Time machine sometimes fails with no space left

Question

Hi,
I'm beginning to notice that sometimes Time Machine is failing to backup saying there is no space left on the Time Machine backup disk.  I'm using OS X 10.9.4 .
My understanding is that when there is not enough space left, Time Machine will automatically delete older backups to make space? Sometimes this happens, sometimes not.
For example:
Aug 10 18:39:49 Barney.local com.apple.backupd[9571]: Starting automatic backup
Aug 10 18:39:50 Barney.local com.apple.backupd[9571]: Backing up to /dev/disk6s2: /Volumes/TimeMachine/Backups.backupdb
Aug 10 18:40:04 Barney.local com.apple.backupd[9571]: Will copy (1 MB) from Macintosh HD
Aug 10 18:40:04 Barney.local com.apple.backupd[9571]: Will copy (Zero KB) from HD5
Aug 10 18:40:04 Barney.local com.apple.backupd[9571]: Will copy (Zero KB) from HD2
Aug 10 18:40:04 Barney.local com.apple.backupd[9571]: Found 64 files (1 MB) needing backup
Aug 10 18:40:04 Barney.local com.apple.backupd[9571]: 1.74 GB required (including padding), 7.41 GB available
Aug 10 18:40:26 Barney.local com.apple.backupd[9571]: Copied 70 items (1 MB) from volume Macintosh HD. Linked 709.
Aug 10 18:40:33 Barney.local com.apple.backupd[9571]: Copied 1 items (Zero KB) from volume HD5. Linked 2.
Aug 10 18:57:34 Barney.local backupd[9571]: Metadata.framework [Error]: server port invalidated for uid 0
Aug 10 19:01:02 Barney.local com.apple.backupd[9571]: Stopping backup.
Aug 10 19:01:02 Barney.local com.apple.backupd[9571]: Error: (-34) SrcErr:NO Copying /Volumes/HD2/Users/gj/Documents/2013.partial.mbox/mbox to /Volumes/TimeMachine/Backups.backupdb/Barney/2014-08-10-184001.inProgress/38868 CAA-6167-40B8-8F15-BF798FADF283/HD2/Users/gj/Documents/2013.partial.mbox
Aug 10 19:01:03 Barney.local com.apple.backupd[9571]: Copied 9 items (16.82 GB) from volume HD2. Linked 21.
Aug 10 19:01:03 Barney.local backupd[9571]: Metadata.framework [Error]: MDBackupCancel failed:-1 (4) (os/kern) invalid argument
Aug 10 19:01:03 Barney.local com.apple.backupd[9571]: Copy stage failed with error:28
Aug 10 19:01:04 Barney.local com.apple.backupd[9571]: Backup failed with error 28: 28
What I'm interpreting here is that initially TM says there isn't anything to copy from HD2, but then spends about 3.5 minutes copying a very large file from HD2 and then files due to being out of space.
Why did TM not realise this file existed in the first place?
Thanks for any help.
Steve

Answer 1

About TM "Backup Drive is Full"
Alert TM only deletes older files if they have been deleted from the source and when TM needs space on the backup drive for a new incremental backup. Time Machine "thins" it's backups; hourly backups over 24 hours old, except the first of the day; those "daily" backups over 30 days old, except the first of the week. The weeklies are kept as long as there's room.
So, how long a backup file remains depends on how long it was on your Mac before being deleted, assuming you do at least one backup per day. If it was there for at least 24 hours, it will be kept for at least a month. If it was there for at least a week, it will be kept as long as there's room.
Note, that on a Time Capsule the sparsebundle grows in size as needed, but doesn't shrink. Thus, from the user's view of the TC it appears that no space has been freed, although there may be space in the sparsebundle.
Once TM has found it cannot free up enough space for a new backup it reports the disk is full. You can either erase the backup drive and start your backups anew or replace the drive with a larger drive.