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Add badger index cache #6258

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SungJin1212
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@SungJin1212 SungJin1212 commented Oct 10, 2024 •
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Introduce a badgerDB (https://github.com/dgraph-io/badger) as a disk index cache. It could be a middle layer cache between in-memory and remote cache (in-memory -> (badger) -> Memcached or Redis).

There are two loop in the badger index cache, retentionLoop and diskStatUpdateLoop.
retentionLoop: periodically run badgerDB GC to sink disk space.
diskStatUpdateLoop: exports metric badger_value_log_space_available_bytes and badger_value_log_space_available_bytes track amount of disk space left on the value and key log used in Badger at mount point in bytes.

Which issue(s) this PR fixes:
Fixes #6241

Checklist

  • Tests updated
  • Documentation added
  • CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

@dosubot dosubot bot added go Pull requests that update Go code type/feature labels Oct 10, 2024
@SungJin1212 SungJin1212 marked this pull request as draft October 10, 2024 11:55
@SungJin1212 SungJin1212 force-pushed the Add-badger-index-cache branch 13 times, most recently from be32122 to 3ed5938 Compare October 11, 2024 06:29
yeya24
yeya24 reviewed Oct 14, 2024
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docs/configuration/config-file-reference.md Outdated

# [Experimental] How long to cache an index for a block.
# CLI flag: -blocks-storage.bucket-store.index-cache.badger.index-ttl
[index_ttl: <duration> | default = 24h]
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Can we make this the default value 24h for now? Let's not expose it if other index caches don't expose this parameter

docs/configuration/config-file-reference.md Outdated

# [Experimental] Keys Data directory in which to cache index data.
# CLI flag: -blocks-storage.bucket-store.index-cache.badger.keys-data-dir
[key_data_dir: <string> | default = "./index-cache/keys"]
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Let's keep only one parameter for the badger DB directory and use <dir>/keys and <dir>/values by default

pkg/storage/tsdb/badger_index_cache.go
// valueLogSpaceAvailable stores the amount of space left on the value log mount point in bytes
valueLogSpaceAvailable prometheus.Gauge
// keyLogSpaceAvailable stores the amount of space left on the key log mount point in bytes
keyLogSpaceAvailable prometheus.Gauge
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How does it calculate the amount of space left?

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It is calculated by # of available block * block size at the mount point.

pkg/storage/tsdb/badger_index_cache.go Outdated
)

const (
defaultDataDir = "./index-cache"
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Can we use badger-index-cache instead?

pkg/storage/tsdb/badger_index_cache.go
c.keyLogSpaceAvailable = promauto.With(reg).NewGauge(prometheus.GaugeOpts{
Name: "badger_key_log_space_available_bytes",
Help: "Amount of space left on the key log mount point in bytes.",
})
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Is there a good way to get used bytes instead of available bytes left?

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There is a method called Size.

pkg/storage/tsdb/badger_index_cache.go
ticker := time.NewTicker(interval)
defer ticker.Stop()

for range ticker.C {
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I recommend calling Size to get the size of the LSM key and values file and check if we need to run GC based on if it exceeds a threshold.

pkg/storage/tsdb/badger_index_cache.go Outdated

// err is raised when nothing to clean
for err == nil {
err = c.cache.RunValueLogGC(0.5)
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Does RunValueLogGC clean up the associated key files the same time when cleaning up the values?
Want to make sure we don't keep the values forever.

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By quickly checking some of the docs I think it doesn't clean up keys. It only discard those stale values after compaction. Not like how cache works to evict the key and value.
So the key and value still there and wait until TTL to be removed. I don't think it is the right behavior tbh. We should probably delete the key ourselves.

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SungJin1212 commented Oct 14, 2024 •
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@yeya24
Using Size, we can track bytes usage of vlog, lsm. But, I should do more digging to delete keys when some threshold reached.

@SungJin1212 SungJin1212 force-pushed the Add-badger-index-cache branch 8 times, most recently from 5c0d04f to 15b0dd0 Compare October 14, 2024 10:55
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This is a very interesting PR.. at some point i was thinking on doing something similar.

@SungJin1212 were you able to do some load test on this to see how it performs compared with the other caching solutions?

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@alanprot
Thank you for your interest in my work.
I haven't done the load test yet, but I was thinking that the load tests are necessary.
Which performance aspects are essential in the load test do you think?

@SungJin1212 SungJin1212 force-pushed the Add-badger-index-cache branch 2 times, most recently from 7ec38fb to 1002047 Compare October 25, 2024 07:46
@SungJin1212
Add badger index cache
34b98f6 
Signed-off-by: SungJin1212 <[email protected]>
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Store Gateway: index cache with local disk based backend
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