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Current by Robert Crockett
on Nov 01, 2012 09:24.

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Block based backups bypass files and file systems almost completely. All operating systems have a specialized component of the O/S called the [File System|]. Examples of popular server file systems are [NTFS |]on Windows and [ext3|] for Linux. The file system divides the hard disk, volume or RAID array (software and hardware RAID) into chunks or groups of bytes called blocks (fixed size) or extents (variable size).  Typically these are ordered 0 - N. The size of a block depends on the file system used and potentially settings used when the file system was created. NTFS and ext3 use fixed block sizes.  Some file systems support a concept of variable length block sizes typically called extents.

The file system is responsible for keeping track of the tree or hierarchy of files. It also stores a file in neat little fixed size blocks on the disk and keeps track of where these blocks are which can be scattered across the disk.  Backup applications that read files use the file system to get at data and are inherently very slow and time consuming no matter what file system is used.
Some backup application vendors use methods for taking snapshots that do not guarantee file system consistency. These should be avoided when possible.  In the case of 2.4 Linux it is unavoidable.  Linux kernels greater than 2.6.8 have low level device driver facilities for making file systems consistent during a snapshot.  Windows XP and greater support Volume Shadow COpy which gauruntees a consistent file system during a snapshot.  Beware of applications that brag about their method for determing a snapshot using a "quiesce  period".  You can identify these by reading about their snapshot process.  They will usually describe a process that waits for a quiesce period (e.g. 5 seconds) to be "observed" and once the disk is quiesced or observed to be quiet it is therefore "assumed" the file system is in a consistent state\!  For an example see the OTM technical paper at []