Differential and Incremental backups

miniman

Hi



Am I correct in saying that there is no difference between a differential and Incremental backup in 2000? They both archive what has been changed since the last full backup. Surely there must be a slight difference otherwise why the diffferent names

miniman

NPA24

There is a main difference. This is what it says at Microsoft's site.

Differential backups - Designed to create backup copies of files that have changed since the last normal backup. The presence of the archive attribute indicates that the file has been modified and only files with this attribute are backed up. However, the archive attribute on files isn't modified. This allows you to perform other types of backups on the files at a later date.

Incremental backups - Designed to create backups of files that have changed since the most recent normal or incremental backup. The presence of the archive attribute indicates that the file has been modified and only files with this attribute are backed up. When a file is backed up, the archive attribute is cleared. If the file is later modified, this attribute is set, which indicates that the file needs to be backed up.

Ref. http://www.microsoft.com/technet/prodtechnol/windows2000serv/maintain/operate/14w2kada.mspx

miniman

Thanks for your reply. So basically the only difference is that the attributes are cleared/marked up for later backup?

miniman

NPA24

You will see more about the differences of these two backup methods when you are studying for your Server+. I don't believe they were on my A+ test when I took it. But to answer your question, the difference of both of them is what the criteria a company wants in restoring the backup the fastest or whether a company doesn't care how long it takes for a daily backup. If you want the backups to be fast at the end of a day, you would want to do incremental backup because it is backing up less data. But if a company wants to restore a backup without using so many backup tapes then you would use differential backup. In both scenarios you would still need to use a Full backup tape. It's a mix and match scenario and like I said you don't really need to know it in your A+ exam.

tromer1

Actually the difference is quite significant. A differential backup records all changes since the last full backup. An incremental backups all changes since the last backup. As noted, to operate in this manner a differential backup DOES NOT clear the archive bit, thus each differential backup archives all files since the last full backup by simply backing up all files with the archive bit set. An incremental backup also backs up all files since the last backup by backing up all files with the archive bit set. However, it clears the archive bit for the files backed up.

Let's examine this with a concrete example. John and Jane both complete full backups to QIC tapes each Saturday. At this point the backup bit of all files is cleared. John creates a differential backup each other weekday. Sunday's backup records 100 files that changed. Monday's backup has 200 files that changed, inclusive of the files recorded on the prior backup. Tuesday's backup has 300 files that changed, inclusive of all 200 files that changed on the prior days. By Thursday the most recent differential backup has perhaps 500 files... all files changed since the prior full backup on Saturday. To restore his system John only needs to restore the backups from the prior Saturday and Thursday, because the Thursday backup includes ALL files changed after Saturday's full backup.

Jane completes a daily incremental backup. Each day about 100 files are changed, and thus each backup has 100 files and thus complete quicker than Jane's differential backups. Some files for both John and Jane's daily backups overlapped, of course, because they changed multiple days. (For example the registry). This had no impact to Jane since the last backup included all changes since the prior full backup. However John must restore hist system by restoring ALL incremental backup tapes in addition to the full backup. Thus although his INCREMENTAL backups completed normally he must restore Saturday's full backup, Sunday's incremental Backup, Monday's backup, Tuesday's backup, Wednesday's Backup, AND Thursday's backup. In addition to the time needed to swap tapes files with multiple changes will be restored each time backed up for Jane while they only needed to be restored once from the differential backup for John.

miniman

Thanks people for the full explanation of both types of backup.
miniman

garv221

Incremental backup faster but restore slower.

Differential backup slower but restore quickly.