[BP to 6.0.4] - couchstore node flushing doesn't respect node quota size

Description

Backport of MB-36424 to 6.0.4

Summary

When couchstore writes out modified node elements during document saves, it it supposed to limit the number of bytes written in a single node to a _chunk_threshold - by default 1279 bytes. If the node is larger than the limit it should be split into multiple sibling nodes.

However, this limit is not respected, resulting in overly-large nodes being written out. In the case of the by-seqno B-Tree (which always writes values to the rightmost leaf as seqnos are increasing), it results in all leaf elements residing in a single leaf node. Moreover, this means that adding another element to the B-Tree effectively re-writes the entire tree, resulting in massive Write Amplification.

Steps to Reproduce

  1. Start 2-node cluster run (single replica):

  2. Start a SyncWrite workload; single threaded client updating the same 100,000 items (each key 10 times) with level=persistMajority:

  3. Observe the op/s and Write Amplification

Expected Results

  • Op/s should be broadly constant (given both key and seqno B-Tree should have a constant number of items in them).

  • Write amplification should also be broadly constant.

Actual Results

  • The op/s quickly drops from a peak of ~600 down to 150:

  • The Write Amplification increase (corresponding with the top in op/s) from 5.6x, up to 743x .

  • Both op/s and Write Amplification temporarily recover when compaction occurs, but same pattern is observed over time:

Components

Fix versions

Environment

None

Link to Log File, atop/blg, CBCollectInfo, Core dump

None

Release Notes Description

None

Attachments

2

Activity

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CB robot June 9, 2020 at 9:08 AM

Build couchbase-server-7.0.0-2304 contains couchstore commit 143858d with commit message:
: Ensure flush_mr_partial() obeys node size quota

CB robot April 29, 2020 at 6:25 AM

Build couchbase-server-1006.5.1-1125 contains couchstore commit 143858d with commit message:
: Ensure flush_mr_partial() obeys node size quota

CB robot February 20, 2020 at 12:24 PM

Build couchbase-server-1006.5.1-1065 contains couchstore commit 143858d with commit message:
: Ensure flush_mr_partial() obeys node size quota

CB robot February 6, 2020 at 4:15 PM

Build couchbase-server-6.5.1-6127 contains couchstore commit 143858d with commit message:
: Ensure flush_mr_partial() obeys node size quota

CB robot November 8, 2019 at 11:12 AM

Build couchbase-server-6.0.4-3020 contains couchstore commit 143858d with commit message:
: Ensure flush_mr_partial() obeys node size quota

Fixed
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Details

Assignee

Ritesh Agarwal

Reporter

Is this a Regression?

Yes

Triage

Triaged

Priority

Instabug

Open Instabug

PagerDuty

Sentry

Zendesk Support

Created November 7, 2019 at 10:45 PM
Updated June 9, 2020 at 9:08 AM
Resolved November 8, 2019 at 10:23 AM
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