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From "Stu Hood (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-674) New SSTable Format
Date Wed, 05 Jan 2011 09:33:51 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-674?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12977697#action_12977697
] 

Stu Hood commented on CASSANDRA-674:
------------------------------------

> How will ranges be stored? The parent ordering would mean the sorting of data at that
level is lost no?
Added some explanation of how I think ranges should work to the wiki. http://wiki.apache.org/cassandra/FileFormatDesignDoc?action=diff&rev1=15&rev2=16

> Are chunks broken up by size only?
Technically "spans" are the largest unit, so they define the boundaries: tried to clarify
this part as well. There are a few possible thresholds, including a max number of rows, columns,
range tombstones or total bytes in the span.

One semi-undefined portion is what happens when a row is larger than can be stuffed in a span.
Most likely we'll want to use the range metadata to indicate the portion of the row covered
by the span (the approach I took in the original implementation attached here).

> Will the metadata be ripe for caching?
I don't think so: the metadata is useless on it's own. It only becomes useful when it is attached
to data (a column or to a range), so there is no reason to cache the meta- independently of
the data.

Thanks!

> New SSTable Format
> ------------------
>
>                 Key: CASSANDRA-674
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-674
>             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 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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