How do spanned disks work?

[Smoblikat]

So I have two 3TB's and two 2TB's. I want to create 2 5TB volumes, my question is this:

How does that work? Does it fill up one drive to the max then automatically switch over to the second, or are different bits of data written to it (like RAID0). I am curious because I want to be able to get some data back if one drive fails. And if the full file is written to the drive then that shouldnt be a problem.

 

[OVerLoRDI]

Windows does it by filling one drive then spilling over to the next. No striping.

 

[Smoblikat]

Windows does it by filling one drive then spilling over to the next. No striping.

Perfect, so if one disk fails I still have ALL the data on the other disk?

 

[dave_the_nerd]

Perfect, so if one disk fails I still have ALL the data on the other disk?

My google-fu says no, short of professional data recovery services.

 

[Mark R]

A spanned drive just means that the volume appears to be 1 large hard drive. Let's say that you have 2x 1 TB drives (each with 2 billion sectors). Sectors 1-2,000,000,000 will be sent to drive 1, sectors 2,000,000,001 - 4,000,000,000 will go to drive 2.

Files will go wherever on those drives that the OS wants to put them - depending on where bits of free space are. This may be on one drive, or both drives, or in multiple fragments scattered over multiple drives.

Also, the directory/index information will go wherever the OS puts it - which is usually at the start of the volume (so first drive). However, NTFS tends to put the critical information in the middle of the volume (so it might get split up); although NTFS does keep a backup of this data at the end of the volume (so last drive).

Generally, however, if you lose one drive in a spanned set, the entire span's data is toast.

You MIGHT get lucky if the drive that fails contains only file data and not directory or index data. However, if index data is on the dead drive, you're hosed.

 

[Nothinman]

Perfect, so if one disk fails I still have ALL the data on the other disk?

No because it's one big filesystem so the MFT will likely be all within the 3TB drive or at best spread across both as well. File data will be effectively randomly placed on both drives because of the NTFS allocater is smarter than the FAT one and doesn't just blindly place data after the last cluster used. Assuming you'll be using NTFS, anyway.

 

[corkyg]

As is indicated above, spanned drives are not striped, but are integrated, and dependent on the whole set. My practice is this - if spanning is indicated, I choose a different form of media.

 

[Smoblikat]

Thanks guys, that cleared things up a lot. Ill just keep all drives single then and use software to do what I need to do.