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src/backup.rs: start explaining different GC algorithm

This commit is contained in:
Dietmar Maurer 2019-03-30 17:21:40 +01:00
parent 8a4757342c
commit c8ec450e37
1 changed files with 27 additions and 5 deletions
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32
src/backup.rs
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@ -17,7 +17,7 @@
//! 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 kink of locking for the
//! 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
@ -39,7 +39,8 @@
//!
//! Acquire shared lock for ChunkStore (process wide).
//!
//! Note: We create temporary (.tmp) file, then do an atomic rename ...
//! Note: When creating .idx files, we create temporary (.tmp) file,
//! then do an atomic rename ...
//!
//!
//! * Garbage Collect:
@ -56,7 +57,7 @@
//! socket.
//!
//!
//! # Garbage Collection
//! # Garbage Collection (GC)
//!
//! Deleting backups is as easy as deleting the corresponding .idx
//! files. Unfortunately, this does not free up any storage, because
@ -69,10 +70,31 @@
//! store.
//!
//! The above locking mechanism makes sure that we are the only
//! process running GC.
//! 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.
//!
//! ## `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 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).
//!
//! Another problem arise when running backups references old
//! chunks. We need to make sure that the sweep does not remove such
//! chunks. Not sure how to implement that.
//!
//! ## Store `marks` in RAM using a HASH
//!
//! Not sure if this is better. TODO
mod chunk_stat;
pub use chunk_stat::*;