 25 additions and 8 deletions
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33
pageserver/src/tenant/layer_map.rs
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@@ -80,15 +80,32 @@ where
L: ?Sized + Layer,
{
///
/// Find the latest layer that covers the given 'key', with lsn <
/// 'end_lsn'.
/// Find the latest layer (by lsn.end) that covers the given
/// 'key', with lsn.start < 'end_lsn'.
///
/// Returns the layer, if any, and an 'lsn_floor' value that
/// indicates which portion of the layer the caller should
/// check. 'lsn_floor' is normally the start-LSN of the layer, but
/// can be greater if there is an overlapping layer that might
/// contain the version, even if it's missing from the returned
/// layer.
/// The caller of this function is the page reconstruction
/// algorithm looking for the next relevant delta layer, or
/// the terminal image layer. The caller will pass the lsn_floor
/// value as end_lsn in the next call to search.
///
/// If there's an image layer exactly below the given end_lsn,
/// search should return that layer regardless if there are
/// overlapping deltas.
///
/// If the latest layer is a delta and there is an overlapping
/// image with it below, the lsn_floor returned should be right
/// above that image so we don't skip it in the search. Otherwise
/// the lsn_floor returned should be the bottom of the delta layer
/// because we should make as much progress down the lsn axis
/// as possible. It's fine if this way we skip some overlapping
/// deltas, because the delta we returned would contain the same
/// wal content.
///
/// TODO: This API is convoluted and inefficient. If the caller
/// makes N search calls, we'll end up finding the same latest
/// image layer N times. We should either cache the latest image
/// layer result, or simplify the api to `get_latest_image` and
/// `get_latest_delta`, and only call `get_latest_image` once.
///
/// NOTE: This only searches the 'historic' layers, *not* the
/// 'open' and 'frozen' layers!