Suppose I have two avro data files containing a number of records. Can I
simply concatenate them together to have a single avro data file without
loosing any records or do I need to actually read them and then write them?

If the schemas are identical, you can append:

http://avro.apache.org/docs/current/api/java/org/apache/avro/file/DataFileWriter.html#appendAllFrom%28org.apache.avro.file.DataFileStream,%20boolean%29

If the compression codec is the same, it will just append block by block without re-serialization or re-compression (very fast).  You can also force it to re-compress if you wish.
On 4/21/11 4:28 PM, "Andrew Hammond" <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:

Suppose I have two avro data files containing a number of records. Can I simply concatenate them together to have a single avro data file without loosing any records or do I need to actually read them and then write them?

> If the schemas are identical, you can append:

You can't, however, use the Unix "cat" command.  That will give you a second Avro file header halfway through your new file, which is invalid.