layer file creation: fsync timeline directories using VirtualFile::sync_all()
#6986
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The `writer.finish()` methods already fsync the inode, using `VirtualFile::sync_all()`. All that the callers need to do is fsync their directory, i.e., the timeline directory. Note that there's a call in the new compaction code that is apparently dead-at-runtime, so, I couldn't fix up any fsyncs there [Link](https://github.com/neondatabase/neon/blob/502b69b33bbd4ad1b0647e921a9c665249a2cd62/pageserver/src/tenant/timeline/compaction.rs#L204-L211). In the grand scheme of things, layer durability probably doesn't matter anymore because the remote storage is authoritative at all times as of #5198. But, let's not break the discipline in htis commit. part of #6663
…nc_all() Except for the involvement of the VirtualFile fd cache, this is equivalent to what happened before at runtime. Future PR #6378 will implement `VirtualFile::sync_all()` using tokio-epoll-uring if that's configured as the io engine. This PR is preliminary work for that.
As pointed out in the comments added in this PR: the in-memory state of the filesystem already has the layer file in its final place. If the fsync fails, but pageserver continues to execute, it's quite easy for subsequent pageserver code to observe the file being there and assume it's durable, where it really isn't. It can happen that we get ENOSPC during the fsync. However, 1. the timeline dir is small (remember, the big layer _file_ has already been synced). Small data means ENOSPC due to delayed allocation races etc are less likely. 2. what elase are we going to do in that case? If we decide to bubble up the error, the file remains on disk. We could try to unlink it and fsync after the unlink. If that fails, we would _definitely_ need to error out. Is it worth the trouble though? Side note: all this logic about not carrying on after fsync failure implies that we `sync` the filesystem successfully before we restart the pageserver. Our systemd unit currently does not do that, but should.
…kio-epoll-uring/layer-write-path-fsync-cleanups
…sync-cleanups' into problame/integrate-tokio-epoll-uring/create-layer-fatal-err-on-fsync
2484 tests run: 2361 passed, 0 failed, 123 skipped (full report)Code coverage* (full report)
* collected from Rust tests only The comment gets automatically updated with the latest test results
c654263 at 2024-03-04T13:20:20.883Z :recycle: |
…kio-epoll-uring/layer-write-path-fsync-cleanups
…sync-cleanups' into problame/integrate-tokio-epoll-uring/create-layer-fatal-err-on-fsync
…-err-on-fsync' into problame/integrate-tokio-epoll-uring/ioengine-par-fsync
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part of #6663 See that epic for more context & related commits. Problem ------- Before this PR, the layer-file-creating code paths were using VirtualFile, but under the hood these were still blocking system calls. Generally this meant we'd stall the executor thread, unless the caller "knew" and used the following pattern instead: ``` spawn_blocking(|| { Handle::block_on(async { VirtualFile::....().await; }) }).await ``` Solution -------- This PR adopts `tokio-epoll-uring` on the layer-file-creating code paths in pageserver. Note that on-demand downloads still use `tokio::fs`, these will be converted in a future PR. Design: Avoiding Regressions With `std-fs` ------------------------------------------ If we make the VirtualFile write path truly async using `tokio-epoll-uring`, should we then remove the `spawn_blocking` + `Handle::block_on` usage upstack in the same commit? No, because if we’re still using the `std-fs` io engine, we’d then block the executor in those places where previously we were protecting us from that through the `spawn_blocking` . So, if we want to see benefits from `tokio-epoll-uring` on the write path while also preserving the ability to switch between `tokio-epoll-uring` and `std-fs` , where `std-fs` will behave identical to what we have now, we need to ***conditionally* use `spawn_blocking + Handle::block_on`** . I.e., in the places where we use that know, we’ll need to make that conditional based on the currently configured io engine. It boils down to investigating all the places where we do `spawn_blocking(... block_on(... VirtualFile::...))`. Detailed [write-up of that investigation in Notion](https://neondatabase.notion.site/Surveying-VirtualFile-write-path-usage-wrt-tokio-epoll-uring-integration-spawn_blocking-Handle-bl-5dc2270dbb764db7b2e60803f375e015?pvs=4 ), made publicly accessible. tl;dr: Preceding PRs addressed the relevant call sites: - `metadata` file: turns out we could simply remove it (#6777, #6769, #6775) - `create_delta_layer()`: made sensitive to `virtual_file_io_engine` in #6986 NB: once we are switched over to `tokio-epoll-uring` everywhere in production, we can deprecate `std-fs`; to keep macOS support, we can use `tokio::fs` instead. That will remove this whole headache. Code Changes In This PR ----------------------- - VirtualFile API changes - `VirtualFile::write_at` - implement an `ioengine` operation and switch `VirtualFile::write_at` to it - `VirtualFile::metadata()` - curiously, we only use it from the layer writers' `finish()` methods - introduce a wrapper `Metadata` enum because `std::fs::Metadata` cannot be constructed by code outside rust std - `VirtualFile::sync_all()` and for completeness sake, add `VirtualFile::sync_data()` Testing & Rollout ----------------- Before merging this PR, we ran the CI with both io engines. Additionally, the changes will soak in staging. We could have a feature gate / add a new io engine `tokio-epoll-uring-write-path` to do a gradual rollout. However, that's not part of this PR. Future Work ----------- There's still some use of `std::fs` and/or `tokio::fs` for directory namespace operations, e.g. `std::fs::rename`. We're not addressing those in this PR, as we'll need to add the support in tokio-epoll-uring first. Note that rename itself is usually fast if the directory is in the kernel dentry cache, and only the fsync after rename is slow. These fsyncs are using tokio-epoll-uring, so, the impact should be small.
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Except for the involvement of the VirtualFile fd cache, this is
equivalent to what happened before at runtime.
Future PR #6378 will implement
VirtualFile::sync_all()usingtokio-epoll-uring if that's configured as the io engine.
This PR is preliminary work for that.
part of #6663