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Date: Thu, 18 Feb 2010 02:37:27 +0000 (UTC)
From: "Stu Hood (JIRA)" <jira@apache.org>
To: cassandra-commits@incubator.apache.org
Subject: [jira] Commented: (CASSANDRA-674) New SSTable Format
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    [ https://issues.apache.org/jira/browse/CASSANDRA-674?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12835095#action_12835095 ] 

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

> it would be easier to skip from one group to another w/ the "slice" indexes next to each other
Since the block might be compressed, you can't assume random access to the whole data file: you might have to scan the block from the beginning anyway. So indexing externally to the data file at a resolution higher than blocks is of questionable value.

> 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
>            Assignee: Stu Hood
>             Fix For: 0.7
>
>         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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