Re: Script to set up sparse device for testing

Hi,

On Thu, 2006-07-06 at 13:47 +0200, Erik Mouw wrote:

> So truncate shall increase the file and zero fill it, but it doesn't
> say it should be a sparse file. I suppose that's left to the
> implementation.

Right.

The main advantage of sparsetmp over sparse files is specifically
helpful for the 48-bit ext3 extents work --- it allows us to create
sparse devices much larger than the 2TB limit of a regular file:

        # lvcreate -n tmp -L 10g LV
          Logical volume "tmp" created
        # sparse_create sparsetmp /dev/LV/tmp  200000000000
        dmsetup create sparsetmp_zero table.4403
        dmsetup create sparsetmp_zero_org table.4403
        dmsetup create sparsetmp table.4403
        # verify-data -Q /dev/mapper/sparsetmp
        BLKGETSIZE64 reports size is 102400000000000 bytes
        ioctl(BLKGETSIZE): File too large
        BLKBSZGET reports logical block size 4096 bytes
        BLKSSZGET reports physical block size 512 bytes
        BLKRAGET reports 256 readahead pages
        BLKROGET reports device is writable
        BLKSECTGET reports IO limit of 255 sectors
        File is a block device: size 102400000000000 bytes (93.13T)
        
which is helpful for very large filesystem testing.  Regular files are
limited to 2TB (at least on ext3.)

Cheers,
 Stephen



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