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From Aaron Morton <aa...@thelastpickle.com>
Subject Re: Cassandra to store 1 billion small 64KB Blobs
Date Mon, 26 Jul 2010 00:13:12 GMT
For what it's worth...

* Many smaller boxes with local disk storage are preferable to 2 with huge NAS storage.
* To cache the hash values look at the KeysCached setting in the storage-config
* There are some row size limits see http://wiki.apache.org/cassandra/CassandraLimitations
* If you wanted to get 1000 blobs, rather then group them in a single row using a super column
consider building a secondary index in a standard column One CF for the blobs using your hash,
one CF that uses whatever they grouping key is with a col for every blobs hash value. Read
from the index first, then from the blobs themselves.

Aaron

On 24 Jul, 2010,at 06:51 PM, Michael Widmann <michael.widmann@gmail.com> wrote:

> Hi Jonathan
>
> Thanks for your very valuable input on this.
>
> I maybe didn't enough explanation - so I'll try to clarify
>
> Here are some thoughts:
>
>     * binary data will not be indexed - only stored. 
>     * The file name to the binary data (a hash) should be indexed for search
>     * We could group the hashes in 62 "entry" points for search retrieving -> i think
suprcolumns (If I'm right in terms) (a-z,A_Z,0-9)
>     * the 64k Blobs meta data (which one belong to which file) should be stored separate
in cassandra
>     * For Hardware we rely on solaris / opensolaris with ZFS in the backend
>     * Write operations occur much more often than reads
>     * Memory should hold the hash values mainly for fast search (not the binary data)
>     * Read Operations (restore from cassandra) may be async - (get about 1000 Blobs)
- group them restore
>
> So my question is too: 
>
> 2 or 3 Big boxes or 10 till 20 small boxes for storage...
> Could we separate "caching" - hash values CFs cashed and indexed - binary data CFs not
...
> Writes happens around the clock - on not that tremor speed but constantly
> Would compaction of the database need really much disk space
> Is it reliable on this size (more my fear)
>
> thx for thinking and answers...
>
> greetings
>
> Mike
>
> 2010/7/23 Jonathan Shook <jshook@gmail.com>
>
>     There are two scaling factors to consider here. In general the worst
>     case growth of operations in Cassandra is kept near to O(log2(N)) Any
>     worse growth would be considered a design problem, or at least a high
>     priority target for improvement.  This is important for considering
>     the load generated by very large column families, as binary search is
>     used when the bloom filter doesn't exclude rows from a query.
>     O(log2(N)) is basically the best achievable growth for this type of
>     data, but the bloom filter improves on it in some cases by paying a
>     lower cost every time.
>
>     The other factor to be aware of is the reduction of binary search
>     performance for datasets which can put disk seek times into high
>     ranges. This is mostly a direct consideration for those installations
>     which will be doing lots of cold reads (not cached data) against large
>     sets. Disk seek times are much more limited (low) for adjacent or near
>     tracks, and generally much higher when tracks are sufficiently far
>     apart (as in a very large data set). This can compound with other
>     factors when session times are longer, but that is to be expected with
>     any system. Your storage system may have completely different
>     characteristics depending on caching, etc.
>
>     The read performance is still quite high relative to other systems for
>     a similar data set size, but the drop-off in performance may be much
>     worse than expected if you are wanting it to be linear. Again, this is
>     not unique to Cassandra. It's just an important consideration when
>     dealing with extremely large sets of data, when memory is not likely
>     to be able to hold enough hot data for the specific application.
>
>     As always, the real questions have lots more to do with your specific
>     access patterns, storage system, etc. I would look at the benchmarking
>     info available on the lists as a good starting point.
>
>
>     On Fri, Jul 23, 2010 at 11:51 AM, Michael Widmann
>     <michael.widmann@gmail.com> wrote:
>     > Hi
>     >
>     > We plan to use cassandra as a data storage on at least 2 nodes with RF=2
>     > for about 1 billion small files.
>     > We do have about 48TB discspace behind for each node.
>     >
>     > now my question is - is this possible with cassandra - reliable - means
>     > (every blob is stored on 2 jbods)..
>     >
>     > we may grow up to nearly 40TB or more on cassandra "storage" data ...
>     >
>     > anyone out did something similar?
>     >
>     > for retrieval of the blobs we are going to index them with an hashvalue
>     > (means hashes are used to store the blob) ..
>     > so we can search fast for the entry in the database and combine the blobs to
>     > a normal file again ...
>     >
>     > thanks for answer
>     >
>     > michael
>     >
>
>
>
>
> -- 
> bayoda.com - Professional Online Backup Solutions for Small and Medium Sized Companies

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