This is a stop-gap solution until we re-write the pruner to support
rules for removing step holds.
Note that disabling step holds for incremental sends does not affect
zrepl's guarantee that incremental replication is always possible:
Suppose you yank the external drive during an incremental @from -> @to step:
* restarting that step or future incrementals @from -> @to_later` will be possible
because the replication cursor bookmark points to @from until the step is complete
* resuming @from -> @to will work as long as the pruner on your internal pool doesn't come around to destroy @to.
* in that case, the replication algorithm should determine that the resumable state
on the receiving side isuseless because @to no longer exists on the sending side,
and consequently clear it, and restart an incremental step @from -> @to_later
refs #288
- drop HintMostRecentCommonAncestor rpc call
- it is wrong to put faith into the active side of the replication to always make that call
(we might not trust it, ref pull setup)
- clean up step holds + step bookmarks + replication cursor bookmarks on
send RPC instead
- this makes it symmetric with Receive RPC
- use a cache (endpoint.sendAbstractionsCache) to avoid the cost of
listing the on-disk endpoint abstractions state on every step
The "create" methods for endpoint abstractions (CreateReplicationCursor, HoldStep) are now fully
idempotent and return an Abstraction.
Notes about endpoint.sendAbstractionsCache:
- fills lazily from disk state on first `Get` operation
- fill from disk is generally only attempted once
- unless the `ListAbstractions` fails, in which case the fill from
disk is retried on next `Get` (the current `Get` will observe a
subset of the actual on-disk abstractions)
- the `Invalidate` method is called
- it is a global (zrepl process-wide) cache
fixes#316
- discovered during investigation of #316
- this is not the fix for #316, as a malicious receiver who doesn't
implement the behavior added by this patch could still cause leakage
of step holds on the sender
refs #316
package trace:
- introduce the concept of tasks and spans, tracked as linked list within ctx
- see package-level docs for an overview of the concepts
- **main feature 1**: unique stack of task and span IDs
- makes it easy to follow a series of log entries in concurrent code
- **main feature 2**: ability to produce a chrome://tracing-compatible trace file
- either via an env variable or a `zrepl pprof` subcommand
- this is not a CPU profile, we already have go pprof for that
- but it is very useful to visually inspect where the
replication / snapshotter / pruner spends its time
( fixes#307 )
usage in package daemon/logging:
- goal: every log entry should have a trace field with the ID stack from package trace
- make `logging.GetLogger(ctx, Subsys)` the authoritative `logger.Logger` factory function
- the context carries a linked list of injected fields which
`logging.GetLogger` adds to the logger it returns
- `logging.GetLogger` also uses package `trace` to get the
task-and-span-stack and injects it into the returned logger's fields
- prior to this patch, context cancellation would leave rpc.Server open
- did not make problems because context was only cancelled by SIGINT,
which was immediately followed by os.Exit
The motivation for this recatoring are based on two independent issues:
- @JMoVS found that the changes merged as part of #259 slowed his OS X
based installation down significantly.
Analysis of the zfs command logging introduced in #296 showed that
`zfs holds` took most of the execution time, and they pointed out
that not all of those `zfs holds` invocations were actually necessary.
I.e.: zrepl was inefficient about retrieving information from ZFS.
- @InsanePrawn found that failures on initial replication would lead
to step holds accumulating on the sending side, i.e. they would never
be cleaned up in the HintMostRecentCommonAncestor RPC handler.
That was because we only sent that RPC if there was a most recent
common ancestor detected during replication planning.
@InsanePrawn prototyped an implementation of a `zrepl zfs-abstractions release`
command to mitigate the situation.
As part of that development work and back-and-forth with @problame,
it became evident that the abstractions that #259 built on top of
zfs in package endpoint (step holds, replication cursor,
last-received-hold), were not well-represented for re-use in the
`zrepl zfs-abstractions release` subocommand prototype.
This commit refactors package endpoint to address both of these issues:
- endpoint abstractions now share an interface `Abstraction` that, among
other things, provides a uniform `Destroy()` method.
However, that method should not be destroyed directly but instead
the package-level `BatchDestroy` function should be used in order
to allow for a migration to zfs channel programs in the future.
- endpoint now has a query facitilty (`ListAbstractions`) which is
used to find on-disk
- step holds and bookmarks
- replication cursors (v1, v2)
- last-received-holds
By describing the query in a struct, we can centralized the retrieval
of information via the ZFS CLI and only have to be clever once.
We are "clever" in the following ways:
- When asking for hold-based abstractions, we only run `zfs holds` on
snapshot that have `userrefs` > 0
- To support this functionality, add field `UserRefs` to zfs.FilesystemVersion
and retrieve it anywhere we retrieve zfs.FilesystemVersion from ZFS.
- When asking only for bookmark-based abstractions, we only run
`zfs list -t bookmark`, not with snapshots.
- Currently unused (except for CLI) per-filesystem concurrent lookup
- Option to only include abstractions with CreateTXG in a specified range
- refactor `endpoint`'s various ZFS info retrieval methods to use
`ListAbstractions`
- rename the `zrepl holds list` command to `zrepl zfs-abstractions list`
- make `zrepl zfs-abstractions list` consume endpoint.ListAbstractions
- Add a `ListStale` method which, given a query template,
lists stale holds and bookmarks.
- it uses replication cursor has different modes
- the new `zrepl zfs-abstractions release-{all,stale}` commands can be used
to remove abstractions of package endpoint
- Adjust HintMostRecentCommonAncestor RPC for stale-holds cleanup:
- send it also if no most recent common ancestor exists between sender and receiver
- have the sender clean up its abstractions when it receives the RPC
with no most recent common ancestor, using `ListStale`
- Due to changed semantics, bump the protocol version.
- Adjust HintMostRecentCommonAncestor RPC for performance problems
encountered by @JMoVS
- by default, per (job,fs)-combination, only consider cleaning
step holds in the createtxg range
`[last replication cursor,conservatively-estimated-receive-side-version)`
- this behavior ensures resumability at cost proportional to the
time that replication was donw
- however, as explained in a comment, we might leak holds if
the zrepl daemon stops running
- that trade-off is acceptable because in the presumably rare
this might happen the user has two tools at their hand:
- Tool 1: run `zrepl zfs-abstractions release-stale`
- Tool 2: use env var `ZREPL_ENDPOINT_SENDER_HINT_MOST_RECENT_STEP_HOLD_CLEANUP_MODE`
to adjust the lower bound of the createtxg range (search for it in the code).
The env var can also be used to disable hold-cleanup on the
send-side entirely.
supersedes closes#293
supersedes closes#282fixes#280fixes#278
Additionaly, we fixed a couple of bugs:
- zfs: fix half-nil error reporting of dataset-does-not-exist for ZFSListChan and ZFSBookmark
- endpoint: Sender's `HintMostRecentCommonAncestor` handler would not
check whether access to the specified filesystem was allowed.
