Added documentation about backup_file_size about memory usage

I just spent a while chasing phantom OOM crashes in the backup
agent; I'm hoping this comment will help future users avoid
my problem.

Change-Id: I58ff10d212283b032cabf623021d3955111ff8f0

cinder/backup/drivers/posix.py

@@ -41,7 +41,10 @@ posixbackup_service_opts = [
                default=BACKUP_FILE_SIZE,
                help='The maximum size in bytes of the files used to hold '
                     'backups. If the volume being backed up exceeds this '
-                    'size, then it will be backed up into multiple files.'
+                    'size, then it will be backed up into multiple files. '
+                    'backup_file_size also determines the buffer size '
+                    'used to build backup files, so should be scaled '
+                    'according to available RAM and number of workers. '
                     'backup_file_size must be a multiple of '
                     'backup_sha_block_size_bytes.'),
     cfg.IntOpt('backup_sha_block_size_bytes',

doc/source/admin/volume-backups.rst

@@ -141,7 +141,9 @@ deduplication or compression on the backup data.
 The option ``backup_file_size`` must be a multiple of
 ``backup_sha_block_size_bytes``. It is effectively the maximum file size to be
 used, given your environment, to hold backup data. Volumes larger than this
-will be stored in multiple files in the backup repository. The
+will be stored in multiple files in the backup repository. ``backup_file_size``
+also determines the buffer size used to produce backup files; on smaller hosts
+it may need to be scaled down to avoid OOM issues. The
 ``backup_sha_block_size_bytes`` option determines the size of blocks from the
 cinder volume being backed up on which digital signatures are calculated in
 order to enable incremental backup capability.