[dm-devel] block: Fix a WRITE SAME BUG_ON

[Christoph Hellwig]

On Fri, Feb 01, 2019 at 05:03:40PM +0100, Heinz Mauelshagen wrote:
> On 2/1/19 3:09 PM, John Dorminy wrote:
> > I didn't know such a thing existed... does it work on any block
> > device? Where do I read more about this?
> 
> 
> Use sg_write_same(8) from package sg3_utils.
> 
> For instance 'sg_write_same --in=foobarfile --lba=0 --num=20000
> --xferlen=512 /dev/sdwhatever'
> 
> will read the pattern to write same from file 'foobarfile' with length
> explicitely set to 512 bytes
> (rather than derived from foobarfile's size) and write it 20000 times
> starting at LBA 0 to /dev/sdwhatever.

Yeah.  Note that this will only work on SCSI disks (and maybe ATA
for a very specific corner case).  But for actual devices and not
remappers the same is true of REQ_OP_WRITE_SAME.