Hi Tim, I'm guessing a lot of these writes are happening on the reduce side. On the JT web interface, there are three columns: map, reduce, overall. Is the 900GB figure from the overall column? The value in the map column will probably be closer to what you were expecting. There are writes on the reduce side too during the shuffle and multi-pass merge.
Ed 2010/2/23 Tim Kiefer <[email protected]>: > Hi Gang, > > thanks for your reply. > > To clarify: I look at the statistics through the job tracker. In the > webinterface for my job I have columns for map, reduce and total. What I > was refering to is "map" - i.e. I see FILE_BYTES_WRITTEN = 3 * Map > Output Bytes in the map column. > > About the replication factor: I would expect the exact same thing - > changing to 6 has no influence on FILE_BYTES_WRITTEN. > > About the sorting: I have io.sort.mb = 100 and io.sort.factor = 10. > Furthermore, I have 40 mappers and map output data is ~300GB. I can't > see how that ends up in a factor 3? > > - tim > > Am 23.02.2010 14:39, schrieb Gang Luo: >> Hi Tim, >> the intermediate data is materialized to local file system. Before it is >> available for reducers, mappers will sort them. If the buffer (io.sort.mb) >> is too small for the intermediate data, multi-phase sorting happen, which >> means you read and write the same bit more than one time. >> >> Besides, are you looking at the statistics per mapper through the job >> tracker, or just the information output when a job finish? If you look at >> the information given out at the end of the job, note that this is an >> overall statistics which include sorting at reduce side. It also include the >> amount of data written to HDFS (I am not 100% sure). >> >> And, the FILE-BYTES_WRITTEN has nothing to do with the replication factor. I >> think if you change the factor to 6, FILE_BYTES_WRITTEN is still the same. >> >> -Gang >> >> >> ----- 原始邮件 ---- >> 发件人： Tim Kiefer <[email protected]> >> 收件人： "[email protected]" <[email protected]> >> 发送日期： 2010/2/23 (周二) 6:44:28 上午 >> 主 题： How are intermediate key/value pairs materialized between map and >> reduce? >> >> Hi there, >> >> can anybody help me out on a (most likely) simple unclarity. >> >> I am wondering how intermediate key/value pairs are materialized. I have a >> job where the map phase produces 600,000 records and map output bytes is >> ~300GB. What I thought (up to now) is that these 600,000 records, i.e., >> 300GB, are materialized locally by the mappers and that later on reducers >> pull these records (based on the key). >> What I see (and cannot explain) is that the FILE_BYTES_WRITTEN counter is as >> high as ~900GB. >> >> So - where does the factor 3 come from between Map output bytes and >> FILE_BYTES_WRITTEN??? I thought about the replication factor of 3 in the >> file system - but that should be HDFS only?! >> >> Thanks >> - tim >> >> >> >> ___________________________________________________________ >> 好玩贺卡等你发，邮箱贺卡全新上线！ >> http://card.mail.cn.yahoo.com/ >> >
