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

[John Haxby <john.haxby@xxxxxxxxxx>]

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.

jch


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