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 -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
