Performance improvements for IBM AIX FC and FCoE device driver stacks
Note: This is a re-post of article that was published on IBM developerworks on January 06, 2017.
Contributors: Yadagiri Rajaboina , Kiran K Anumalasetty, Vinod Kumar Boddukuri, Prashantha Subbarao
Introduction
This article describes performance improvements for IBM® AIX® Fibre Channel (FC) / Fibre Channel over Ethernet (FCoE) device drivers stack for 16 Gb FC (Feature Code: EN0A) and10 Gb FCoE (Feature Code: EN0H) HBAs. The AIX FC driver stack includes an initiator mode Small Computer System Interface (SCSI) protocol driver and an adapter driver. The existing AIX FC adapter driver maintains a SCSI I/O queue for submitting all I/O requests to the FC HBA. Scaling issues have been observed with the existing FC stack with extreme I/O transactions per second (IOPS) and small I/O sizes. This is due to I/O serialization through a single I/O queue.
To improve the number of IOPS on smaller block size I/O requests, the multiqueue functionality is introduced with the 16 Gb FC or 10 Gb FCoE adapter driver starting from:
- AIX releases – AIX 7.2 TL01 SP1, AIX 7.1 TL04 SP3 and AIX 6.1 TL09 SP8
- VIOS release – VIOS 2.2.4.30 and VIOS 2.2.5.0
Figure 1 depicts how I/O is parallelized on multiple I/O queues with the improvements discussed in this article.
Figure 1: A traditional FC driver stack versus an improved FC driver stack
Implementation details
To support multiple I/O queue feature, a new Object Data Manager (ODM) attribute, num_io_queues, is introduced for FC/FCoE devices (fcs) to indicate the number of I/O queues configured in the FC adapter driver. Each I/O queue is associated with a hardware work queue in the FC HBA. All the I/O requests issued to a particular hdisk will be mapped to the same SCSI I/O queue. Each SCSI I/O queue can service multiple hdisks, However, I/O request to a given hdisk cannot be distributed to multiple SCSI I/O queues.
Example: For a 16 Gb FC HBA, the ODM stanza for num_io_queues attribute is shown below:
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# lsdev | grep fcs
fcs0 Available 00-00 PCIe2 2-Port 16Gb FC Adapter (df1000e21410f103)
fcs1 Available 00-01 PCIe2 2-Port 16Gb FC Adapter (df1000e21410f103)
# odmget -q name=fcs0 CuDv
CuDv:
name = "fcs0"
status = 1
chgstatus = 0
ddins = "pci/emfcdd"
location = "00-00"
parent = "pci0"
connwhere = "0"
PdDvLn = "adapter/pciex/df1000e21410f10"
#
# odmget -q uniquetype="adapter/pciex/df1000e21410f10" PdAt | grep -p num_io_queues
PdAt:
uniquetype = "adapter/pciex/df1000e21410f10"
attribute = "num_io_queues"
deflt = "8"
values = "1-16,1"
width = ""
type = "R"
generic = "DU"
rep = "nr"
nls_index = 67
#
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The value of this attribute can be changed using the chdev command or the System Management Interface Tool (SMIT) interface. The possible values are:
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# lsattr -l fcs0 -a num_io_queues -R
1...16 (+1)
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To enable multiple I/O queues, the HBA’s direct memory access (DMA) resources should be sufficient to distribute I/O requests across multiple queues. The existing ODM attribute, io_dma, controls the amount of I/O DMA region that the adapter driver requests while configuring the HBA.
Default ODM attribute values
This section provides the default values for the ODM attributes related to the multiple I/O queue feature.
For AIX 7.2 TL01 SP1 and VIOS 2.2.5 releases
The default value of the num_io_queues attribute is set to 8 and to have sufficient DMA resources, the default value of the io_dma attribute is increased to 256, starting with AIX 7.2 TL01 SP1 and VIOS 2.2.5 releases.
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# lsattr -El fcs0 | grep -e num_io_queues -e io_dma
io_dma 256 IO_DMA True
num_io_queues 8 Desired number of IO queues True
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For AIX 7.1 TL04 SP3, AIX 6.1 TL09 SP8 and VIOS 2.2.4.30 releases
The default value of the num_io_queues attribute is set to 1 and io_dma is set to 64 for the AIX 7.1 TL04 SP3, AIX 6.1 TL09 SP8 and VIOS 2.2.4.30 releases.
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# lsattr -El fcs0 | grep -e num_io_queues -e io_dma
io_dma 64 IO_DMA True
num_io_queues 1 Desired number of IO queues True
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As mentioned earlier in this article, there should be sufficient DMA resources to enable support for multiple I/O queues. Therefore, the value of the io_dma attribute should be increased from 64 to 256. In case the user changes only the num_io_queues' value without increasing the io_dma value to 256, the adapter instance will be configured with a single SCSI I/O queue and the following informational error will be logged with the AIX error log.
# errpt | grep fcs0
29FA8C20 0629173616 I O fcs0 Additional FC Adapter Information
Steps for tuning the num_io_queues attributes using the chdev command:
You need to perform the following steps to tune the num_io_queues attributes using the chdev command for 16 Gb FC HBAs:
1. Unconfigure the device instance.
#rmdev -Rl fcs0
2. Change the attribute to the required value (say 16).
# chdev -l fcs0 -a num_io_queues=16
fcs0 changed
3. Configure the device instance.
# cfgmgr -l fcs0
4. Verify if the attribute is set to the required value.
# lsdev | grep fcs0
fcs0 Available 00-00 PCIe2 2-Port 16Gb FC Adapter (df1000e21410f103)
# lsattr -El fcs0 | grep num_io_queues
num_io_queues 16 Desired number of IO queues True
Performance results – random read operations with a block size of 4 KB
The following results are for AIX native [that is, physical HBAs owned by the logical partition (LPAR)] case. I/O requests are running in parallel on the
FlashSystem 900 storage targets using the default shortest_queue algorithm on the hdisk devices.
Figure 2: AIX (native) results for 16 Gb FC HBA
Figure 3: AIX (native) results for 10 Gb FCoE HBA
The following results are for the NPIV (that is, VIOS owning physical HBAs and num_io_queues tuned on VIOS) case. I/O requests are running in parallel on all the I/O paths for a given disk on each NPIV client using the default shortest_queue algorithm on the Flash System 900 storage disks.
Conclusion
In native configuration, the number of IOPS for random read operations with a block size of 4KB, for a single FC HBA port case increased by approximately 2.5 times with the improved FC stack, which is a significant improvement. The achieved IOPS count of 390,000 is very close to the line speed for a single FC HBA port.
The IOPS gain for random read operations with a block size of 4 KB in the NPIV configuration is almost equivalent to that of the native configuration when the number of clients is six or more.
