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)
## Problem
There are some common types that we pass into tenants and timelines as
we construct them, such as remote storage and the broker client.
Currently the list is small, but this is likely to grow -- the deletion
queue PR (#4960) pushed some methods to the point of clippy complaining
they had too many args, because of the extra deletion queue client being
passed around.
There are some shared objects that currently aren't passed around
explicitly because they use a static `once_cell` (e.g.
CONCURRENT_COMPACTIONS), but as we add more resource management and
concurreny control over time, it will be more readable & testable to
pass a type around in the respective Resources object, rather than to
coordinate via static objects. The `Resources` structures in this PR
will make it easier to add references to central coordination functions,
without having to rely on statics.
## Summary of changes
- For `Tenant`, the `broker_client` and `remote_storage` are bundled
into `TenantSharedResources`
- For `Timeline`, the `remote_client` is wrapped into
`TimelineResources`.
Both of these structures will get an additional deletion queue member in
#4960.
## Problem
IndexPart contains two redundant lists of layer names: a set of the
names, and then a map of name to metadata.
We already required that all the layers in `timeline_layers` are also in
`layers_metadata`, in `initialize_with_current_remote_index_part`, so if
there were any index_part.json files in the field that relied on these
sets being different, they would already be broken.
## Summary of changes
`timeline_layers` is made private and no longer read at runtime. It is
still serialized, but not deserialized.
`disk_consistent_lsn` is also made private, as this field only exists
for convenience of humans reading the serialized JSON.
This prepares us to entirely remove `timeline_layers` in a future
release, once this change is fully deployed, and therefore no
pageservers are trying to read the field.
I'm still a bit nervous about attach -> crash case. But it should work.
(unlike case with timeline). Ideally would be cool to cover this with
test.
This continues tradition of adding bool flags for Tenant::set_stopping.
Probably lifecycle project will help with fixing it.
## Problem
Before, DeltaLayer dumping (via `cargo run --release -p pagectl --
print-layer-file` ) would crash as one can't call `Handle::block_on` in
an async executor thread.
## Summary of changes
Avoid the problem by using `DeltaLayerInner::load_keys` to load the keys
into RAM (which we already do during compaction), and then load the
values one by one during dumping.
- move them to pageserver which is the only dependant on the crate fail
- "move" the exported macro to the new module
- support at init time the same failpoints as runtime
Found while debugging test failures and making tests more repeatable by
allowing "exit" from pageserver start via environment variables. Made
those changes to `test_gc_cutoff.py`.
---------
Co-authored-by: Christian Schwarz <christian@neon.tech>
(This PR is the successor of https://github.com/neondatabase/neon/pull/4984 )
## Summary
The current way in which `EphemeralFile` uses `PageCache` complicates
the Pageserver code base to a degree that isn't worth it.
This PR refactors how we cache `EphemeralFile` contents, by exploiting
the append-only nature of `EphemeralFile`.
The result is that `PageCache` only holds `ImmutableFilePage` and
`MaterializedPage`.
These types of pages are read-only and evictable without write-back.
This allows us to remove the writeback code from `PageCache`, also
eliminating an entire failure mode.
Futher, many great open-source libraries exist to solve the problem of a
read-only cache,
much better than our `page_cache.rs` (e.g., better replacement policy,
less global locking).
With this PR, we can now explore using them.
## Problem & Analysis
Before this PR, `PageCache` had three types of pages:
* `ImmutableFilePage`: caches Delta / Image layer file contents
* `MaterializedPage`: caches results of Timeline::get (page
materialization)
* `EphemeralPage`: caches `EphemeralFile` contents
`EphemeralPage` is quite different from `ImmutableFilePage` and
`MaterializedPage`:
* Immutable and materialized pages are for the acceleration of (future)
reads of the same data using `PAGE_CACHE_SIZE * PAGE_SIZE` bytes of
DRAM.
* Ephemeral pages are a write-back cache of `EphemeralFile` contents,
i.e., if there is pressure in the page cache, we spill `EphemeralFile`
contents to disk.
`EphemeralFile` is only used by `InMemoryLayer`, for the following
purposes:
* **write**: when filling up the `InMemoryLayer`, via `impl BlobWriter
for EphemeralFile`
* **read**: when doing **page reconstruction** for a page@lsn that isn't
written to disk
* **read**: when writing L0 layer files, we re-read the `InMemoryLayer`
and put the contents into the L0 delta writer
(**`create_delta_layer`**). This happens every 10min or when
InMemoryLayer reaches 256MB in size.
The access patterns of the `InMemoryLayer` use case are as follows:
* **write**: via `BlobWriter`, strictly append-only
* **read for page reconstruction**: via `BlobReader`, random
* **read for `create_delta_layer`**: via `BlobReader`, dependent on
data, but generally random. Why?
* in classical LSM terms, this function is what writes the
memory-resident `C0` tree into the disk-resident `C1` tree
* in our system, though, the values of InMemoryLayer are stored in an
EphemeralFile, and hence they are not guaranteed to be memory-resident
* the function reads `Value`s in `Key, LSN` order, which is `!=` insert
order
What do these `EphemeralFile`-level access patterns mean for the page
cache?
* **write**:
* the common case is that `Value` is a WAL record, and if it isn't a
full-page-image WAL record, then it's smaller than `PAGE_SIZE`
* So, the `EphemeralPage` pages act as a buffer for these `< PAGE_CACHE`
sized writes.
* If there's no page cache eviction between subsequent
`InMemoryLayer::put_value` calls, the `EphemeralPage` is still resident,
so the page cache avoids doing a `write` system call.
* In practice, a busy page server will have page cache evictions because
we only configure 64MB of page cache size.
* **reads for page reconstruction**: read acceleration, just as for the
other page types.
* **reads for `create_delta_layer`**:
* The `Value` reads happen through a `BlockCursor`, which optimizes the
case of repeated reads from the same page.
* So, the best case is that subsequent values are located on the same
page; hence `BlockCursor`s buffer is maximally effective.
* The worst case is that each `Value` is on a different page; hence the
`BlockCursor`'s 1-page-sized buffer is ineffective.
* The best case translates into `256MB/PAGE_SIZE` page cache accesses,
one per page.
* the worst case translates into `#Values` page cache accesses
* again, the page cache accesses must be assumed to be random because
the `Value`s aren't accessed in insertion order but `Key, LSN` order.
## Summary of changes
Preliminaries for this PR were:
- #5003
- #5004
- #5005
- uncommitted microbenchmark in #5011
Based on the observations outlined above, this PR makes the following
changes:
* Rip out `EphemeralPage` from `page_cache.rs`
* Move the `block_io::FileId` to `page_cache::FileId`
* Add a `PAGE_SIZE`d buffer to the `EphemeralPage` struct.
It's called `mutable_tail`.
* Change `write_blob` to use `mutable_tail` for the write buffering
instead of a page cache page.
* if `mutable_tail` is full, it writes it out to disk, zeroes it out,
and re-uses it.
* There is explicitly no double-buffering, so that memory allocation per
`EphemeralFile` instance is fixed.
* Change `read_blob` to return different `BlockLease` variants depending
on `blknum`
* for the `blknum` that corresponds to the `mutable_tail`, return a ref
to it
* Rust borrowing rules prevent `write_blob` calls while refs are
outstanding.
* for all non-tail blocks, return a page-cached `ImmutablePage`
* It is safe to page-cache these as ImmutablePage because EphemeralFile
is append-only.
## Performance
How doe the changes above affect performance?
M claim is: not significantly.
* **write path**:
* before this PR, the `EphemeralFile::write_blob` didn't issue its own
`write` system calls.
* If there were enough free pages, it didn't issue *any* `write` system
calls.
* If it had to evict other `EphemeralPage`s to get pages a page for its
writes (`get_buf_for_write`), the page cache code would implicitly issue
the writeback of victim pages as needed.
* With this PR, `EphemeralFile::write_blob` *always* issues *all* of its
*own* `write` system calls.
* Also, the writes are explicit instead of implicit through page cache
write back, which will help #4743
* The perf impact of always doing the writes is the CPU overhead and
syscall latency.
* Before this PR, we might have never issued them if there were enough
free pages.
* We don't issue `fsync` and can expect the writes to only hit the
kernel page cache.
* There is also an advantage in issuing the writes directly: the perf
impact is paid by the tenant that caused the writes, instead of whatever
tenant evicts the `EphemeralPage`.
* **reads for page reconstruction**: no impact.
* The `write_blob` function pre-warms the page cache when it writes the
`mutable_tail` to disk.
* So, the behavior is the same as with the EphemeralPages before this
PR.
* **reads for `create_delta_layer`**: no impact.
* Same argument as for page reconstruction.
* Note for the future:
* going through the page cache likely causes read amplification here.
Why?
* Due to the `Key,Lsn`-ordered access pattern, we don't read all the
values in the page before moving to the next page. In the worst case, we
might read the same page multiple times to read different `Values` from
it.
* So, it might be better to bypass the page cache here.
* Idea drafts:
* bypass PS page cache + prefetch pipeline + iovec-based IO
* bypass PS page cache + use `copy_file_range` to copy from ephemeral
file into the L0 delta file, without going through user space
## Problem
The performance benchmark in `test_runner/performance/test_layer_map.py`
is currently failing due to the warning added in #4888.
## Summary of changes
The test mentioned has a `compaction_target_size` of 8192, which is just
one page size. This is an unattainable goal, as we generate at least
three pages: one for the header, one for the b-tree (minimally sized
ones have just the root node in a single page), one for the data.
Therefore, we add two pages to the warning limit. The warning text
becomes a bit less accurate but I think this is okay.
This sidesteps clippy complaining about function arg counts,
and will enable introducing more shared structures in future
without the noise of adding extra args to all the functions
involved in timeline setup.
