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etcdserver: keep a min number of entries in memory #2403

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Mar 1, 2015
Merged

etcdserver: keep a min number of entries in memory #2403

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9 changes: 9 additions & 0 deletions etcdserver/raft.go
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,15 @@ import (
"github.com/coreos/etcd/wal/walpb"
)

const (
// Number of entries for slow follower to catch-up after compacting
// the raft storage entries.
// We expect the follower has a millisecond level latency with the leader.
// The max throughput is around 10K. Keep a 5K entries is enough for helping
// follower to catch up.
numberOfCatchUpEntries = 5000
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Why did we pick 5000 here?

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add a comment for why we choose 5000.

)

var (
// indirection for expvar func interface
// expvar panics when publishing duplicate name
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10 changes: 8 additions & 2 deletions etcdserver/server.go
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Expand Up @@ -836,8 +836,14 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
if err := s.r.storage.SaveSnap(snap); err != nil {
log.Fatalf("etcdserver: save snapshot error: %v", err)
}
log.Printf("etcdserver: saved snapshot at index %d", snap.Metadata.Index)

err = s.r.raftStorage.Compact(snapi)
// keep some in memory log entries for slow followers.
compacti := uint64(1)
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Does this mean we will always keep 1 log entry in memory? Why 1 and not say 10?

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compact 1 means do not nothing rather than keeping 1.

if the snapshot index is smaller than what we should keep, we should keep all of them.

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I wonder if we can express that fact in the comment, it's not clear what's going on here.

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i think the code is clear enough. and we do not comment code path in general.
you need to check the compact comment to see what is going on.

// Compact discards all log entries prior to i.
// It is the application's responsibility to not attempt to compact an index
// greater than raftLog.applied.

so it is clear that compact(1) = discard nothing.

if snapi > numberOfCatchUpEntries {
compacti = snapi - numberOfCatchUpEntries
}
err = s.r.raftStorage.Compact(compacti)
if err != nil {
// the compaction was done asynchronously with the progress of raft.
// raft log might already been compact.
Expand All @@ -846,7 +852,7 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
}
log.Panicf("etcdserver: unexpected compaction error %v", err)
}
log.Printf("etcdserver: saved snapshot at index %d", snap.Metadata.Index)
log.Printf("etcdserver: compacted raft log at %d", compacti)
}()
}

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