Re: Long IO Latency
- From: Christo Kutrovsky <kutrovsky.oracle@xxxxxxxxx>
- To: shivan@xxxxxxxxxxxxxxxx
- Date: Tue, 16 Aug 2005 12:36:43 -0400
Welcome to the world of disk IO.
You approach is generally correct. There are a few things you need to consider:
- Random IO is much slower due to seek times
- Sequencial IO benefits from large reads
- OS cache
You approach of finding out whether the latency you get is acceptable
is on the right way.
Keep in mind disk manufacturer always give you the maximum sequencial
IO speed. Sequencial IO speed is faster in the beggining of the disk
drive, and almost 2x slower towards the end.
Usually sequencial IO (such as full table scans, or copying a file, or
taking a backup) is measured in Mb/sec. Larger reads yeld faster
speeds.
Random IO however (table access after index range scan, nested loop
joins) are measured in IO / sec.
Doing Random IO in specific disk region is much faster then doing
random IO over the entire disk.
Some approximate number are:
Sequencial IO: 55 Mb/sec
Random IO: 120 IO/sec (1 sec = 1000 ms , 1000 ms / 120 IO = 8.33 ms latency)
Depending on how fast your disks are (7200 rpm, 10000 rpm, 15 000 rpm)
you will get different latencies.
Be carefull with OS cache or SAN (controller) cache.
You dont need to write programs, www.iometer.org.
--
Christo Kutrovsky
Database/System Administrator
The Pythian Group
On 8/16/05, Shivanischal A <shivan@xxxxxxxxxxxxxxxx> wrote:
> Hi All,
>
> I'm an application developer on a huge OLTP/DSS product. During my
> performance tuning efforts using Statspack, I happened to notice huge
> latencies under the "Tablespace I/O" and "Disk I/O" sections. The delays
> sometimes were as huge as 700ms even for Undo Tablespace. I immediately
> took up this with the client's DBA team.
>
> And I found that they use RAID-5 for all their ".dbf" files. I know that
> that is a strict no-no. The client's DBA team seems to agree. Just
> wanted to know from you all, how difficult will it be to migrate all
> disks down to RAID 0+1? Are there any best-of-breed methods to do it?
>
> Just one more question, while reading the I/O latency in the Statspack
> report, how slow is slow? I mean lets say the latency is 50ms, how do I
> determine that it is slow?
>
> I multiplied the "Avg Blocks/Read" field with DB Block Size to arrive at
> "Avg Bytes/Read" and then used the below equation:
> (("Avg Bytes/Read" * 1000)/("Av Rd")) = X Bytes/Second.
>
> I then compared this X with the disk manufacturer's specification to
> determine if its slow or not.
>
> Could anyone advise me on how to write test scripts/programs to test
> disk latency? I've written a multi-threaded C++ program that does a lot
> of I/O on available disks. But, when I ran it on the same system, the
> delays my program observed were much smaller than what Statspack showed.
> Is my approach correct? Are there any other parameters that I should
> also consider, like our partitioning policy, separate RAID levels for
> log and undo tablespaces?
>
> Thanks,
> Shiva
>
>
> --
> //www.freelists.org/webpage/oracle-l
>
--
Christo Kutrovsky
Database/System Administrator
The Pythian Group
--
//www.freelists.org/webpage/oracle-l
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