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Re: AW: [Xen-users] Understanding sparse-files

Rustedt, Florian wrote:
 Thank you!

That was exactly what i needed ;)

My pleasure.

Does the choosen blocksize has impact on the formatting, so do i need to take 
smaller blocksizes if i want to use the space with the filesystem i am choosing 
more efficiently?
Or is it just for calculation, so formatting a "dd bs=512K seek=2048" results exactly in 
the same filesystemlayout after formatting as a "dd bs=1M seek=1024" would do?
So in both cases, i can use a "mkfs.ext2 -b 512 huge" and the resulting file 
mounts in both cases equally with a 256 Byte blocksize?

Files don't have block sizes :-) dd just multiples the block size and seek to find the starting offset. In fact, as the count is zero (write nothing at all to the file), dd bs=1M seek=20480 count=0 turns into this:

   open("/dev/zero", O_RDONLY)             = 3
   dup2(3, 0)                              = 0
   close(3)                                = 0
   lseek(0, 0, SEEK_CUR)                   = 0
   open("huge", O_RDWR|O_CREAT, 0666)      = 3
   dup2(3, 1)                              = 1
   close(3)                                = 0
   ftruncate(1, 21474836480)               = 0

The important call is the last one.


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