> Your analogy with meteorites is not correct. > -- > Peter van der Does >
You are so close to the trees that you are missing the forest. The comparison is between a full CD that has low or no compression versus a full CD that has very high compression being written using the same physical device. My original observation was that there is a tradeoff between getting more on the CD via higher compression on the one hand, versus getting lower failure rates on CDs with lower compression. Such is in accord with observations and with theoretical analysis. When speaking of compression, we aren't actually speaking about disk surface that doesn't have data written to it. Instead we are really referring to whole sections of disk real estate that are filled with zeros or other repetitive and unimportant data. If you need to make better use of areas that are literally unpopulated, you employ defragmentation, which is a related but different technique needed for antiquated file systems. On a typical CD, the entire CD is populated, as Soren rightly mentioned, with zeros and ones. The physical device makes the same number of read/writes whether the data is compressed or not, and the error rate is the same either way. On the surface, it would appear that compression doesn't introduce more significant error. The difference is that on an uncompressed CD, much of what is written is not important. Text files, for instance, are mostly a bunch of zeros at the physcial layer. Compression uses algorhythms to represent all those zeros in a shorthand way, so that the device doesn't actually have to write each one of them. This frees up disk real estate for more information. The consequence is that the compressed disk has a higher density of important bits and bytes on the same disk. Assuming that the device has a constant error rate and assuming that the CD is filled to the same capacity, it is more likely that the errors on compressed disks will affect something important and cause a failure, simply because there is more important data on the CD. I actually remember when "floppy disks" were flexible 12 inch disks and how amazed everyone was to get so much information on 5 1/2 inch disks. Engineers have made remarkable progress over the last 30 years. Much of the heavy lifting to make that possible was the improvements in error prevention, detection, and correction necessitated by the compression. -- ubuntu-server mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-server More info: https://wiki.ubuntu.com/ServerTeam
