size on disk

[xfactordomine]

newbie question

In windows, when you right click on a file or folder and go to properties, there are 2 entries for the size. Size and Size on Disk. What the heck is Size on Disk?
At first there were such discrepancies between those 2 sizes because it depended on the File System the file was stored on (i've only taken note of this Size on Disk thing when checking properties for network folders and files). But the discrepancies remain locally as well, albeit smaller.

Thanks,

-X

 

[CTho9305]

A file takes up disk drive space in blocks of 8k or 16k or 64k (and so on). If it does'nt completely fill a block, the size on disk will be bigger (two fiels can't share a block unless you use disk compression). With compression the size on disk can be smaller.

 

[Nothinman]

A file takes up disk drive space in blocks of 8k or 16k or 64k (and so on)

Actually it starts at 512b because that's the physical size of a sector on the disk (most of the time...) and works its way up to whatever the OS and filesystem driver support, NTFS goes up to 64K I believe.

If it does'nt completely fill a block, the size on disk will be bigger (two fiels can't share a block unless you use disk compression).

In Windows 2 files can never share the same cluster, compression will just make a single file take up fewer clusters. NetWare does support cluster suballocation and I believe they recommend 64K clusters, not sure how it works or why they even do it.

The filesystem is divided up into clusters, normally 4K each with NTFS. Any single file takes up a certain number of clusters (could even be 0 because really small files are stored in the MFT). If a file is 4K it takes up exactly 1 cluster, if it's 5K it takes up 2 clusters and wastes 3K of the second cluster so the file size would be 5K but the size on disk would be 8K.

 

[xfactordomine]

Oh ok thanks, but why such huge discrepancies sometimes? E.g., i'm running windows xp with NTFS, and a folder size locally is about 30MB, but the size on disk of the same folder on a Netware Volume is 170MB? WTF?

And which is the one I should be paying attention to, when i'm say, burning a CD, since the size on disk may be larger than 700MB's.

Thanks.

-X

 

[Nothinman]

Size on disk is different for CDs because they use a different filesystem, trust the program you're burning with and the size it calculates.