[libvirt] [PATCH v2 2/8] storage_backend_iscsi_direct: Simplify vol zeroing

[Pavel Hrdina]

On Fri, Mar 15, 2019 at 04:12:17PM +0100, Michal Privoznik wrote:
> On 3/15/19 2:53 PM, Pavel Hrdina wrote:
> > On Wed, Mar 06, 2019 at 03:59:12PM +0100, Michal Privoznik wrote:
> > > So far we have two branches: either we zero BLOCK_PER_PACKET
> > > (currently 128) block at one, or if we're close to the last block
> > > then we zero out one block at the time. This is very suboptimal.
> > > We know how many block are there left. Might as well just write
> > > them all at once.
> > > 
> > > Also, SCSI protocol has WRITESAME command which allows us to
> > > write a single buffer multiple times. This means, that we have to
> > > transfer the buffer far less frequently and have the destination
> > > write it over and over again. Ideally, we would allocate a buffer
> > > with size equivalent to block size of the LUN and then write it
> > > as many times as there are blocks on the LUN. Sadly, this works
> > > well in theory but in practise I've found out that it's possible
> > > to write only 4096 blocks at once. Any higher value leads to a
> > > failure. But still better than Twili^Wwhat we have now.
> > 
> > s/Twili^W//
> > 
> > > Since 4096 seems like an arbitrary number, I'm also implementing
> > > a mechanism that exponentially decreases the number of blocks
> > > we're trying to write at once. It starts at 4096 blocks and if
> > > that's too much the number is halved, and so on.
> > 
> > According to documentation if you set 0 as the number of blocks it
> > should write the same block starting at the LBA to the end of the
> > device.  Would be nice to try it out and improve the algorithm to
> > use only single transfer.
> 
> I've tried several values and found there are some differencies between docs
> and actual IMPL. What you suggest would be ideal of course, but the Linux
> Kernel I've tested doesn't allow me to specify anything else but [1, 4096].
> Maybe it has something to do with the fact that LUN is virtualized and in
> fact a file on a filesystem. At any rate, we should be able to zero that
> too, shouldn't we?

So I was able to find the limit in linux source code and the limit is
4096 for file-based block stores.  It looks like that the WRITESAME
command is not supported by ramdisk backstore so we probably cannot
use it to wipe the iscsi volumes.

Pavel

> 
> Michal
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