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Some docs updates

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git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@118 d57e44dd-8a1f-0410-8b47-8ef2f437770f
This commit is contained in:
Vladislav Bolkhovitin
2007-05-31 15:13:13 +00:00
parent 1c5144105c
commit 1dd534e2aa
2 changed files with 21 additions and 13 deletions
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29
scst/README
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@@ -423,22 +423,25 @@ IMPORTANT: Since data in BLOCKIO and FILEIO modes are not consistent between
Pass-through mode
-----------------
As any other hardware, a local SCSI hardware for devices on target,
which are exported in the pass-through mode, can not handle commands
with amount of data and/or segments count in scatter-gather array
bigger, than some values. Therefore, when using the pass-through mode
you should note that the corresponding values on the corresponding
devices on initiators can not be bigger, than values, which the hardware
on the target can support. Otherwise you will see symptoms like small
transfers work well, but large ones stall and messages like: "Unable to
complete command due to SG IO count limitation" are printed in the
kernel logs.
In the pass-through mode (i.e. using the pass-through device handlers
scst_disk, scst_tape, etc) SCSI commands, coming from remote initiators,
are passed to local SCSI hardware on target as is, without any
modifications. As any other hardware, the local SCSI hardware can not
handle commands with amount of data and/or segments count in
scatter-gather array bigger some values. Therefore, when using the
pass-through mode you should note that values for maximum number of
segments and maximum amount of transferred data for each SCSI command on
devices on initiators can not be bigger, than corresponding values of
the corresponding SCSI devices on the target. Otherwise you will see
symptoms like small transfers work well, but large ones stall and
messages like: "Unable to complete command due to SG IO count
limitation" are printed in the kernel logs.
You can't control from the user space limit of the scatter-gather
segments, but for block devices usually it is sufficient if you set on
the initiator /sys/block/DEVICE_NAME/queue/max_sectors_kb in the same or
lower value as in /sys/block/DEVICE_NAME/queue/max_hw_sectors_kb for the
corresponding device on the target.
the initiators /sys/block/DEVICE_NAME/queue/max_sectors_kb in the same
or lower value as in /sys/block/DEVICE_NAME/queue/max_hw_sectors_kb for
the corresponding devices on the target.
For not-block devices SCSI commands are usually generated directly by
applications, so, if you experience large transfers stalls, you should

5
scst/src/scst.c
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@@ -32,6 +32,11 @@
#include "scst_priv.h"
#include "scst_mem.h"
#ifndef CONFIG_NOHIGHMEM
#warning HIGHMEM kernel configurations are supported, but not recommended. \
Consider changing VMSPLIT option or using 64-bit configuration instead.
#endif
/*
* All targets, devices and dev_types management is done under
* this mutex.