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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?

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I believe you're seeing higher IOPS in the TPS value because of disk cache. –  ceejayoz Dec 1 '13 at 20:48
    
Ok, I tried a 10GB file through dd with 256kB block to actually fill the cache and after ~90 seconds tps drops to ~200, so maybe you're right. But still 80 and 200 is quite a difference... Is it possible that read and write IOPS differ? And is there any way to figure out required IOPS from this value? –  pystole Dec 1 '13 at 21:34
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Can you describe why your are after IOPS? read and write are quite a different pair of shoes that get thrown into the same pot here. –  Nils Dec 1 '13 at 22:18
    
The reason is I need to describe a minimum HW requirements. I have a server that receives data over network (we can assume constant bitrate here) and writes received data to disk. Data is written to files sequentially but there could be hundreds (e.g. 800) of them in parallel. I have figured out that when number of clients reach some point I start to get large iowaits. Actual disk throughput I can achieve is about 25MB/s which is quite low, less clients with higher bitrate can do 35MB/s, pure sequential about 130MB/s. So I guess IOPS is what matters here... –  pystole Dec 1 '13 at 22:44
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2 Answers

up vote 2 down vote accepted

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:

  • client-connection: If the network is Gigabit you will never have more than 100 MB/s input
  • disk-controller: If this is a 3 Gb controller you will never have more than 300 MB/s throughput
  • disk: Look up the manufacturers value for sustained write performance
  • Filesystem: There is a little overhead since the OS needs to process data - test that in a RAM-disk...

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.

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Ok, you're right IOPS does not make any sense for writes as there are lot of caches, reordering and merges involved. Closing... Thanks. –  pystole Dec 2 '13 at 21:25
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Please also be aware that TPS value represents reads and writes, you can use -x switch for extended view where reads and writes are separated (r/s = read IOPS, w/s = write IOPS):

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
vda               0.07    24.65    0.30   18.95    30.65   330.22    18.74     0.07    3.61   0.98   1.89
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Yeah, you're right. But in my case there's almost 100% of writes, reads are almost none. –  pystole Dec 2 '13 at 12:24
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