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From "Stu Hood (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-674) New SSTable Format
Date Thu, 06 Jan 2011 07:13:50 GMT


Stu Hood commented on CASSANDRA-674:

> If we assume we keep the datamodel as is how can we simplify the open ended-ness of your
design to make the approach fit our current data model.
To keep this from becoming a point of contention, I'll remove that goal from the design doc:
the design so far has this feature as a side effect though.

> New SSTable Format
> ------------------
>                 Key: CASSANDRA-674
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>             Fix For: 0.8
>         Attachments: 674-v1.diff, perf-674-v1.txt, perf-trunk-2f3d2c0e4845faf62e33c191d152cb1b3fa62806.txt
> 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 class, but
>  * 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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