Stefan Richter wrote:
Erez Zilber wrote:
> I'm not sure that I understand the meaning of max_sectors in
> scsi_host_template.
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
I will go over it. Thanks for the link.
> Is it the maximum data length of a single SCSI command?
Yes.
> Is it in bytes?
No, it is in units of 512 bytes.
> What's the size of a sector?
"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
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
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