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From Viktor Jevdokimov <vjevdoki...@gmail.com>
Subject Re: Reads and memory usage clarification
Date Wed, 23 Feb 2011 10:12:35 GMT
Everything as I thought, thank you!

2011/2/23 Matthew Dennis <mdennis@datastax.com>

> Data is in Memtables from writes before they get flushed (based on first
> threshold of ops/size/time exceeded; all are configurable) to SSTables on
> disk.
>
> There is a keycache and a rowcache.  The keycache caches offsets into
> SSTables for the rows.  the rowcache caches the entire row.  There is also
> the OS page cache which is heavily used.
>
> When a read happens, the keycache is updated with the information for the
> SSTables the row was eventually found in.  If there are too many entries now
> in the keycache, some are ejected.  Overall the keycache uses very little
> memory per entry and can cut your disk IO in half so it's a pretty big win.
>
> If you read an entire row it goes in the row cache.  Like the keycache,
> this may result in older entries being ejected from the cache.  If you
> insert lots of really large rows in the rowcache you can OOM your JVM.  The
> rowcache is kept up to date with the memtables as writes come in.
>
> When a read comes in, C* will collect the data from the SSTables and
> Memtables and merge them together but data only goes into Memtables from
> writes.
>
>
> On Tue, Feb 22, 2011 at 3:32 AM, Viktor Jevdokimov <vjevdokimov@gmail.com>wrote:
>
>> Hello,
>>
>> Write path is perfectly documented in architecture overview.
>>
>> I need Reads to be clarified:
>>
>> How memory is used
>> 1. When data is in the Memtable
>> 2. When data is in the SSTable
>>
>> How cache is used alongside with Memtable?
>>
>> Are records created in the Memtable from writes only or from reads also?
>>
>> What I need to know is, how Cassandra uses memory and Memtables for reads?
>>
>>
>> Thenk you,
>> Viktor
>>
>
>

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