usage of max_sectors in scsi_host_template

I'm not sure that I understand the meaning of max_sectors in scsi_host_template. Is it the maximum data length of a single SCSI command? Is it in bytes? What's the size of a sector?

Thanks,

____________________________________________________________

Erez Zilber | 972-9-971-7689

Software Engineer, Storage Solutions

Voltaire – _The Grid Backbone_

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www.voltaire.com <http://www.voltaire.com/>


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Did you have a look at scsi_mid_low_api.txt?
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/scsi/scsi_mid_low_api.txt;h=6f70f2b9327e1f0db7bc05bdbf2d6ce3b2fcbdcf#l1232

Yes.

No, it is in units of 512 bytes.

"Usually" 512 bytes according to above doc. Always 512 bytes from the point of view of block/ll_rw_blk.c::blk_queue_max_sectors().
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=block/ll_rw_blk.c;h=75c98d58f4ddf7252e2717e0924b9d6a8925b4e5#l590

Stefan Richter
-=====-=-=== =-== --==-
http://arcgraph.de/sr/

I will go over it. Thanks for the link.

So, ll_rw_blk actually uses the max_sectors value to chop requests larger than max_sectors. Am I right? If yes, I have a problem:

I'm running sgp_dd (on RHAS 4 up4 - kernel version is 2.6.9), so it calls scsi-ml directly (without going through ll_rw_blk). I ran it with the following parameters:

sgp_dd bs=512 of=/dev/null if=/dev/sg1 bpt=2048 thr=4 time=1 count=100k deb=9

I see that a single 1MB command is generated. Here's the debug info from sgp_dd:

sgp_dd: if=/dev/sg1 skip=0 of=/dev/null seek=0 count=102400 Start of loop, count=102400, in_num_sect=0, out_num_sect=0 Starting worker thread k=0
sg_start_io: SCSI READ, blk=0 num_blks=2048
Read (10) [28 00 00 00 00 00 00 08 00 00 ]
dir=-3, len=1048576, dxfrp=0x2a9558a000, cmd_len=10

Now, the low-level driver below scsi-ml is open-iscsi over iSER. max_sectors is set to 1024 (i.e. 512 kB). Still, the iSER driver receives a 1MB command. I guess that the max_sectors value is never used. Am I right?

Thanks,
Erez

...

I have no idea. However you might be able to track how max_sectors trickles through the layers via LXR:
http://lxr.free-electrons.com/ident?i=max_sectors
(Not all of the LXR sites out there support search for struct members but free-electrons' does.)

Stefan Richter
-=====-=-=== =-== --===
http://arcgraph.de/sr/

Well, there is q->max_sectors and q->max_hw_sectors. In current kernels q->max_hw_sectors is scsi_host_template->max_sectors. And q->max_sectors is sort of a block layer limit to make sure commands to not get too big.

RHEL4's sg.c does not take into account q->max_sectors or q->max_hw_sectors.

In later kernels like in RHEL5 (probably upstream 2.6.16+), sg.c and st.c goes through llw_rw_blkc and obeys the sector limit. For pass through like sg and block layer sg, the scsi command is limited by q->max_hw_sectors which like I said above is scsi_host_template->max_sectors. And normal FS commands are limited by min(q->max_hw_sectors, q->max_sectors).