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Backing up your data using the Zip64 format or single zip archives

by Roberto Grassi ( GRSoftware President/CEO )

Zip archives.

The ZIP file format is a data compression and archive format. The Zip64 format was born few years ago to overcome some of the limitation of standard Zip (version 2). The first big limitation was the 2 or 4 GB max file size limit (this limit depends on the implementation; some software has a 2GB limit and others have a 4GB limit). You cannot add a file that is greater than this limit and also the zip itself cannot grow over that limit. Another limitation of version 2 specification is that you cannot have more than 65535 files inside a zip archive.

Zip64 archives.

The Zip64 format is an extension to the standard zip format that effectively removes limits in sizes and the number of files inside of a zip archive.
In these modern days we frequently manage big files and thus the zip2 limitations are easy to reach. The Zip64 format is backward compatible with version 2 specification and allows you to start as a zip2 archive and then automatically change to Zip64 when a limit is reached. So the inner working of this is completely transparent to the user and automatically handled by the archiver software.

Compare Standard Format Versus Zip64 Format.

The maximum values allowed in each format are summarized below:

Parameter Standard Format Zip64 Format
Number of Files Inside an Archive 65,535 18,446,744,070 TB
Size of a File Inside an Archive [bytes] 4.295 GB 18,446,744,070 TB
Size of an Archive [bytes] 4.295 GB 18,446,744,070 TB
Number of Segments in a Segmented Archive    999 (spanning) 65,535 (splitting 4.295 GB
Central Directory Size [bytes] 4.295 GB 18,446,744,070 TB

Other than these limitations what is the importance of having a well known and widely used compression standard? Well, this means that you have a full choice of software utilities that are able to read and write those compressed files. This is a great advantage. Even if all major backup software are able to read and write zip archives you still have the choice to handle them with separate software on different machines and operating systems. Not all implementations are equal however, so you have to select and carefully test them.
Microsoft Windows has included support for ZIP compression in Explorer since the Plus! pack that was released for Windows 98. Microsoft calls this feature "Compressed Folders". Not all ZIP features are supported by the Windows Compressed Folders capability. For example, ZIP64, AES Encryption, split or spanned archives, and Unicode entry encoding are not known to be readable or writable by the Compressed Folders feature in Windows XP or Windows Vista. In this case you need to get a copy of a Zip utility that explicitly supports these features.

Single Zip feature.

The single zip feature provides the option of storing the folders inside a single zip archive. This lets you create a whole file system inside a zip archive and also allows you to store there a folder tree with relative path names. Of course you cannot store the source disk drive letter or network root path but you can store all folders below the root. This opens the possibility to store a whole backup inside a single zip archive

What are the advantages or disadvantages of having a complete backup inside a single zip archive?

Having a single zip file lets you handle it easily. If, however, the backup is composed of many files this zip archive may be extremely large. It's true thatthe copy speed of a single zip64 archive file will be higher than having a zip archive inside each folder as the folder change / open file / close file operations are avoided. On the other hand we have to consider the difficulty of updating such a big zip file when we want to update only few files inside it. This means that in a single zip archive, updating a single zip file requires that the whole zip archive is read, a new temporary zip archive is created by copy and then the file that need to be changed is replaced in the temporary file. Then the original zip file is overwritten by the newly created temporary zip file. Doing the same on a smaller zip archive inside the folder where the file need to be updated is by far quicker. Considering that you could update your backup hourly this could make a huge difference in the time needed. Another disadvantage of the single zip backup is that if you have disk problems and you lose pieces of the zip archive data (due to disk sectors that cannot be read back, for example) you run a serious risk of losing the whole backup! Having a zip archive in every folder is not that dangerous. You risk to losing some archives but you would be able to recover the majority of your zip archive in case of disk problems. Of course, if you lose the whole disk everything will be lost anyway. In summary, from a backup point of view there are big advantages to have Zip64 support and possibly avoid single zip archives.

The professional back up solution

GRSoftware announces the availability of a newprofessional server backup software product -- GRBackPro (TM) -- that backs up and restores large servers faster and more reliably than any other utility of its type. Time savings for system administrators and network managers are impressive.

Observes GRSoftware President Roberto Grassi: "While the amount of data being connected to workstations and file servers is steadily increasing, the amount of time when systems are available for backup is decreasing. GRBackPro solves this problem by providing a flexible backup and restore solution that sends data to any drives faster than other backup utilities. A further virtue of GRBackPro is that it allows more productive use of servers. That's because backups require less time and fewer system resources, so there's less impact on other applications running on the host."