blambov commented on code in PR #2836:
URL: https://github.com/apache/cassandra/pull/2836#discussion_r1374242013
##########
src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.md:
##########
@@ -363,6 +423,24 @@ UCS accepts these compaction strategy parameters:
minimum compaction concurrency for the lowest levels. A low number would
result in larger L0 sstables but may limit
the overall maximum write throughput (as every piece of data has to go
through L0).
The default value is 4 (1 for system tables, or when multiple data locations
are defined).
+* **sstable_growth** The sstable growth component $\lambda$, applied as a
factor in the shard exponent calculation.
+ This is a number between 0 and 1 that controls what part of the density
growth should apply to individual sstable
+ size and what part should increase the number of shards. Using a value of 1
has the effect of fixing the shard
+ count to the base value. Using 0.5 makes the shard count and sstable size
grow with the square root of the density
+ growth.
+ This is useful to decrease the sheer number of sstables that will be created
for very large data sets. For
+ example, without growth correction a data set of 10TiB with 1GiB target size
would result in over 10k sstables,
+ which may present as too much overhead both as on-heap memory used by
per-sstable structures as well as time to look
+ for intersecting sstables and tracking overlapping sets during compaction.
Applying $\lambda=0.5$
+ in this scenario (with base count 4) will reduce the potential number of
sstables to ~160 of ~64GiB, which is still
+ manageable both as memory overhead and individual compaction duration and
space overhead. The balance between the
+ two can be further tweaked by increasing $\lambda$ to get fewer but bigger
sstables on the top level, and decreasing
+ it to favour a higher count of smaller sstables. The default value is 0.333
meaning the sstable size
+ grows with the square root of the growth of the shard count
Review Comment:
End the sentence with a period.
##########
src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.md:
##########
@@ -214,6 +214,66 @@ deletions, the resulting sstables will be of size 75 MiB,
token share 1/16 and d
This sharding mechanism is independent of the compaction specification.
+## Full sharding scheme
+
+This sharding scheme easily admits extensions. In particular, when the size of
the data set is expected to grow very
+large, to avoid having to pre-specify a high enough target size to avoid
problems with per-sstable overhead, we can
+apply an "sstable growth" parameter, which determines what part of the density
growth should be assigned to increased
+SSTable size, reducing the growth of the number of shards (and hence
non-overlapping sstables).
+
+Additionally, to allow for a mode of operation with a fixed number of shards,
and splitting conditional on reaching
+a minimum size, we provide for a "minimum sstable size" that reduces the base
shard count whenever that would result
+in sstables smaller than the provided minimum.
+
+Generally, the user can specify four sharding parameters:
+
+- base shard count $b$
+- target sstable size $t$
+- minimum sstable size $m$
+- sstable growth component $\lambda$
+
+The number of shards $S$ for a given density $d$ is then calculated as
+
+$$
+S =
+\begin{cases}
+1
+ & \text{if } d < m \\
+min(2^{\left\lfloor \log_2 \frac d m \right\rfloor}, x)
+ & \text{if } d < mb \text{, where } x \text{ is the largest power of 2
divisor of } b \\
+b
+ & \text{if } d < tb \\
+2^{\left\lfloor (1-\lambda) \cdot \log_2 \left( {\frac d t \cdot \frac 1
b}\right)\right\rceil} \cdot b
+ & \text{otherwise}
+\end{cases}
+$$
+
+Some useful combinations of these parameters:
+
+- The basic scheme above uses a sstable growth $\lambda=0$, and a minimum
sstable size $m=0$. The graph below
+ illustrates it for base shard count $b=4$ and target sstable size
$t=1\mathrm{GB}$:
+
+
+
+- Using $\lambda = 0.5$ makes the strategy grow the shard count and sstable
size evenly. When the density
+ quadruples, both the shard count and the expected sstable size for that
density band will double. The example
+ below uses $b=8$, $t=1\mathrm{GB}$ and also applies a minimal size
$m=100\mathrm{MB}$:
+
+
+
+- Similarly, $\lambda = 1/3$ makes the sstable growth the cubic root of the
density growth, i.e. the sstable size
+ grows with the square root of the growth of the shard count. The graph below
uses $b=1$ and $t = 1\mathrm{GB}$
+ (note: when $b=1$ the minimal size has no effect):
+
+
+
+- A growth component of 1 constructs a hierarchy with exactly $b$ shards at
every level. Combined with a minumum
+ sstable size, this defines a mode of operation similar to UCS V1 (as used in
DSE 6.8), where we use a pre-specified
Review Comment:
Drop "similar to UCS V1 (as used in DSE 6.8),".
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