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    SubjectRe: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16
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    On Wed, 2014-07-16 at 14:45 -0400, Martin K. Petersen wrote:
    > >>>>> "James" == James Bottomley <jbottomley@parallels.com> writes:
    >
    > >> I don't have a problem with a BLIST_PREFER_UNMAP flag or something
    > >> like that. But BLIST_TRY_VPD_PAGES seems more generally useful and it
    > >> does fix the problem at hand. That's why I went that route.
    >
    > James> Hang on ... unless we apply Christoph or my fix, we'll get the
    > James> same issue with every raid driver (that's about 10 I think) that
    > James> set no_write_same when they hit a >2TB RAID volume,
    >
    > no_write_same is set for all the RAID controller drivers (54b2b50c20a6).
    >
    > If a WRITE SAME(10/16) command fails and the UNMAP bit is not set we'll
    > set no_write_same=1 and disable REQ_WRITE_SAME support.
    >
    > If a WRITE SAME(10/16) command fails and the UNMAP bit is set we'll
    > disable REQ_DISCARD support.
    >
    > Not sure where the 10 vs. 16 byte 2TB limitation comes into play here?

    It's the code we identified in sd.c:read_capacity_16():

    if (buffer[14] & 0x80) { /* LBPME */
    sdkp->lbpme = 1;

    if (buffer[14] & 0x40) /* LBPRZ */
    sdkp->lbprz = 1;

    sd_config_discard(sdkp, SD_LBP_WS16);
    }

    If the inquiry shows LBPME set, we'll do write same regardless of the
    no_write_same bit. That's why for SPC-2 devices it only shows up on
    >2TB devices because they force RC16.

    James



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