Overwrite or Append on differential Backups?

Posted on 2006-05-10
Last Modified: 2008-01-09
I am using Microsoft Backup to backup the Documents and Settings folder and an application folder to an NAS drive. I created a baseline backup using the Normal method and then set a "Differential backup" to a different file name telling it to overwrite each time.

That seems to be working but I got to thinking....

What if there are files that change less often than the backups. If I have a file that gets backed up on the differential for say three or four nights, and then doesn't change for a few days. Does using the overwrite option mean that the file in question would not be backed up?

What is the best way to provide a good backup without creating a bunch of little files that I would need to restore individually in the event of a failure?
Question by:RandyTippetts
    LVL 16

    Expert Comment

    The BEST way to backup and know that you only need to restore from a single location is to OVERWRITE the entire backup each time it runs.

    Assuming you have enough drive space or tapes AND you can run this after hours, the fact that it takes longer is not important.

    Differential backup IMHO is useless when you need to restore.
    LVL 1

    Author Comment

    I prefer that option as well but I have 40gig of info being backup up on this server and several other automated backups running at night on the network. The longer each of them take the more they cut in to the off time which in our case is less than four hours. Even copying the folders to another location on the drive to be backed up during the active part of the day takes a considerable amount of time. Using the differential backup reduced that to between 15 seconds and three minutes.

    I am hoping that there is some combination that will make them useful.
    LVL 16

    Expert Comment

    Can you go to a DLT tape drive. That can hold lots of data.

    or 2....
    Also, you could backup Documents and Settings to the local PC....daily, then just copy it to the server once a week or so.
    That way you have two simultaneous backups and  less network traffic.

    0r 3...
    For \documents and settings\ you could also automate
    xcopy "c:\documents and settings\*.*"  "drive_letter:\\documents and settings\*.* " /y /s /d

    That will overwrite file automatically, include subdirectories and only copy files with a newer date stamp.

    Put it in a batch file and into the PC Scheduled tasks.
    LVL 1

    Expert Comment

    Whoa... Randy, the reason for backups is to provide for fault-tolerance first, incidental restore second.

    First question you should ask yourself is "How painful would it be if I lost my entire hard drive?" If your pucker factor just maxed out, then you should rethink your process.

    Backups should always be moved OFF disk to another location. That way, if your disk dies during the week, it does not kill your differential backups located on the same disk.

    Full backups means just that... everything on your system (and should include system state). Fulls reset (turns off) the archive bit on every file backed up.
    Incremental backs up the files changed since the last backup and resets the archive bit (turns it off) on files it backs up.
    Differential backs up the files changed since the last backup BUT DOES NOT RESET THE ARCHIVE BIT. So it collects ALL the files from the last FULL backup. These backups grow in size over the week or month.

    Best practice is to keep one Full backup and at least the last two full Diffs. So you could erase the second to the last Diff file every time but not the last. This way, if the system were to die in the middle of the Diff backup, you didn't erase your last Diff. Then you can restore your last full and your last Diff and be right back to sqare.

    BTW... are you backing up a workstation or a server? If its a server, you should be backing up the system state as well. Plus... if your using Win2k3 then you can use Snapshot restores for incidental restore of deleted/lost files.

    Hope that helps

    LVL 1

    Author Comment

    The reason I copy the data to another spot on the drive is to provide a static dataset that I can backup during active periods. I am trying to develop a backup strategy for both servers and workstations using NT backup. I have Novastor Novanet for some of my servers now but they don’t allow you to backup to a network NAS. My data is getting too large for the DAT drives we have on the servers.

    I have decided to run a primary “normal” backup weekly and follow up with differential backups nightly. My confusion is with the append or overwrite option on the differential backup. What am I gaining or loosing with each? What are the dangers?
    LVL 1

    Accepted Solution

    Then you already know that an append will create one file that grows a lot but does not eliminate the previous data. Overwrite will remove your last diff backup. Append is fine when you have plenty of disk space. Overwrite will save you disk space at the cost of fault-tolerance (redundant backups).

    Additionally, append allows you to stream several diff backups to the same file... i.e. \docs then \databse then System State... etc. It is also possible to run the ntbackup from a batch file that will allow you to script different file names for your diff backups depending on the day of the week. That way, if you have enough disk space, you can simply script out an entire weeks worth of backups. Of course, you could also use the backup scheduler as well to do the same thing. With the scripts, you could right a scipt that will change the name of the file each time you backup and delete the files that are older than say 2 weeks prior to current day.

    More directly... overwrite is risky since your removing the backups for the week prior to starting the current backup. If the system should die, your left with the last full backup. Append retains previous backups for the week but takes a lot of space on disk.

    Hope that helps...
    LVL 1

    Author Comment

    All good points. I think as far as having the storage space in concerned, it it better to do what you need to do to make the storage space available. I am going to go with append for a while on a weekly schedule and see what it does to my storage.

    Thanks all.

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