If all samples in consecutive chunks have the same timestamp, the way
we used to load chunks will fail. With this change, the persist
watermark is used to load the right amount of chunkDescs from disk.
This bug is a possible reason for the rare storage corruption we have
observed.
Fixes https://github.com/prometheus/prometheus/issues/481
While doing so, clean up and fix a few other things:
- Fix `go vet` warnings (@fabxc to blame ;).
- Fix a racey problem with unarchiving: Whenever we unarchive a
series, we essentially want to do something with it. However, until
we have done something with it, it appears like a series that is
ready to be archived or even purged. So e.g. it would be ignored
during checkpointing. With this fix, we always load the chunkDescs
upon unarchiving. This is wasteful if we only want to add a new
sample to an archived time series, but the (presumably more common)
case where we access an archived time series in a query doesn't
become more expensive.
- The change above streamlined the getOrCreateSeries ond
newMemorySeries flow. Also, the modTime is now always set correctly.
- Fix the leveldb-backed implementation of KeyValueStore.Delete. It
had the wrong behavior of still returning true, nil if a
non-existing key has been passed in.
This change is conceptually very simple, although the diff is large. It
switches logging from "github.com/golang/glog" to
"github.com/prometheus/log", while not actually changing any log
messages. V(1)-style logging has been changed to be log.Debug*().
Also, clean up some things in the code (especially introduction of the
chunkLenWithHeader constant to avoid the same expression all over the place).
Benchmark results:
BEFORE
BenchmarkLoadChunksSequentially 5000 283580 ns/op 152143 B/op 312 allocs/op
BenchmarkLoadChunksRandomly 20000 82936 ns/op 39310 B/op 99 allocs/op
BenchmarkLoadChunkDescs 10000 110833 ns/op 15092 B/op 345 allocs/op
AFTER
BenchmarkLoadChunksSequentially 10000 146785 ns/op 152285 B/op 315 allocs/op
BenchmarkLoadChunksRandomly 20000 67598 ns/op 39438 B/op 103 allocs/op
BenchmarkLoadChunkDescs 20000 99631 ns/op 12636 B/op 192 allocs/op
Note that everything is obviously loaded from the page cache (as the
benchmark runs thousands of times with very small series files). In a
real-world scenario, I expect a larger impact, as the disk operations
will more often actually hit the disk. To load ~50 sequential chunks,
this reduces the iops from 100 seeks and 100 reads to 1 seek and 1
read.
A number of mostly minor things:
- Rename chunk type -> chunk encoding.
- After all, do not carry around the chunk encoding to all parts of
the system, but just have one place where the encoding for new
chunks is set based on the flag. The new approach has caveats as
well, but the polution of so many method signatures is worse.
- Use the default chunk encoding for new chunks of existing
series. (Previously, only new _series_ would get chunks with the
default encoding.)
- Use an enum for chunk encoding. (But keep the version number for the
flag, for reasons discussed previously.)
- Add encoding() to the chunk interface (so that a chunk knows its own
encoding - no need to have that in a different top-level function).
- Got rid of newFollowUpChunk (which would keep the existing encoding
for all chunks of a time series). Now only use newChunk(), which
will create a chunk encoding according to the flag.
- Simplified transcodeAndAdd.
- Reordered methods of deltaEncodedChunk and doubleDeltaEncoded chunk
to match the order in the chunk interface.
- Only transcode if the chunk is not yet half full. If more than half
full, add a new chunk instead.
In that commit, the 'maintainSeries' call was accidentally removed.
This commit refactors things a bit so that there is now a clean
'maintainMemorySeries' and a 'maintainArchivedSeries' call.
Straighten the nomenclature a bit (consistently use 'drop' for
chunks and 'purge' for series/metrics).
Remove the annoying 'Completed maintenance sweep through archived
fingerprints' message if there were no archived fingerprints to do
maintenance on.
- Parallelize AppendSamples as much as possible without breaking the
contract about temporal order.
- Allocate more fingerprint locker slots.
- Do not run early checkpoints if we are behind on chunk persistence.
- Increase fpMinWaitDuration to give the disk more time for more
important things.
Also, switch math.MaxInt64 and math.MinInt64 to the new constants.
persistence.go is way too long anyway, and a lot of code is just crash
recovery, which is not important to understand the normal operation.
Also, remove unused `exists` function.
