On 05/03/2010 06:51 AM, Jack Z wrote:
Hi group,
I have been tracing the code related to sending PDUs from iscsi
initiator (ver 2.0-871).
And through some printk()s i realize that starting from
iscsi_sw_tcp_pdu_init(), all the functions using scatterlist (struct
scatterlist *sg) seem to use 4096 as the length (sg->length).
But I was not able to trace down where this 4096 is initially assigned
to sg->length... I searched through the code for "4096" and only two
spots came up: ".sg_tablesize = 4096" in struct scsi_host_template
iscsi_sw_tcp_sht and "#define ISCSI_TOTAL_CMDS_MAX 4096". But changing
these two values did not affect the sg->length value, which was still
4096.
I was guessing this 4096 had something to do with the fs block size
and this value was somewhat from "struct scsi_data_buffer *sdb =
scsi_out(task->sc);" in iscsi_sw_tcp_pdu_init()... but still don't
have a clue about how and why iscsi initiator gets this value as the
length for the scatterlist...
Could anyone maybe explain a bit or point me to some relevant
document?
The fs/block layer is going to send down some struct called a bio, which
has a mapping of pages to some sectors to read/write. The block layer's
elevator code is then going to try and make large IO requests by merging
bios. So if there was a bio to read sectors 0 - 7 into page0 and a bio
to read sectors 8 - 15 into page1, then they would be merged into the
same request to read sector 0 - 15.
At some point this request is then sent to the scsi layer, which will
use some block layer helper to create a scatterlist from the pages in
the requests's bios. The sg->page pointer points to the first page in a
group of pages that are contiguous in memory, and sg->length is then the
total length in bytes of all those pages. So in my example, if the 2
pages in each bio were next to each other then they could be merged into
1 sg entry. This does not happen for iscsi_tcp though. In your case, you
see sg->lenth as at most 4096 because iscsi_tcp only supports 1 page per
sg entry and the page size on your arch is PAGE_SIZE=4096 (we set the
scsi_host_template->use_clustering flag to indicate that we only want
one page per sg entry btw).
Next is where sg_tablesize comes into play. Here, we are setting it to
4096 to indicate that at most we want 4096 entries on that scatterlist
that is made (4096 being the page size and sg_tablesize is just a
coincidence). So for us in your setup we can have at most 4096 sg
entries, with each entry having 4096 bytes. We could actually have a
smaller sg list, because there are other settings that limit the size of
the request like the sht->max_sectors.
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