Disk Imaging in Windows — Increases User Productivity And Permits Broader Range of Backup DevicesPart 2: Leading-Edge Technology
Acronis overcomes this issue of coping open Windows files by effectively freezing a moment of time. A special filter driver layers between file system drivers and volume drivers, allowing the software to create and backup consistent views of all files, including open files. The driver is installed above the volume drivers so it can see all the read and write requests passing to a volume — or partition. When the IT manager or user initiates imaging of the hard disk drive containing the system volume, the filter driver flushes the file system mounted to that volume; then all the operations on the system volume are temporarily frozen. Immediately thereafter, the filter driver creates a point-in-time view of the system volume and a bitmap describing the used sectors on this volume. Once the bitmap is created, the filter driver unfreezes the I/O operations on the system volume. It generally takes just several seconds to create a point-in-time view of the volume. After that, the operating system continues working as usual and the software can start imaging of the system volume. Acronis True Image reads the sectors on the system volume according to the created bitmap. Once a sector is read, the appropriate bit in the bitmap is reset. In its turn, the filter driver continues working to hold the point-in-time view of the system volume. Whenever the filter driver sees a write operation directed at the system volume, it checks whether these sectors are not backed-up yet, if they are not, the filter driver reads a copy of the sectors that will be overwritten into a special buffer created by the software, then it allows the sectors to be overwritten. Acronis True Image backups the sectors from the special buffer so that all the sectors of the point-in-time view of the system volume will be backed-up intact. Meanwhile, the operating system continues working and the user will not notice anything unusual in their operating system functionality. Since Windows XP supports the Universal Disk Format (UDF) file system, Acronis True Image can take advantage of this file system and create images via the UDF. Once the file system is mounted in the read/write mode, the software detects it and writes the image to a file created on this file system. If the UDF is not available in the operating system, then the backup software will burn the image directly to blank CDs. All of the popular imaging applications have at least some support for writing to CDs — although the DOS-based applications might not have support for the latest CD-RW devices. However, only programs that can call Windows-based drivers can write to DVD drives. All products have to reboot the imaged system in order to restore an image. Again, many of the popular products reboot to DOS, just as they did when they imaged the disk in the first place. Acronis True Image, however, reboots into a customized version of Linux and presents the user with a Windows-like interface. By booting to Linux, the software is able to support the latest storage devices, such as CD-RW and writable DVD devices. After all, it wouldn't make much sense to save the image to DVD if the software couldn't restore it from it as well. To give you an idea of the capacity differences between CD and DVD technology, popular CD-R disks today boast 700MB of capacity. A DVD-18 drive (double-sided/double-layered) supports 16GB, or nearly 23 times more capacity than the poplar blank CDs today. That's an extremely significant difference when you need to image a large disk and do not want multiple disks per volume. The advanced technology in leading-edge imaging products work is not how they create a partition, but rather, how they image the disk. As noted above, the Acronis offering takes a bitmapped image of the disk while still in Windows and then monitors subsequent write operations. As files are written or updated, those portions of the disk are re-imaged so that a complete and up-to-date image is maintained. Products that force the user to reboot back to DOS do so because they are unable to image open files, particularly files on the system partition. Remember, when a computer is running, it has open system files even if all of the applications are closed. Being unable to image those files means that the product cannot work in the Windows environment. As a result, the user needs to use the slower, more cumbersome DOS approach. Additionally, products in this category are differentiated by the file systems they support. All disk imaging products support the Windows file systems-File Allocation Table (FAT) 16, FAT 32, and NT File System (NTFS) — and some provided limited support for Linux file systems Ext2, Ext3 and Linux Swap, but today only Acronis supports ReiserFS, the default file system used by SuSE Linux. To review, creating a full and complete disk image is the best way to create a backup of a system disk, be it on a Windows 2000 server or on a Windows 95/95/XP/2000 desktop. From a performance standpoint, operating in Windows is not only the fastest method, but it also allows the user to continue to be productive while the disk image is being made. A Windows/Linux backup and restore strategy is faster than simply booting down to DOS for all backup and restore operations. It was bad enough when IT managers had to worry about their systems being hit by pernicious viruses, such as Nimda; today they realized that other catastrophic events could cause systems to crash. And consumers too realize that their systems are not immune from damage, even if they are not connected to the Internet. Backup traditionally has been thought of an insurance policy. Today, it's also part of corporate security and productivity plans. Max Lyadvinsky is director of engineering at Acronis (San Francisco)
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