Missing files in pg_xlog, pg_subtrans and pg_clog directoriesduring file system based backup

Hi,

While performing file system based backup, after running pg_start_backup, we create a list of files in data directory called collect file.

Then backup the files listed in collect file.

This is approximately 35TB database. Very frequently we are noticing there are failed files in pg_xlog, pg_subtrans and pg_clog directory. It means, the files were present when list of files is created (Collect file). However, it is not there while backing up.

Is the backup valid with these failed files. Any suggestion would help us.

Thanks in advance!!

Regards,

Preeti