Botond Dénes bc1fcd3db2 multishard_combining_reader: only read from needed shards ...

The multishard combining reader currently assumes that all shards have
data for the read range. This however is not always true and in extreme
cases (like reading a single token) it can lead to huge read
amplification. Avoid this by not pushing shards to
`_shard_selection_min_heap` if the first token they are expected to
produce falls outside of the read range. Also change the read ahead
algorithm to select the shards from `_shard_selection_min_heap`, instead
of walking them in shard order. This was wrong in two ways:
* Shards may be ordered differently with respect to the first partition
  they will produce; reading ahead on the next shard in shard order
  might not bring in data on the next shard the read will continue on.
  Shard order is only correct when starting a new range and shards are
  iterated over in the order they own tokens according to the sharding
  algorithm.
* Shards that may not have data relevant to the read range are also
  considered for read ahead.

After this patch, the multishard reader will only read from shards that
have data relevant to the read range, both in the case of normal reads
and also for read-ahead.

Fixes: #8161

Tests: unit(release)
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20210226132536.85438-1-bdenes@scylladb.com>

2021-02-26 23:29:20 +02:00

110 KiB

Raw Blame History

View Raw