On 3/11/22 00:16, Javier González wrote: > On 10.03.2022 07:07, Keith Busch wrote: >> On Thu, Mar 10, 2022 at 02:58:07PM +0000, Matias Bjørling wrote: >>> >> Yes, these drives are intended for Linux users that would use the >>>>>> zoned block device. Append is supported but holes in the LBA space >>>>>> (due to diff in zone cap and zone size) is still a problem for these users. >>>>> >>>>> With respect to the specific users, what does it break specifically? What are >>>> key features are they missing when there's holes? >>>> >>>> What we hear is that it breaks existing mapping in applications, where the >>>> address space is seen as contiguous; with holes it needs to account for the >>>> unmapped space. This affects performance and and CPU due to unnecessary >>>> splits. This is for both reads and writes. >>>> >>>> For more details, I guess they will have to jump in and share the parts that >>>> they consider is proper to share in the mailing list. >>>> >>>> I guess we will have more conversations around this as we push the block >>>> layer changes after this series. >>> >>> Ok, so I hear that one issue is I/O splits - If I assume that reads >>> are sequential, zone cap/size between 100MiB and 1GiB, then my gut >>> feeling would tell me its less CPU intensive to split every 100MiB to >>> 1GiB of reads, than it would be to not have power of 2 zones due to >>> the extra per io calculations. >> >> Don't you need to split anyway when spanning two zones to avoid the zone >> boundary error? > > If you have size = capacity then you can do a cross-zone read. This is > only a problem when we have gaps. > >> Maybe this is a silly idea, but it would be a trivial device-mapper >> to remap the gaps out of the lba range. > > One thing we have considered is that as we remove the PO2 constraint > from the block layer is that devices exposing PO2 zone sizes are able to > do the emulation the other way around to support things like this. > > A device mapper is also a fine place to put this, but it seems like a > very simple task. Is it worth all the boilerplate code for the device > mapper only for this? Boiler plate ? DM already support zoned devices. Writing a "dm-unhole" target would be extremely simple as it would essentially be a variation of dm-linear. There should be no DM core changes needed. -- Damien Le Moal Western Digital Research
