Basically as in README-IET, where file names are changed as specified
above.
+If you experience problems during kernel module load or running, check
+your system and/or kernel logs (or run dmesg command for the few most
+recent kernel messages).
+
To use full power of TCP zero-copy transmit functions, especially
dealing with user space supplied via scst_user module memory, iSCSI-SCST
needs to be notified when Linux networking finished data transmission.
handlers) usage of SGV cache on transmit path (READ-type commands)
will be disabled. The performance hit will be not big, but performance
will still remain better, than for IET, because SGV cache will remain
- used on receive path and IET doesn't have such feature.
+ used on receive path while IET doesn't have such feature.
- For user space allocated memory (scst_user handler) all transmitted
data will be additionally copied into temporary TCP buffers. The
"Default_iqn.2007-05.com.example:storage.disk1.sys1.xyz", and add there
all necessary LUNs. Check SCST README file for details.
+Check SCST README file how to tune for the best performance.
+
If under high load you experience I/O stalls or see in the kernel log
-abort or reset messages then try to reduce QueuedCommands parameter in
-iscsi-scstd.conf file for the corresponding target. Particularly, it is
-known that the default value 32 sometimes too high if you do intensive
-writes from VMware on a target disk, which use LVM in the snapshot mode.
-In this case value like 16 or even 10 depending of your backstorage
-speed could be more appropriate.
+abort or reset messages, then try to reduce QueuedCommands parameter in
+iscsi-scstd.conf file for the corresponding target. See also SCST README
+file for more details about that issue.
CAUTION: Working of target and initiator on the same host isn't
======== supported. See SCST README file for details.
TRACE_ENTRY();
- TRACE((scst_mcmd->fn == SCST_ABORT_TASK) ? TRACE_MGMT_MINOR : TRACE_MGMT,
- "scst_mcmd (%p) status %#x state %#x", scst_mcmd,
+ TRACE_MGMT_DBG("scst_mcmd (%p) status %#x state %#x", scst_mcmd,
scst_mcmd->status, scst_mcmd->state);
mcmd = scst_mgmt_cmd_get_tgt_priv(scst_mcmd);
are seen remotely. There must be LUN 0 in each security group, i.e. LUs
numeration must not start from, e.g., 1.
+If you experience problems during modules load or running, check your
+kernel logs (or run dmesg command for the few most recent messages).
+
IMPORTANT: Without loading appropriate device handler, corresponding devices
========= will be invisible for remote initiators, which could lead to holes
in the LUN addressing, so automatic device scanning by remote SCSI
See also important notes about setting block sizes >512 bytes
for VDISK FILEIO devices above.
+What if target's backstorage is too slow
+----------------------------------------
+
+If under high load you experience I/O stalls or see in the kernel log on
+the target abort or reset messages, then your backstorage is too slow
+for your target link speed and amount of simultaneously queued commands.
+Simply processing of one or more commands takes too long, so initiator
+decides that they are stuck on the target and tries to recover.
+Particularly, it is known that the default amount of simultaneously
+queued commands (48) is sometimes too high if you do intensive writes
+from VMware on a target disk, which uses LVM in the snapshot mode. In
+this case value like 16 or even 10 depending of your backstorage speed
+could be more appropriate.
+
+Unfortunately, currently SCST lacks dynamic I/O flow control, when the
+queue depth on the target is dynamically decreased/increased based on
+how slow/fast the backstorage speed comparing to the target link. So,
+there are only 4 possible actions, which you can do to workaround or fix
+this issue:
+
+1. Ignore incoming task management (TM) commands. It's fine if there are
+not too many of them, i.e. if the backstorage isn't too slow.
+
+2. Decrease /sys/block/sdX/device/queue_depth on the initiator in case
+if it's Linux (see below how) and/or SCST_MAX_TGT_DEV_COMMANDS constant
+in scst_priv.h file until you stop seeing incoming TM commands.
+
+3. Insrease speed of the target's backstorage.
+
+4. Implement in SCST the dynamic I/O flow control.
+
+To decrease device queue depth on Linux initiators run command:
+
+# echo Y >/sys/block/sdX/device/queue_depth
+
+where Y is the new number of simultaneously queued commands, X - your
+imported device letter, like 'a' for sda device. There are no special
+limitations for Y value, it can be any value from 1 to possible maximum
+(usually, 32), so start from dividing the current value on 2, i.e. set
+16, if /sys/block/sdX/device/queue_depth contains 32.
+
Credits
-------
the page cache (in order to avoid data copy between it and internal
buffers). Requires modifications of the kernel.
+ - Dynamic I/O flow control, when the device queue depth on the target
+ will be dynamically decreased/increased based on how slow/fast the
+ backstorage speed comparing to the target link for current IO
+ pattern.
+
- Fix in-kernel O_DIRECT mode.
- Close integration with Linux initiator SCSI mil-level, including
* Set if the prev cmd was ORDERED. Size must allow unprotected
* modifications
*/
- unsigned long prev_cmd_ordered;
+ unsigned long prev_cmd_ordered;
- int num_free_sn_slots;
+ int num_free_sn_slots; /* if it's <0, then all slots are busy */
atomic_t *cur_sn_slot;
- atomic_t sn_slots[10];
+ atomic_t sn_slots[15];
/* Used for storage of dev handler private stuff */
void *dh_priv;
INIT_LIST_HEAD(&tgt_dev->deferred_cmd_list);
INIT_LIST_HEAD(&tgt_dev->skipped_sn_list);
tgt_dev->expected_sn = 1;
- tgt_dev->num_free_sn_slots = ARRAY_SIZE(tgt_dev->sn_slots);
+ tgt_dev->num_free_sn_slots = ARRAY_SIZE(tgt_dev->sn_slots)-1;
tgt_dev->cur_sn_slot = &tgt_dev->sn_slots[0];
for(i = 0; i < (int)ARRAY_SIZE(tgt_dev->sn_slots); i++)
atomic_set(&tgt_dev->sn_slots[i], 0);
** Maximum count of uncompleted commands that an initiator could
** queue on any device. Then it will start getting TASK QUEUE FULL status.
**/
-#define SCST_MAX_TGT_DEV_COMMANDS 32
+#define SCST_MAX_TGT_DEV_COMMANDS 48
/**
** Maximum count of uncompleted commands that could be queued on any device.
TRACE_SN("Slot is 0 (num_free_sn_slots=%d)",
tgt_dev->num_free_sn_slots);
- if (tgt_dev->num_free_sn_slots != ARRAY_SIZE(tgt_dev->sn_slots)) {
+ if (tgt_dev->num_free_sn_slots < (int)ARRAY_SIZE(tgt_dev->sn_slots)-1) {
spin_lock_irq(&tgt_dev->sn_lock);
- if (tgt_dev->num_free_sn_slots != ARRAY_SIZE(tgt_dev->sn_slots)) {
+ if (likely(tgt_dev->num_free_sn_slots < (int)ARRAY_SIZE(tgt_dev->sn_slots)-1)) {
+ if (tgt_dev->num_free_sn_slots < 0)
+ tgt_dev->cur_sn_slot = slot;
+ smp_mb(); /* to be in-sync with SIMPLE case in scst_cmd_set_sn() */
tgt_dev->num_free_sn_slots++;
TRACE_SN("Incremented num_free_sn_slots (%d)",
tgt_dev->num_free_sn_slots);
- if (tgt_dev->num_free_sn_slots == 0)
- tgt_dev->cur_sn_slot = slot;
+
}
spin_unlock_irq(&tgt_dev->sn_lock);
}
tgt_dev->prev_cmd_ordered = 0;
} else {
- TRACE(TRACE_MINOR, "%s", "Not enough SN slots");
+ TRACE(TRACE_MINOR, "***WARNING*** Not enough SN slots "
+ "%d", ARRAY_SIZE(tgt_dev->sn_slots));
goto ordered;
}
break;
ordered:
if (!tgt_dev->prev_cmd_ordered) {
spin_lock_irqsave(&tgt_dev->sn_lock, flags);
- tgt_dev->num_free_sn_slots--;
- smp_mb();
- if ((tgt_dev->num_free_sn_slots >= 0) &&
- (atomic_read(tgt_dev->cur_sn_slot) > 0)) {
- do {
- tgt_dev->cur_sn_slot++;
- if (tgt_dev->cur_sn_slot ==
- tgt_dev->sn_slots +
- ARRAY_SIZE(tgt_dev->sn_slots))
- tgt_dev->cur_sn_slot = tgt_dev->sn_slots;
- } while(atomic_read(tgt_dev->cur_sn_slot) != 0);
- TRACE_SN("New cur SN slot %zd",
- tgt_dev->cur_sn_slot-tgt_dev->sn_slots);
- } else
- tgt_dev->num_free_sn_slots++;
+ if (tgt_dev->num_free_sn_slots >= 0) {
+ tgt_dev->num_free_sn_slots--;
+ if (tgt_dev->num_free_sn_slots >= 0) {
+ int i = 0;
+ /*
+ * Commands can finish in any order, so we don't
+ * know, which slot is empty.
+ */
+ while(1) {
+ tgt_dev->cur_sn_slot++;
+ if (tgt_dev->cur_sn_slot == tgt_dev->sn_slots +
+ ARRAY_SIZE(tgt_dev->sn_slots))
+ tgt_dev->cur_sn_slot = tgt_dev->sn_slots;
+
+ if (atomic_read(tgt_dev->cur_sn_slot) == 0)
+ break;
+
+ i++;
+ sBUG_ON(i == ARRAY_SIZE(tgt_dev->sn_slots));
+ }
+ TRACE_SN("New cur SN slot %zd",
+ tgt_dev->cur_sn_slot-tgt_dev->sn_slots);
+ }
+ }
spin_unlock_irqrestore(&tgt_dev->sn_lock, flags);
}
tgt_dev->prev_cmd_ordered = 1;
http://git.etherboot.org / mirror / scst / .git / commitdiff