In logs it is confusing to see seqwait timeouts which seemingly arise
from the branched lsn but actually are about the ancestor, leading to
questions like "has the last_record_lsn went back".
Noticed by @problame.
## Problem
`read_blk` does I/O and thus we would like to make it async. We can't
make the function async as long as the `PageReadGuard` returned by
`read_blk` isn't `Send`. The page cache is called by `read_blk`, and
thus it can't be async without `read_blk` being async. Thus, we have a
circular dependency.
## Summary of changes
Due to the circular dependency, we convert both the page cache and
`read_blk` to async at the same time:
We make the page cache use `tokio::sync` synchronization primitives as
those are `Send`. This makes all the places that acquire a lock require
async though, which we then also do. This includes also asyncification
of the `read_blk` function.
Builds upon #4994, #5015, #5056, and #5129.
Part of #4743.
## Problem
I saw these things while working on #5111.
## Summary of changes
* Add a comment explaining why we use `Vec::leak` instead of
`Vec::into_boxed_slice` plus `Box::leak`.
* Add another comment explaining what `valid` is doing, it wasn't very
clear before.
* Add a function `set_usage_count` to not set it directly.
Before this patch, when dropping an EphemeralFile, we'd scan the entire
`slots` to proactively evict its pages (`drop_buffers_for_immutable`).
This was _necessary_ before #4994 because the page cache was a
write-back cache: we'd be deleting the EphemeralFile from disk after,
so, if we hadn't evicted its pages before that, write-back in
`find_victim` wouldhave failed.
But, since #4994, the page cache is a read-only cache, so, it's safe
to keep read-only data cached. It's never going to get accessed again
and eventually, `find_victim` will evict it.
The only remaining advantage of `drop_buffers_for_immutable` over
relying on `find_victim` is that `find_victim` has to do the clock
page replacement iterations until the count reaches 0,
whereas `drop_buffers_for_immutable` can kick the page out right away.
However, weigh that against the cost of `drop_buffers_for_immutable`,
which currently scans the entire `slots` array to find the
EphemeralFile's pages.
Alternatives have been proposed in #5122 and #5128, but, they come
with their own overheads & trade-offs.
Also, the real reason why we're looking into this piece of code is
that we want to make the slots rwlock async in #5023.
Since `drop_buffers_for_immutable` is called from drop, and there
is no async drop, it would be nice to not have to deal with this.
So, let's just stop doing `drop_buffers_for_immutable` and observe
the performance impact in benchmarks.
Unrelated fixes noticed while integrating #4938.
- Stop leaking future layers in remote storage
- We schedule extra index_part uploads if layer name to be removed was
not actually present
## Problem
The `metadata_bytes` field of IndexPart required explicit
deserialization & error checking everywhere it was used -- there isn't
anything special about this structure that should prevent it from being
serialized & deserialized along with the rest of the structure.
## Summary of changes
- Implement Serialize and Deserialize for TimelineMetadata
- Replace IndexPart::metadata_bytes with a simpler `metadata`, that can
be used directly.
---------
Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>
## Problem
We want to make `read_blk` an async function, but outside of
`async_trait`, which allocates, and nightly features, we can't use async
fn's in traits.
## Summary of changes
* Remove all uses of `BlockReader::read_blk` in favour of using block
cursors, at least where the type of the `BlockReader` is behind a
generic
* Introduce a `BlockReaderRef` enum that lists all implementors of
`BlockReader::read_blk`.
* Remove `BlockReader::read_blk` and move its implementations into
inherent functions on the types instead.
We don't turn `read_blk` into an async fn yet, for that we also need to
modify the page cache. So this is a preparatory PR, albeit an important
one.
Part of #4743.
## Problem
The `EphemeralFile::write_blob` function accesses the page cache
internally. We want to require `async` for these accesses in #5023.
## Summary of changes
This removes the implementaiton of the `BlobWriter` trait for
`EphemeralFile` and turns the `write_blob` function into an inherent
function. We can then make it async as well as the `push_bytes`
function. We move the `SER_BUFFER` thread-local into the
`InMemoryLayerInner` so that the same buffer can be accessed by
different threads as the async is (potentially) moved between threads.
Part of #4743, preparation for #5023.
Current implementation first calls `load_layer_map`, which loads all
local layers, cleans up files, leave cleaning up stuff to "second
function". Then the "second function" is finally called, it does not do
the cleanup and some of the first functions setup can torn down. "Second
function" is actually both `reconcile_with_remote` and
`create_remote_layers`.
This change makes it a bit more verbose but in one phase with the
following sub-steps:
1. scan the timeline directory
2. delete extra files
- now including on-demand download files
- fixes#3660
3. recoincile the two sources of layers (directory, index_part)
4. rename_to_backup future layers, short layers
5. create the remaining as layers
Needed by #4938.
It was also noticed that this is blocking code in an `async fn` so just
do it in a `spawn_blocking`, which should be healthy for our startup
times. Other effects includes hopefully halving of `stat` calls; extra
calls which were not done previously are now done for the future layers.
Co-authored-by: Christian Schwarz <christian@neon.tech>
Co-authored-by: John Spray <john@neon.tech>
## Problem
Currently, anything that uses backoff::retry will delay the join of its
task by however long its backoff sleep is, multiplied by its max
retries.
Whenever we call a function that sleeps, we should be passing in a
CancellationToken.
## Summary of changes
- Add a `Cancel` type to backoff::retry that wraps a CancellationToken
and an error `Fn` to generate an error if the cancellation token fires.
- In call sites that already run in a `task_mgr` task, use
`shutdown_token()` to provide the token. In other locations, use a dead
`CancellationToken` to satisfy the interface, and leave a TODO to fix it
up when we broaden the use of explicit cancellation tokens.
Accidentially giving is_incremental=true for ImageLayers costs a lot of
debugging time. Removes all API which would allow to do that. They can
easily be restored later *when needed*.
Split off from #4938.
When doing global queries in VictoriaMetrics, the per-timeline
histograms make us run into cardinality limits.
We don't want to give them up just yet because we don't
have an alternative for drilling down on timeline-specific
performance issues.
So, add a pre-aggregated histogram and add observations to it
whenever we add observations to the per-timeline histogram.
While we're at it, switch to using a strummed enum for the operation
type names.
#4938 will make on-demand download of layers in compaction possible, so
it's not suitable for our "policy" of no `spawn_blocking(|| ...
Handle::block_on(async { spawn_blocking(...).await })` because this
poses a clear deadlock risk. Nested spawn_blockings are because of the
download using `tokio::fs::File`.
- Remove `spawn_blocking` from caller of `compact_level0_phase1`
- Remove `Handle::block_on` from `compact_level0_phase1` (indentation
change)
- Revert to `AsLayerDesc::layer_desc` usage temporarily (until it
becomes field access in #4938)
