Svccg nosql 2011_sri-cassandra

Editor's Notes

1. Typical write operation involves a write into a commit log for durability and recoverability and an update into an in-memory data structure. The write into the in-memory data structure is performed only after a successful write into the commit log. We have a dedicated disk on each machine for the commit log since all writes into the commit log are sequential and so we can maximize disk throughput. When the in-memory data structure crosses a certain threshold, calculated based on data size and number of objects, it dumps itself to disk. This write is performed on one of many commodity disks that machines are equipped with. All writes are sequential to disk and also generate an index for ecient lookup based on row key. These indices are also persisted along with the data le. Over time many such les could exist on disk and a merge process runs in the background to collate the different les into one le. This process is very similar to the compaction process that happens in the Bigtable system
2. “ A typical read operation rst queries the in-memory data structure before looking into the les on disk. The files are looked at in the order of newest to oldest. When a disk lookup occurs we could be looking up a key in multiple les on disk. In order to prevent lookups into les that do not contain the key, a bloom lter, summarizing the keys in the le, is also stored in each data le and also kept in memory. This bloom lter is rst consulted to check if the key being looked up does indeed exist in the given le. A key in a column family could have many columns. Some special indexing is required to retrieve columns which are further away from the key. In order to prevent scanning of every column on disk we maintain column indices which allow us to jump to the right chunk on disk for column retrieval. As the columns for a given key are being serialized and written out to disk we generate indices at every 256K chunk boundary. This boundary is congurable, but we have found 256K to work well for us in our production workloads.”
3. ConsistencyLevel