Re: Disk performance

On Tuesday 15 June 2010 15:16:19 Ivan Voras wrote:
> On 06/15/10 14:59, Janning wrote:
> > Hi all,
> >
> > as we encountered some limitations of our cheap disk setup, I really
> > would like to see how cheap they are compared to expensive disk setups.
> >
> > We have a 12 GB RAM machine with intel i7-975 and using
> > 3 disks "Seagate Barracuda 7200.11, ST31500341AS (1.5 GB)"
> > One disk for the system and WAL etc. and one SW RAID-0 with two disks for
> > postgresql data.
> >
> > Now I ran a few test as described in
> > http://www.westnet.com/~gsmith/content/postgresql/pg-disktesting.htm
> >
> > # time sh -c "dd if=/dev/zero of=bigfile bs=8k count=3000000 && sync"
> > 3000000+0 records in
> > 3000000+0 records out
> > 24576000000 bytes (25 GB) copied, 276.03 s, 89.0 MB/s
> >
> > real    4m48.658s
> > user    0m0.580s
> > sys    0m51.579s
> >
> > # time dd if=bigfile of=/dev/null bs=8k
> > 3000000+0 records in
> > 3000000+0 records out
> > 24576000000 bytes (25 GB) copied, 222.841 s, 110 MB/s
> >
> > real    3m42.879s
> > user    0m0.468s
> > sys    0m18.721s
>
> The figures are ok if the tests were done on a single drive (i.e. not
> your RAID-0 array).

Ahh, I meant raid-1, of course.  Sorry for this.
I tested my raid 1 too and it looks quite the same. Not much difference.

> > IMHO it is looking quite fast compared to the values mentioned in the
> > article. What values do you expect with a very expensive setup like many
> > spindles, scsi, raid controller, battery cache etc. How much faster will
> > it be?
>
> For start, you are attempting to use RAID-0 with two disks here. This
> means you have twice as much risk that a drive failure will cause total
> data loss. In any kind of serious setup this would be the first thing to
> replace.

I did it already :-)

> > Of yourse, you can't give me exact results, but I would just like to get
> > a an idea about how much faster an expensive disk setup could be.
> > Would it be like 10% faster, 100% or 1000% faster? If you can give me any
> > hints, I would greatly appreciate it.
>
> There is no magic here - scalability of drives can be approximated
> linearly:
>
> a) faster drives: 15,000 RPM drives will be almost exactly 15000/7200
> times faster at random access

ok.

> b) more drives: depending on your RAID schema, each parallel drive or
> drive combination will grow your speed linearly. For example, a 3-drive
> RAID-0 will be 3/2 times faster than a 2-drive RAID-0. Of course, you
> would not use RAID-0 anywhere serious. But an 8-drive RAID-10 array will
> be 8/4=2 times faster than a 4-drive RAID-10 array.

So RAID-10 with 4 disks is 2 times faster than a RAID-1, I got it. So as I
need much more power I should look for a RAID-10 with 8 or more 15k RPM disks.

> Finally, it all depends on your expected load vs budget. If you are
> unsure of what you want and what you need, but don't expect serious
> write loads, make a 4-drive RAID-10 array of your cheap 7200 RPM drives,
> invest in more RAM and don't worry about it.

ok, I will look for a hoster who can provide this. Most hosters normaly offer
lots of ram and cpu but no advanced disk configuration.

> Drive controllers are another issue and there is somewhat more magic
> here. If the above paragraph describes you well, you probably don't need
> a RAID controller. There are many different kinds of these with
> extremely different prices, and many different configuration option so
> nowadays it isn't practical to think about those until you really need to.

thanks very much for your help.
It gave me a good idea of what to do. If you have further recommendations, I
would be glad to here them.

kind regards
Janning