I'm trying to estimate IOPS requirements of my application running on 32-bit CentOS 6.2. I started to take some measurement on a machine with SATA disks and I'm quite confused of difference between IOPS and tps measured by sar.
According to wikipedia SATA disk should perform 75-100 IOPS. ioping utility seems to confirm this for random access test:
# ./ioping -R /dev/sda
--- /dev/sda (device 931.0 Gb) ioping statistics ---
279 requests completed in 3.0 s, 92 iops, 371.3 kb/s
min/avg/max/mdev = 2.7 ms / 10.8 ms / 130.8 ms / 7.9 ms
But tps values produced by sar are much higher (/dev/sda):
# iostat 1
avg-cpu: %user %nice %system %iowait %steal %idle
0.17 0.00 2.02 14.86 0.00 82.96
Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sda 559.00 0.00 142600.00 0 142600
dm-0 18433.00 0.00 147464.00 0 147464
dm-1 0.00 0.00 0.00 0 0
dm-2 0.00 0.00 0.00 0 0
It does not really mind if this load is sequential (dd with various block sizes) or random access (ioping), value is still the same. I thought tps actually is IOPS and I would expect it go down with larger chunks transferred.
So what exactly does the tps value mean? And how does it relate to IOPS?
Best Answer
Transactions are single IO-commands (fetch block/write block) that are written to the RAW-disk (in your example dm-0). The linux-kernel tries to order those commands into a better sequence or tries to compress them into more efficient commands (like: get two blocks at once instead of get one block and get another block right after this one). These are the transactions that go out to the disk-controller (tps for sda).
Good controllers migth have a logic of their own that reduce the real number of transactions further.
A transaction might be the SCSI-command "write 2 GB to crontoller 1 target 2 lun 3 starting from sector 22). As you can see this can not be brought into direct correlation with throughput-numbers.
What you are after is the sustained write-rate. You have a couple of limiting factors here:
My guess for your system is: Get a good hardware-raid-controller that is capable of doing raid 10 or 5 and get at least 6 fast (15k) disks.
For professional use use SAS instead of SATA.