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rustdoc: overhaul backup rustdoc and add locking table

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Rewrite most of the documentation to be more readable and correct
(according to the current implementations).

Add a table visualizing all different locks used to synchronize
concurrent operations.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
This commit is contained in:
Stefan Reiter 2020-10-15 12:49:16 +02:00 committed by Dietmar Maurer
parent 254b1f2213
commit b64e9a97f3
1 changed files with 119 additions and 80 deletions
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199
src/backup.rs
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@ -1,107 +1,146 @@
//! This module implements the proxmox backup data storage
//! This module implements the data storage and access layer.
//!
//! Proxmox backup splits large files into chunks, and stores them
//! deduplicated using a content addressable storage format.
//! # Data formats
//!
//! A chunk is simply defined as binary blob, which is stored inside a
//! `ChunkStore`, addressed by the SHA256 digest of the binary blob.
//! PBS splits large files into chunks, and stores them deduplicated using
//! a content addressable storage format.
//!
//! Index files are used to reconstruct the original file. They
//! basically contain a list of SHA256 checksums. The `DynamicIndex*`
//! format is able to deal with dynamic chunk sizes, whereas the
//! `FixedIndex*` format is an optimization to store a list of equal
//! sized chunks.
//! Backup snapshots are stored as folders containing a manifest file and
//! potentially one or more index or blob files.
//!
//! # ChunkStore Locking
//! The manifest contains hashes of all other files and can be signed by
//! the client.
//!
//! We need to be able to restart the proxmox-backup service daemons,
//! so that we can update the software without rebooting the host. But
//! such restarts must not abort running backup jobs, so we need to
//! keep the old service running until those jobs are finished. This
//! implies that we need some kind of locking for the
//! ChunkStore. Please note that it is perfectly valid to have
//! multiple parallel ChunkStore writers, even when they write the
//! same chunk (because the chunk would have the same name and the
//! same data). The only real problem is garbage collection, because
//! we need to avoid deleting chunks which are still referenced.
//! Blob files contain data directly. They are used for config files and
//! the like.
//!
//! * Read Index Files:
//! Index files are used to reconstruct an original file. They contain a
//! list of SHA256 checksums. The `DynamicIndex*` format is able to deal
//! with dynamic chunk sizes (CT and host backups), whereas the
//! `FixedIndex*` format is an optimization to store a list of equal sized
//! chunks (VMs, whole block devices).
//!
//! Acquire shared lock for .idx files.
//!
//!
//! * Delete Index Files:
//!
//! Acquire exclusive lock for .idx files. This makes sure that we do
//! not delete index files while they are still in use.
//!
//!
//! * Create Index Files:
//!
//! Acquire shared lock for ChunkStore (process wide).
//!
//! Note: When creating .idx files, we create temporary a (.tmp) file,
//! then do an atomic rename ...
//!
//!
//! * Garbage Collect:
//!
//! Acquire exclusive lock for ChunkStore (process wide). If we have
//! already a shared lock for the ChunkStore, try to upgrade that
//! lock.
//!
//!
//! * Server Restart
//!
//! Try to abort the running garbage collection to release exclusive
//! ChunkStore locks ASAP. Start the new service with the existing listening
//! socket.
//! A chunk is defined as a binary blob, which is stored inside a
//! [ChunkStore](struct.ChunkStore.html) instead of the backup directory
//! directly, and can be addressed by its SHA256 digest.
//!
//!
//! # Garbage Collection (GC)
//!
//! Deleting backups is as easy as deleting the corresponding .idx
//! files. Unfortunately, this does not free up any storage, because
//! those files just contain references to chunks.
//! Deleting backups is as easy as deleting the corresponding .idx files.
//! However, this does not free up any storage, because those files just
//! contain references to chunks.
//!
//! To free up some storage, we run a garbage collection process at
//! regular intervals. The collector uses a mark and sweep
//! approach. In the first phase, it scans all .idx files to mark used
//! chunks. The second phase then removes all unmarked chunks from the
//! store.
//! regular intervals. The collector uses a mark and sweep approach. In
//! the first phase, it scans all .idx files to mark used chunks. The
//! second phase then removes all unmarked chunks from the store.
//!
//! The above locking mechanism makes sure that we are the only
//! process running GC. But we still want to be able to create backups
//! during GC, so there may be multiple backup threads/tasks
//! running. Either started before GC started, or started while GC is
//! running.
//! The locking mechanisms mentioned below make sure that we are the only
//! process running GC. We still want to be able to create backups during
//! GC, so there may be multiple backup threads/tasks running, either
//! started before GC, or while GC is running.
//!
//! ## `atime` based GC
//!
//! The idea here is to mark chunks by updating the `atime` (access
//! timestamp) on the chunk file. This is quite simple and does not
//! need additional RAM.
//! timestamp) on the chunk file. This is quite simple and does not need
//! additional RAM.
//!
//! One minor problem is that recent Linux versions use the `relatime`
//! mount flag by default for performance reasons (yes, we want
//! that). When enabled, `atime` data is written to the disk only if
//! the file has been modified since the `atime` data was last updated
//! (`mtime`), or if the file was last accessed more than a certain
//! amount of time ago (by default 24h). So we may only delete chunks
//! with `atime` older than 24 hours.
//!
//! Another problem arises from running backups. The mark phase does
//! not find any chunks from those backups, because there is no .idx
//! file for them (created after the backup). Chunks created or
//! touched by those backups may have an `atime` as old as the start
//! time of those backups. Please note that the backup start time may
//! predate the GC start time. So we may only delete chunks older than
//! the start time of those running backup jobs.
//! mount flag by default for performance reasons (and we want that). When
//! enabled, `atime` data is written to the disk only if the file has been
//! modified since the `atime` data was last updated (`mtime`), or if the
//! file was last accessed more than a certain amount of time ago (by
//! default 24h). So we may only delete chunks with `atime` older than 24
//! hours.
//!
//! Another problem arises from running backups. The mark phase does not
//! find any chunks from those backups, because there is no .idx file for
//! them (created after the backup). Chunks created or touched by those
//! backups may have an `atime` as old as the start time of those backups.
//! Please note that the backup start time may predate the GC start time.
//! So we may only delete chunks older than the start time of those
//! running backup jobs, which might be more than 24h back (this is the
//! reason why ProcessLocker exclusive locks only have to be exclusive
//! between processes, since within one we can determine the age of the
//! oldest shared lock).
//!
//! ## Store `marks` in RAM using a HASH
//!
//! Not sure if this is better. TODO
//! Might be better. Under investigation.
//!
//!
//! # Locking
//!
//! Since PBS allows multiple potentially interfering operations at the
//! same time (e.g. garbage collect, prune, multiple backup creations
//! (only in seperate groups), forget, ...), these need to lock against
//! each other in certain scenarios. There is no overarching global lock
//! though, instead always the finest grained lock possible is used,
//! because running these operations concurrently is treated as a feature
//! on its own.
//!
//! ## Inter-process Locking
//!
//! We need to be able to restart the proxmox-backup service daemons, so
//! that we can update the software without rebooting the host. But such
//! restarts must not abort running backup jobs, so we need to keep the
//! old service running until those jobs are finished. This implies that
//! we need some kind of locking for modifying chunks and indices in the
//! ChunkStore.
//!
//! Please note that it is perfectly valid to have multiple
//! parallel ChunkStore writers, even when they write the same chunk
//! (because the chunk would have the same name and the same data, and
//! writes are completed atomically via a rename). The only problem is
//! garbage collection, because we need to avoid deleting chunks which are
//! still referenced.
//!
//! To do this we use the
//! [ProcessLocker](../tools/struct.ProcessLocker.html).
//!
//! ### ChunkStore-wide
//!
//! * Create Index Files:
//!
//! Acquire shared lock for ChunkStore.
//!
//! Note: When creating .idx files, we create a temporary .tmp file,
//! then do an atomic rename.
//!
//! * Garbage Collect:
//!
//! Acquire exclusive lock for ChunkStore. If we have
//! already a shared lock for the ChunkStore, try to upgrade that
//! lock.
//!
//! Exclusive locks only work _between processes_. It is valid to have an
//! exclusive and one or more shared locks held within one process. Writing
//! chunks within one process is synchronized using the gc_mutex.
//!
//! On server restart, we stop any running GC in the old process to avoid
//! having the exclusive lock held for too long.
//!
//! ## Locking table
//!
//! Below table shows all operations that play a role in locking, and which
//! mechanisms are used to make their concurrent usage safe.
//!
//! | starting ><br>v during | read index file | create index file | GC mark | GC sweep | update manifest | forget | prune | create backup | verify | reader api |
//! |-|-|-|-|-|-|-|-|-|-|-|
//! | **read index file** | / | / | / | / | / | mmap stays valid, oldest_shared_lock prevents GC | see forget column | / | / | / |
//! | **create index file** | / | / | / | / | / | / | / | /, happens at the end, after all chunks are touched | /, only happens without a manifest | / |
//! | **GC mark** | / | Datastore process-lock shared | gc_mutex, exclusive ProcessLocker | gc_mutex | /, GC only cares about index files, not manifests | tells GC about removed chunks | see forget column | /, index files don’t exist yet | / | / |
//! | **GC sweep** | / | Datastore process-lock shared | gc_mutex, exclusive ProcessLocker | gc_mutex | / | /, chunks already marked | see forget column | chunks get touched; chunk_store.mutex; oldest PL lock | / | / |
//! | **update manifest** | / | / | / | / | update_manifest lock | update_manifest lock, remove dir under lock | see forget column | /, “write manifest” happens at the end | /, can call “write manifest”, see that column | / |
//! | **forget** | / | / | removed_during_gc mutex is held during unlink | marking done, doesn’t matter if forgotten now | update_manifest lock, forget waits for lock | /, unlink is atomic | causes forget to fail, but that’s OK | running backup has snapshot flock | /, potentially detects missing folder | shared snap flock |
//! | **prune** | / | / | see forget row | see forget row | see forget row | causes warn in prune, but no error | see forget column | running and last non-running can’t be pruned | see forget row | shared snap flock |
//! | **create backup** | / | only time this happens, thus has snapshot flock | / | chunks get touched; chunk_store.mutex; oldest PL lock | no lock, but cannot exist beforehand | snapshot flock, can’t be forgotten | running and last non-running can’t be pruned | snapshot group flock, only one running per group | /, won’t be verified since manifest missing | / |
//! | **verify** | / | / | / | / | see “update manifest” row | /, potentially detects missing folder | see forget column | / | /, but useless (“update manifest” protects itself) | / |
//! | **reader api** | / | / | / | /, open snap can’t be forgotten, so ref must exist | / | prevented by shared snap flock | prevented by shared snap flock | / | / | /, lock is shared |!
//! * / = no interaction
//! * shared/exclusive from POV of 'starting' process
use anyhow::{bail, Error};