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Print the pool or container UUID at the start of the message, since there are usually multiple targets running on a single server or multiple mountpoints on a client. The error messages should also include enough information to make some useful diagnosis of the problem (e.g. FID and/or filename, client NID, actual values that caused failures, etc). Otherwise, there is little value in having printed an error, but then needing to reproduce the problem to diagnose it:

right:

        D_ERROR(DF_UUID"%s: unsupported incompat filesystem feature(s) %x\n", DP_UUID(po_uuid), incompat);
        D_ERROR(DF_UUID"%s: cannot create root entry: rc = %d"DF_RC"\n", DP_UUID(co_uuid), DP_RC(rc));
        D_ERROR(DF_UUID"%s: error initializing object: rc = %d"DF_RC"\n",DP_UUID(co_uuid), DP_RC(rc));
        D_ERROR(DF_UUID"%s: cannot start coordinator thread: rc = %d"DF_RC"\n", mdt_obd_name(mdt), DP_RC(rc));

wrong:
        D_ERROR("Cannot get handle\n");
        D_ERROR("NULL bitmap!\n");
        D_ERROR("invalid event\n");
        D_ERROR("allocation failed\n");

Error messages that print a numeric error value should print it at the end of the line in a consistent format using ": rc =  %d%d\n":

right:

        D_ERROR("%s: error creating pool %s %s %s: rc = %d",
        D_ERROR("%s: cannot destroy container: rc = %d\n", DP_UUID(co_uuid) ,rc);

wrong:
        D_ERROR("err %d on param %d\n", rc, ptr);
        D_ERROR("Can't get index (%d)\n", rc);

DAOS specific error messages (e.g. DER_*) should be printed using the DF_RC macro:

right:

        D_ERROR(DF_UUID": cannot create root entry: rc = "DF_RC"\n", DP_UUID(co_uuid), DP_RC(rc));

wrong:
        D_ERROR(DF_UUID": cannot create root entry: rc = %d\n", DP_UUID(co_uuid), rc);