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Incremental and Differential Backups with Linux

Novell Cool Solutions: Feature
By Hanny Kraa

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Posted: 30 Nov 2007
 

In the OES documentation (to be more precise, in the OES Storage Management Services Administration Guide) there's a note that incremental and differential backups are not supported for non-NSS file systems on OES Linux, as these file systems do not support the Modify bit. 'Use the SBCON option to filter data sets based on the modification time to perform incremental or differential backups' is the advice.

When first reading this note you would think that this maybe is a minor drawback and has no impact on your Linux environment whatsoever, because you have an NSS filesystem. And because the NSS filesystem supports the Modify bit, incremental or differential backups should be no problem.

In fact it's a bit more complicated then that. Especially when your organization uses Windows based workstations!

First let's have a look why support of the Modify bit is needed.

Didn't backup solutions use the archive bit to see if a file has been previously backed up? If a file is modified, the bit is set. The backup application can remove the bit when performing a full backup .This is a common way for backup solutions to determine which files should be included in an incremental or differential backup.

Only the archive bit is a typical Windows filesystem property, just like the 'read only', 'system' and 'hidden' bits. Linux filesystems don't have an archive bit so it's impossible to use it when performing a differential or incremental backup.

So to backup a Linux file system you have to use another way to determine if a file has been touched since the last backup. Linux files have several modified time stamps and the backup software can use these instead of the archive bit. Of course the backup software has to support differential of incremental backups based on the Modified Time bit.

When both conditions are met (1: NSS filesystem, 2: backup solution which supports modified time) there still are some considerations to make.

When your organization still uses a Windows desktop environment, probably a lot of the files on your data servers will be created from Windows filesystems. All the documents, spreadsheets, photo's and all other kinds of files which your users create from their Windows desktops, are a Windows based file and have the Windows filesystem properties.

This means that these files do have the archive bit, but this bit won't be set by your Linux servers, like we already noticed. On the other hand, these Windows based files don't have the same kind of Modified Time property that the Linux filesystem uses.

Also, when you copy a new file to the network, the Modified Time bit will NOT be changed. For example: at home you create a new file and you save it on your USB stick. The next day at work you copy the file from your USB stick to your 'home directory' which is a drive mapping to a volume on the Linux environment. The modified time bit of the file won't be changed because the file itself isn't modified, only the location where it resides. Therefore the file won't be backed up until a full backup job runs. This is also the case when you receive files by email or when you download them from the internet. Or in any other case where you place a new, existing, windows-created file on a Linux server.

To prevent all these new files from not being backed up, there are two options that you need to add to a file on your Linux servers. However, contact the backup vendor first to check this with them as well. The options need to be added to the end of the file nssstart.cfg. This file can be found in /opt/novell/nss/conf/nssstart.cfg. The options are:

/ListXattrNWmetadata
/CtimeIsMetadataModTime

You may also need to have the latest kernel, NSS and NCP modules installed and also the latest NSS kernel modules package (km_nss).

Summarized, when you want to make a success of incremental and differential backups of your Linux system, here are the five steps you have to take:

Steps:

  1. Determine the need for making differential or incremental backups, now and in the future. Do you have enough storage space to make full backups of all data every day? Is there enough time to make a full backup at night and does the job end in time before the new day starts? Is there enough room for data growth?

    There are a lot of choices to make in the area of back-up and restore, which I will not discuss here. If you're not sure what to do, consult other sources or, preferably, consult a backup specialist.

  2. If you need to make differential or incremental backups, make sure you use the NSS filesystem, at least for the volumes which need to be backed up.

  3. Use the latest kernel, NSS and NCP modules installed, install the latest km_nss module.

  4. Add the two options to the nssstart.cfg file.

  5. Make sure your backup software supports differential of incremental backups based on the Modified Time bit. If not, implement another backup solution that does.


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