New SSTable Format
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Key: CASSANDRA-674
URL: https://issues.apache.org/jira/browse/CASSANDRA-674
Project: Cassandra
Issue Type: Improvement
Components: Core
Affects Versions: 0.9
Reporter: Stu Hood
Assignee: Stu Hood
Fix For: 0.9
Various tickets exist due to limitations in the SSTable file format, including
#16, #47 and #328. Attached is a proposed design/implementation of a new file
format for SSTables that addresses a few of these limitations. The
implementation has a bunch of issues/fixmes, which I'll describe in the
comments.
The file format is described in the javadoc for the o.a.c.io.SSTableWriter
class, but briefly:
* Blocks are opaque (except for their header) so that they can be compressed.
The index file contains an entry for the first key in every Block. Blocks
contain Slices.
* Slices are series of columns with the same parents and (deletion) metadata.
They can be used to represent ColumnFamilies or SuperColumns (or a slice of
columns at any other depth). A single CF can be split across multiple Slices,
which can be split across multiple blocks.
* Neither Slices nor Blocks have a fixed size or maximum length, but they each
have target lengths which can be stretched and broken by very large columns.
The most interesting concepts from this patch are:
* Block compression is possible (currently using GZIP, which has one bug
mentioned in the comments),
* Compaction involves merging intersecting Slices from input SSTables. Since
large rows will be broken down into multiple slices, only the portions of rows
that intersect between tables need to be deserialized/merged/held-in-memory,
* Indexes for individual rows are gone, since the global index allows random
access to the middle of column families that span Blocks, and Slices allow
batches of columns to be skipped within a Block.
* Bloom filters for individual rows are gone, and the global filter contains
ColumnKeys instead, meaning that a query for a column that doesn't exist in a
row that does will often not need to seek to the row.
* Metadata (deletion/gc time) and ColumnKeys (key, colname1, colname2...) for
columns are defined recursively, so deeply nested slices are possible,
* Slices representing a single parent (CF, SC, etc) can have different
Metadata, meaning that a tombstone Slice from d-f could sit between Slices
containing columns a-c and g-h. This allows for eventually consistent range
deletes of columns.
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