several questions about pg_dumpall, pg_start_backup, pg_basebackup and WAL

Hi all,

I have some questions about backup on PostgreSQL:

1) pg_dumpall
I am sure that this is asked in somewhere: why the pg_dumpall does not support custom or tar format like pg_dump.
I have heard that the custom format could be faster and may generate a smaller dump file.
Is the feature enhancement being developed?

2) pg_start_backup in postgresql 8.4
After issuing pg_start_backup, we can take the file based backup (by tar/snapshot) on the postgresql data directory.
If the use of file system snapshot is not possible.
It is possible that there is file changes (added or file size changed) between the pg_start_backup and pg_stop_backup.
Is the backup consistent?

3) extension of 2): If the postgresql version is 9.2
Is pg_basebackup better than pg_start_backup + file based backup + pg_stop_backup?

4) For the WAL backup in postgresql 8.4
After the archive mode is on and WAL is backup, how do I remove the old WAL files?
How about newer version (for example 9.2?)

Thanks and regards,
Patrick