In order to remember what settings limit the transfer size for st, I did the list below. I hope it helps others, too. The field .max_sectors in struct Scsi_Host sets the queue maximum transfer size in 512 byte sectors. The type of the field is unsigned short, limiting the value to 65535 * 512 = 32768 kB - 512 B. The default for SCSI midlevel is 1024 resulting in limit of 512 kB unless the HBA driver sets a larger limit. The transfer size is limited by the number of scatter/gather segments and the maximum size of the segments. The number of segments is limited by the parameter .sg_tablesize in struct Scsi_Host (limiting what HBA accepts) _and_ the queue maximum number of segments (limiting what the HBA driver is given). The HBA parameter .sg_tablesize is usually set to SG_ALL limiting the number of segments to 255. The data type of .sg_tablesize is unsigned short and so the absolute limit is 65535. The SCSI midlevel sets the queue maximum number of segments to SCSI_MAX_PHYS_SEGMENTS. This is currently 128. Note that this is currently the number limiting the number of segments. If the HBA driver sets .use_clustering in struct Scsi_Host to DISABLE_CLUSTERING, the block layer sends requests containing only one page segments. This limits the transfer size to 128 * 4 kB = 512 kB if .sg_tablesize >= 128. If .use_clustering set to ENABLE_CLUSTERING, the block layer tries to combine adjacent pages to larger segments. The st and sg internal buffers are allocated so that this succeeds. This enables passing larger request down to the HBA. The maximum size of the segments is a queue parameter. The default is MAX_SEGMENT_SIZE in include/linux/blkdev.h (currently 65536). The SCSI midlevel could increase this using blk_queue_max_segment_size() but it does not do this. The maximum transfer size according to these queue limits is currently 128 * 64 kB = 8192 kB. This may be lower if the HBA's .sg_tablesize is below 128. Here are some practical examples from my experiments verifying some of the above. The experiments used a tape drive connected to sym53c1010 HBA. My baseline is to hack the driver to enable clustering and to set .max_sectors to 0xffff. This setup experimentally allows block size of 6144 kB. The driver sets sg_tablesize to 96. According to the above, this together with the queue maximum segment size sets the limit to 96 * 64 kB = 6144 kB. OK. Next I hacked the HBA driver to use sg_tablesize = 128. Now the limit is 128 * 8192 kB. Verified experimentally. The next thing was to increase the queue maximum segment size to 128 kB by editing linux/include/blkdev.h (#define MAX_SEGMENT_SIZE (2 * 65536)). This enabled transfers up to 16383 kB. This is below the theoretical limit of 16384 kB but in this case the limiting factor was the maximum block size of the drive. -- Kai - : 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
