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Disk block changes following a save

gamesmeister
gamesmeister asked
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Last Modified: 2010-04-03
Hi,

I'm not sure if this is more of an application specific question, but I'll try here first...

Let's say I have an MS Word document sitting on a disk, and it's currently occupying 10 physical blocks on the disk. I then open the document and make a one word change to it, and click Save. Can anyone tell me which of the following occurs:

1) The physical block on the disk containing the portion of the document that was changed is updated with the new data, and presumbably has a timestamp updated to show the time of its last change. All other physical blocks containing data related to the document remain unchanged.

2) The entire document is written back to the disk. The physical blocks may remain the same, or may change depending on various factors determined by the operating system.

The reason I need to know this is that I'm currently involved in the design of a very large system which will have a lot of data replicated across a WAN. The replication is block-based, so I'm trying to find out how common applications like MS Word save their files.

Many thanks for all your help
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Most Valuable Expert 2015
Commented:
This depends on the application, but most of them, and word as an example, will write the whole file back to disk. Actually this is normally handled by the OS, so if you need an app that only writes the changed blocks back it will have to override what the OS usually does. This means that only apps that circumvent what the OS does can do that. Such apps could do harm to the system because of that, so you'll mainly find them as utilities or tools.

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