Re: [Drbd-dev] [PATCH 05/30] drbd: Introduce new disk config option rs-discard-granularity

[Bart Van Assche]

On 04/25/2016 09:42 AM, Philipp Reisner wrote:
> Am Montag, 25. April 2016, 08:35:26 schrieb Bart Van Assche:
> > On 04/25/2016 05:10 AM, Philipp Reisner wrote:
> > > As long as the value is 0 the feature is disabled. With setting
> > > it to a positive value, DRBD limits and aligns its resync requests
> > > to the rs-discard-granularity setting. If the sync source detects
> > > all zeros in such a block, the resync target discards the range
> > > on disk.
> >
> > Can you explain why rs-discard-granularity is configurable instead of
> > e.g. setting it to the least common multiple of the discard
> > granularities of the underlying block devices at both sides?
>
> we had this idea as well. It seems that real world devices like larger
> discards better than smaller discards. The other motivation was that
> a device mapper logical volume might change it on the fly...
> So we think it is best to delegate the decision on the discard chunk
> size to user space.

Hello Phil,

Are you aware that for aligned discard requests the discard granularity 
does not affect the size of discard requests at all?

Regarding LVM volumes: if the discard granularity for such volumes can 
change on the fly, shouldn't I/O be quiesced by the LVM kernel driver 
before it changes the discard granularity? I think that increasing 
discard granularity while I/O is in progress should be considered as a bug.

Bart.