Messages in this thread | | | Subject | Re: [Drbd-dev] [PATCH 05/30] drbd: Introduce new disk config option rs-discard-granularity | From | Bart Van Assche <> | Date | Mon, 25 Apr 2016 11:48:30 -0700 |
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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.
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