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From "Stu Hood (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2319) Promote row index
Date Sat, 24 Mar 2012 09:12:27 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2319?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13237483#comment-13237483
] 

Stu Hood commented on CASSANDRA-2319:
-------------------------------------

bq. We ended up making compression at the SequentialWriter level, i.e. transparent from the
data and index format, so this end up having no interaction whatsoever with compression.
So, this isn't _quite_ true: due to the column/tuple index being unaware of blocks, and vice-versa,
the current scheme has to deal with alignment mismatches: tuples which fall onto block boundaries
involve reading two blocks worth of data in order to decompress one tuple. Not a very frequent
occurrence when the tuple size is small enough... but anyway.
                
> Promote row index
> -----------------
>
>                 Key: CASSANDRA-2319
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2319
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Assignee: Sylvain Lebresne
>              Labels: index, timeseries
>             Fix For: 1.2
>
>         Attachments: 2319-v1.tgz, 2319-v2.tgz, promotion.pdf, version-f.txt, version-g-lzf.txt,
version-g.txt
>
>
> The row index contains entries for configurably sized blocks of a wide row. For a row
of appreciable size, the row index ends up directing the third seek (1. index, 2. row index,
3. content) to nearby the first column of a scan.
> Since the row index is always used for wide rows, and since it contains information that
tells us whether or not the 3rd seek is necessary (the column range or name we are trying
to slice may not exist in a given sstable), promoting the row index into the sstable index
would allow us to drop the maximum number of seeks for wide rows back to 2, and, more importantly,
would allow sstables to be eliminated using only the index.
> An example usecase that benefits greatly from this change is time series data in wide
rows, where data is appended to the beginning or end of the row. Our existing compaction strategy
gets lucky and clusters the oldest data in the oldest sstables: for queries to recently appended
data, we would be able to eliminate wide rows using only the sstable index, rather than needing
to seek into the data file to determine that it isn't interesting. For narrow rows, this change
would have no effect, as they will not reach the threshold for indexing anyway.
> A first cut design for this change would look very similar to the file format design
proposed on #674: http://wiki.apache.org/cassandra/FileFormatDesignDoc: row keys clustered,
column names clustered, and offsets clustered and delta encoded.

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