Running out of options.... try changing the iSCSI daemon settings in the Tuning options - did you enable the WB on the Target? Not sure if we covered that area - checking....
Try using a different version of client initiator.
a) MaxRecvDataSegmentLength - Sets the maximum data segment length that can be received. This value should be set to multiples of PAGE_SIZE. Currently the maximum supported value is 64 * PAGE_SIZE, e.g. 262144 if PAGE_SIZE is 4kB.
Configuring too large values may lead to problems allocating sufficient memory, which in turn may lead to SCSI commands timing out at the initiator host. The default value is 8192.
b) MaxBurstLength - Sets the maximum amount of either unsolicited or solicited data the initiator may send in a single burst. Any amount of data exceeding this value must be explicitly solicited by the target. This value should be set to multiples of PAGE_SIZE. Configuring too large values may lead to problems allocating sufficient memory, which in turn may lead to SCSI commands timing out at the initiator host. The default value is 262144.
c) MaxXmitDataSegmentLength - Sets the maximum data segment length that can be sent. This value actually used is the minimum of MaxXmitDataSegmentLength and the MaxRecvDataSegmentLength announced by the initiator. It should be set to multiples of PAGE_SIZE. Currently the maximum supported value is 64 * PAGE_SIZE, e.g. 262144 if PAGE_SIZE is 4kB. Configuring too large values may lead to problems allocating sufficient memory, which in turn may lead to SCSI commands timing out at the initiator host. The default value is 8192.
d) DataDigest <CRC32C|None> - If set to "CRC32C" and the initiator is configured accordingly, the integrity of an iSCSI PDU's data segment will be protected by a CRC32C checksum. The default is "None". Note that data digests are not supported during discovery sessions.
e) MaxOutstandingR2T <value> - Controls the maximum number of data transfers the target may request at once, each of up to MaxBurstLength bytes. The default is 1.
f) InitialR2T <Yes|No> - If set to "Yes" (default), the initiator has to wait for the target to solicit SCSI data before sending it. Setting it to "No"
allows the initiator to send a burst of FirstBurstLength bytes unsolicited right after and/or (depending on the setting of ImmediateData together with the command. Thus setting it to "No" may improve performance.
g) ImmediateData <Yes|No> - This allows the initiator to append unsolicited data to a command. To achieve better performance, this should be set to "Yes".
The default is "No".
h) DataPDUInOrder <Yes|No> - It tells initiator if data has to be sent in order. Default is "Yes", which is also recommended.
i) DataSequencerInOrder <Yes|No> - It tells initiator if data has to be sent in order. Default is "Yes", which is also recommended.
j) HeaderDigest <CRC32C|None> - If set to "CRC32C" and the initiator is configured accordingly, the integrity of an iSCSI PDU's header segments will be protected by a CRC32C checksum. The default is "None".
Note that header digests are not supported during discovery sessions.
k) Wthreads - The iSCSI target employs several threads to perform the actual block I/O to the device. Depending on your hardware and your (expected) workload, the number of these threads may be carefully adjusted. The default value of 8 should be sufficient for most purposes.
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