Hello, When a dm is created, it will adjust queue limits from underlying devices. If the no_cluster member is not set, QUEUE_FLAG_CLUSTER will be set. When a bio is constructed, QUEUE_FLAG_CLUSTER enables segment merge. Later, however, when the request is mapped to scatterlist, if the underlying (scsi) device does not enable clustering, the mergeable segments will be separated, and unexpected sg count surge follows. If the segment count is around the limit, the final sg count could break the limit due to the extra addition. This is caused by mismatch of cluster setting between dm and underlying devices. For the stex driver, this problem was made obvious when code in blk_set_default_limits() (block/blk-settings.c) was changed from lim->max_sectors = lim->max_hw_sectors = SAFE_MAX_SECTORS; in 2.6.31, to lim->max_sectors = BLK_DEF_MAX_SECTORS; lim->max_hw_sectors = INT_MAX; in 2.6.32. The dm already has function to adjust limits based on underlying device limits. So, is it good to add the no_cluster setting to the queue limits at the scsi side? For example, some code change like the following? Thanks, Ed Lin diff -rupN a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c --- a/drivers/scsi/scsi_lib.c 2010-11-19 16:45:56.000000000 -0800 +++ b/drivers/scsi/scsi_lib.c 2010-11-19 16:46:27.000000000 -0800 @@ -1641,8 +1641,10 @@ struct request_queue *__scsi_alloc_queue blk_queue_max_segment_size(q, dma_get_max_seg_size(dev)); /* New queue, no concurrency on queue_flags */ - if (!shost->use_clustering) + if (!shost->use_clustering) { queue_flag_clear_unlocked(QUEUE_FLAG_CLUSTER, q); + q->limits.no_cluster = 1; + } /* * set a reasonable default alignment on word boundaries: the -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html