Am 02.05.2016 um 19:14 hat Eric Blake geschrieben: > On 05/02/2016 09:42 AM, Eric Blake wrote: > > On 05/02/2016 09:35 AM, Kevin Wolf wrote: > >> Am 30.04.2016 um 23:48 hat Eric Blake geschrieben: > >>> NBD has situations where it can support FUA but not ZERO_WRITE; > >>> when that happens, the generic block layer fallback was losing > >>> the FUA flag. The problem of losing flags unrelated to > >>> ZERO_WRITE has been latent in bdrv_co_do_write_zeroes() since > >>> aa7bfbff, but back then, it did not matter because there was no > >>> FUA flag. But ever since 93f5e6d8 added bdrv_co_writev_flags(), > >>> the loss of flags can impact correctness. > >>> > > > > >> then we still don't get the necessary flush unless the driver's > >> .bdrv_co_write_zeroes() implementation explicitly handles FUA. As far as > >> I know, we don't have any driver that implements FUA there. > > > > NBD will, once we get to that part of my series. > > And looking further, it looks like SCSI does NOT support FUA during > WRITESAME(10/16), only during WRITE(10/16). Which means we probably > want to start having both .supported_write_flags AND > .supported_write_zero flags, so that at least the iscsi driver can > properly report that it does NOT natively support FUA on a write_zeroes > request.
Hm, not sure if it makes sense, but let me write that thought down: You're going to convert .supported_write_flags to a function anyway. Instead of adding a second function, how about passing a set of flags there and the function returns a subset that it can support? For write zeroes with FUA you would pass in BDRV_REQ_ZERO_WRITE | BDRV_REQ_FUA and in this case the iscsi driver would return only BDRV_REQ_ZERO_WRITE. If we ever decided to get rid of .bdrv_co_write_zeroes() and instead use .bdrv_co_pwritev() with BDRV_REQ_ZERO_WRITE, this would probably be the most consistent interface. Kevin
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