Windows Server Backup Software

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Using DVD-RW or CD-RW for your Windows data backups

by Eng. Roberto Grassi

GRSoftware President / CEO


GRBackPro can back up directly to a DVD-RW or CD-RW only if you have installed a "packet-write" software driver.

What is packet write ?

Today writable CD or DVD drives support the RW media type. This allows you to write on the same CD or DVD for as many as 1000 times or even more. From now all considerations made for CD media or drive will also apply to DVD. You can write to any RW media the normal CD-R (or DVD-R, DVD+R) way using your burning ROM software. This software is able to cache the data to be written to the CD media so that the drive can supply to the laser head the constant data flow it needs. If you interrupt this flow for any reason (a hard disk drive synchronize operation or a hard disk drive read failure or many other causes) your destination CD-R will be lost because your CD drive can't find the point where the break happened. This condition is known as buffer under-run. Here's where help comes from the packet-write software driver. This piece of system level running software can handle your CD-RW in a different manner and overcome the problem of the previously described under-run error. This software needs to format your CD-RW so that it will be able to write to this media in little piece of data blocks (normally in tracks – Track At Once mode). The software has a sufficiently large cache buffer where it will store the data coming from any software in your system and when this buffer level reaches an internally specified threshold it will write the whole buffer into a track of the CD-RW media. As you may suspect, this advantage certainly doesn't comes for free.

Advantages and disadvantages with a CD-RW.

The first problem of the packet-write system is that the format process takes a little longer (for example more than 30 minutes with a 2x-speed CD drive). This is not a big disadvantage because you can format your CD-RW media when you have time. The bigger disadvantage is that your 650MB unformatted CD becomes a 540MB when it has been packet-write formatted. You lost about the 17% of your CD's space so you have to really plan what to put in a single CD-RW. The bigger advantage, however, is that you can consider your CD-RW like a big floppy disk and write all size files at any time to it. This is a great advantage because the floppy disk size (max 1,44MB or 2,88MB) is really unusable to store anything these days. Another disadvantage is that the packet-write depends on the software firm that built it. This means that many software producers have developed different packed write formats that are not compatible each other. Some of them have also produced different versions that are not compatible each other. Some packed-write software also does not support the file update. Another little, but important, disadvantage of the CD-RW is the speed. The speed is good compared to a floppy disk but is not as high as a fixed hard drive or a removable hard drive or other similar products. CD-RW media speed has always improved from the original 1x speed to 2x and 4x. Also a new set of CD-RW media is now available that allows you to go from 6x speed to 12x and more. These new CD-RW media are labeled as "High Speed" rewritable CD. The same has happened for the DVD-RW too. Of course your CD or DVD drive must be updated via firmware updates or even by changing it in order to follow the speed increments.

How to take the best from your CD-RW media.

To better utilize your CD-RW media, in the light of the previous notes, you need to write to them once after you have formatted or cleared the disk (clearing the disk is deleting all folder entries in the disk and normally it takes few minutes to complete). This means that if you have planned to use your CD-RW as the destination for your backup, it is, by far, more convenient to use the FULL backup mode. If you use the update mode you will waste CD free space at every run (if your packet-write software does not support the file update) and very quickly you will reach the disk full condition. If, instead, your packet-write software supports the file update, you will lose more time because every time GRBackPro updates a file the destination CD-RW track has to be deleted and then written again (i.e. the real write-speed will be less than half the normal speed). Moreover your CD-RW life will be reduced in this way. A better approach is to run a full backup to a previously cleared disk every week (or month) and then use a quicker drive for your daily backup (a cross backup between two hard disks for example). In this case you can use the archive bit feature when you run the full backup so that all your source files will have this archive bit off at the end of the full backup. At this point you can run an incremental backup of only those files that have the archive bit attribute on again (i.e. that has been modified). This will save a lot of space and time in your daily backup. When your daily backup destination disk is full or when a sufficient time period is passed you can run another full backup to another CD-RW media and start to run daily backups.