Monday, 31 May 2010

Types of data recovery

There are two distinct types of data recovery, namely “in-place” and “read-only” recovery.

The in-place recovery is the attempt to fix the errors and bring the filesystem to the consistent state. This is done by changing the damaged filesystem itself.
The read-only recovery, as the name implies, does not change the damaged filesystem. Instead, the data is extracted and copied to the separate dedicated storage.

The prevalence of each type of data recovery has been changing over time.

In the days of DOS, Windows 3.11, and then Windows 95, in-place repair prevailed. Actually, it was the only option available before Ontrack released their “Tiramisu Data Recovery” circa 1999. So, you had Norton Disk Doctor (which was quite good in fixing errors), Norton Unerase, and Norton Unformat and that was about it. Norton Utilities worked with FAT filesystem under DOS or Windows 95. The prevalence of in-place repair is understandable if you consider the simplicity of the filesystem and the cost of the storage in those days. The most widespread filesystem was FAT, which is rather simple and well-documented. On the other hand, a spare hard drive cost was prohibitively high.

The release of Windows 2000 changed the things significantly. NTFS filesystem was quickly established as the standard, but it lacked documentation badly (in fact, it is still not fully understood by developers outside Microsoft). As far as in-place repair is concerned, you were left with the Microsoft CHKDSK. Since NTFS is not documented, it is not possible to fix it in-place because you do not know what the consistent state of the filesystem should be. Even a minor deviation from the standard which is unknown to the developers causes the NTFS driver to reject the volume or produce otherwise bizarre behavior. However, the storage cost dropped, making it easier to find a high capacity spare storage. So the simplest route of just extracting what is really needed – the file content, and stop worrying about filesystem consistency became the most cost-effective approach. Nowadays, the read-only data recovery software dominates the do-it-yourself data recovery market and the in-place repair is left to servicemen in the recovery labs. In-place repair is only used in some specific cases where high capacity disk arrays are involved.

No comments:

Post a Comment