On Mon, Mar 30, 2020 at 05:18:15PM +0300, Vladimir Sementsov-Ogievskiy wrote: > Hi all! > > There is an idea to make NBD protocol extension to support 64bit > write-zero/discard/block-status commands (commands, which doesn't > transfer user data). It's needed to increase performance of zeroing > large ranges (up to the whole image). Zeroing of the whole image is used > as first step of mirror job, qemu-img convert, it should be also used at > start of backup actually.. > > We need to support it in block-layer, so we want 64bit write_zeros. > Currently driver handler now have int bytes parameter. > > write_zeros path goes through normal pwritev, so we need 64bit write, > and then we need 64bit read for symmetry, and better, let's make all io > path work with 64bit bytes parameter. > > Actually most of block-layer already have 64bit parameters: offset is > sometimes int64_t and sometimes uint64_t. bytes parameter is one of > int64_t, uint64_t, int, unsigned int... > > I think we need one type for all of this, and this one type is int64_t. > Signed int64_t is a bit better than uint64_t: you can use same variable > to get some result (including error < 0) and than reuse it as an > argument without any type conversion. > > So, I propose, as a first step, convert all uint64_t parameters to > int64_t. > > Still, I don't have good idea of how to split this into more than 3 > patches, so, this is an RFC. > > What's next? > > Converting write_zero and discard is not as simple: we can't just > s/int/uint64_t/, as some functions may use some int variables for > calculations and this will be broken by something larger than int. > > So, I think the simplest way is to add .bdrv_co_pwritev_zeros64 and > .bdrv_co_pdiscard64 and update drivers one-by-one. If at some point all > drivers updated - drop unused 32bit functions, and then drop "64" suffix > from API. If not - we'll live with both APIs. > > Another thing to do is updating default limiting of request (currently > they are limited to INT_MAX). > > Then we may move some drivers to 64bit discard/write_zero: I think about > qcow2, file-posix and nbd (as a proof-of-concept for already proposed > NBD extension).
Sounds good to me. This is a good series to merge early in the QEMU 5.1 release cycle. That way we'll have time to catch any issues. Stefan
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