On 12/14/2012 08:04 AM, Fernando Lozano wrote:
I suppose they use something like inotify (or their own virtual file system driver over a real file system, like NFS or a loop fs) to learn about changed blocks, but they find to which file each block belongs to and salve this info in their backup catalog. If the changed block is filesystem (or md device, or lvm) metadata, they have to understand this and eithert log the change apropriately or ignore it as it's not file data. I can imagine something like this working and even how to program. And I'm a little scared about some backup tool being monitoring my file accesses all the time. ;-)
Conceptually, that is too complicated. All that the software has to do is track block changes and replicate them to a block device or virtual block device at the backup server. The software doesn't have to care whether it's data or metadata, the destination server just needs to be able to mount the block device / virtual block device. Windows systems probably implement that with VHD. Free Software systems can do that with a variety of filesystems, though typically replication would need to stop while the block device is mounted.
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