On 11/17/2016 05:44 PM, Max Reitz wrote: >> >> Since the SCSI specification says nothing about a minimum >> discard granularity, and only documents the preferred >> alignment, it is best if the block layer gives the driver >> every bit of information about discard requests, rather than >> rounding it to alignment boundaries early. > > Is this series supposed to address this issue? Because if so, I fail to > see where it does. If the device advertises 15 MB as the discard > granularity, then the iscsi driver will still drop all discard requests > that are not aligned to 15 MB boundaries, no? >
I don't have access to the device in question, so I'm hoping Peter chimes in (oops, how'd I miss him on original CC?). Here's all the more he said on v1: https://lists.gnu.org/archive/html/qemu-devel/2016-11/msg02223.html My gut feel, however, is that the iscsi code is NOT rounding (the qcow2 code rounded, but that's different); the regression happened in 2.7 because the block layer also started rounding, and this patch gets the block layer rounding out of the way. If nothing changed in the iscsi code in the meantime, then the iscsi code should now (once again) be discarding all sizes, regardless of the 15M advertisement. Meanwhile, I _did_ test this patch with blkdebug (see 9/9), as well as on a slightly modified NBD client that forced 15M alignments, and for those cases, it definitely made the difference on whether all bytes were passed (spread across several calls), vs. just the aligned bytes in the middle of a request larger than 15M. > The only difference is that it's now the iscsi driver that drops the > request instead of the generic block layer. If the iscsi driver was ever dropping it in the first place. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature
