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Disk to LUN I/O performance on SAN and NAS

Oracle Tips by Burleson Consulting 

Many Oracle professionals note that network attached storage (NAS) and Storage

Area Networks (SAN) can result in slower I/O throughput.

These disk mapping concepts are discussed in the book "Oracle Disk I/O Tuning", but

lets review the Device Media Control Language (DMCL) between the logical devices

(the logical unit, or LUN), and the physical disk drives, I/O buffers, SCSI controllers

and device drivers. Mike Ault notes in his book:

"It has also been reported that some hardware R AIDs support a number 

of different LUNs (logical disks), but these LUNs share a common set of 

I/O buffers between them.

This can cause SCSI QFULL conditions on those devices that do not

have commands queued."

Steve Karam notes this relationship between ASM and LUN ma pping:

One thing to remember is that ASM is not RAID. Oracle portrays ASM

as a Volume Manager, filesystem, miracle, whatever you would like to

call it, but in reality it is no more than extent management and load balancing; it scatters extents across your LUNs (1MB stripe size for 

datafiles/archivelogs, 128k stripe size for redo/controlfiles). It also

 provides extent-based mirroring for extra redundancy.

This benefits us in a couple ways. First, remember that your OS, HBA, or 

other parts of the host driver stack may have limits per LUN on I/O.

Distributing your extents across multiple LUNs with ASM will provide

 better I/O concurrency by load balancing across them, eliminating this

 bottleneck.

Second, carving into multiple LUNs allows multiple ASM volumes.

Multiple volumes help us if our hardware has any LUN-based migrationutilities for snapshots or cloning.

Third, you may end up with multiple LUNs if you need to add capacity.

ASM allows us to resize a diskgroup on-the-fly AND rebalance our 

extents evenly at the same time when we add a new LUN. Even if you

only start with a single LUN, you may end up with more in the long run.

Fourth, because an ASM diskgroup is not true RAID, you are able to use

it to stripe across volumes. This means that in a SAN with 3 trays, you

can carve a LUN from each tray and use it to form a single ASM

diskgroup. This further distributes your storage and reduces throughput

  ��

8/11/2019 Disk to LUN I_O Performance on SAN and NAS

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8/14/2014 Disk to LUN I/O performance on SAN and NAS

http://www.dba-oracle.com/t_disk_lun_san_nas_performance_bottleneck.htm

 

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 bottlenecks.

I have not seen any tried and true formula for the number of LUNs per 

ASM diskgroup, but you can calculate it based on your throughput per 

capacity. Make sure the LUNs provide maximum and equivalent I/O

operations per second per gigabyte.

 

LUN Mapping for Oracle disks

The central issue the interface between the logical "LUN" and the mapping of the disk 

device drivers and disk controllers to the LUN's:

 

The DISK array is usually sliced up by a controller into what are known as LUNs

(logical units) and these LUNs are then used to create the logical volumes upon which

we place our virtual disks. Usually there is at least one layer of abstraction (disks-

LUNS) and at least two (disks-LUNS-logical volumes). These layers of abstraction

make pinpointing disk performance issues difficult and may cause masking of 

symptoms. Sometimes there is a third level of abstraction, RAID, where multiple

logical disks are sliced into relatively thin (8k to 1 megabyte) stripes and then combinedto form RAID arrays.

8/11/2019 Disk to LUN I_O Performance on SAN and NAS

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8/14/2014 Disk to LUN I/O performance on SAN and NAS

http://www.dba-oracle.com/t_disk_lun_san_nas_performance_bottleneck.htm

As you can see, tracking a disk performance problem from a database file to the

underlying RAID volume, to the logical disk, to the LUN and finally to the physical disk 

level can be daunting. Further complications arise by some automated tuning features of 

the more advanced arrays where stripes or LUNs may be migrated from "hot" physical

disks to cooler ones. Luckily, many RAID manufacturers are now providing graphical

interfaces that track IO and other performance metrics down to the physical disk level.

Data transfer rates for disks

The data transfer rates for individual disks varies widely depending on the storage

device. Note: Your disk I/O mileage will vary greatly as a function of on-board RAM

caching, your data buffer size, db_file_multiblock_read_count, parallel access arrays,

&c. See details here:

STORAGE DEVICEAvg I/O per second

(IOPS)TRANSFER RATE MEG/SEC

TMS Solid State Disk 400,000 1,600

IEEE1394 1,100 400-800

 

PC Solid State Disk 7,000 62

Typical PC disk 150 15-30

 

The interface also has an effect on I/O throughput speed:

Interface type Speed

Serial 115 kb/s

Parallel(standard) 115 kb/s

Parallel(ECP/EPP) 3.0 Mb/s

SCSI 5-320 Mb/sec

ATA 3.3 - 133Mb/sec

USB1.1 1.5 Mb/s

USB2.x 60 Mb/s

IEEE1394(b) 50-400 Mb/s

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8/14/2014 Disk to LUN I/O performance on SAN and NAS

http://www.dba-oracle.com/t_disk_lun_san_nas_performance_bottleneck.htm

Original IDE 3.3-33 Mb/sec

SATA 150 Mb/s

Fibre Channel 2 Gb/sec

Detecting bottlenecks in SAN and NAS

The location of a bottleneck depends upon many factors, including LUN's sharing

common I/O buffers and a LUN mapped to a device with few controllers.

Solaris LUN bottlenecks

The sd_max_throttle variable sets the maximum number of commands that the SCSI sd

driver will attempt to queue to a single HBA driver. The default value is 256. This

variable must be set to a value less than or equal to the maximum queue depth of each

LUN connected to each instance of the sd driver. If this is not done, then commands

may be rejected because of a full queue condition and the sd driver instance that

receives the queue full message will throttle down sd_max_throttle to 1.

If you like Oracle tuning, see the book " Oracle Tuning: The Definitive

Reference", with 950 pages of tuning tips and s cripts.

You can buy it direct from the publisher for 30%-off and get instant

access to the code depot of Oracle tuning scripts.

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8/14/2014 Disk to LUN I/O performance on SAN and NAS

http://www.dba-oracle.com/t_disk_lun_san_nas_performance_bottleneck.htm

 

Burleson is the American Team

 Note: This Oracle documentation was created as a support and Oracle training

reference for use by our DBA performance tuning consulting professionals.

Feel free to ask questions on our Oracle forum.

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