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Re: tablespaces and DB administration

From: pgsql(at)mohawksoft(dot)com
To: "Andreas Pflug" <pgadmin(at)pse-consulting(dot)de>
Cc: "James Robinson" <jlrobins(at)socialserve(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: tablespaces and DB administration
Date: 2004-05-27 17:34:52
Message-ID: 16436.24.91.171.78.1085679292.squirrel@mail.mohawksoft.com (view raw or flat)
Thread:
Lists: pgsql-hackers
> In the age of inexpensive RAID, tablespaces have more or less lost their
> relevance regarding performance. pgsql's philosophy respects this by
> leaving the storage work up to the OS and disk subsystem. Even having
> the xlog on a different spindle won't help too much; you'll probably be
> better off if you stuff all your spindles in one raid on most systems.
> For worse, splitting two disks into separate storage areas to have xlog
> separated would degrade safety for very little performance gain. So the
> advise is: one disk, no alternative. 2 to 20 disks: use a single raid.
> more disks: examine your access patterns carefully before you believe
> you can do the job better than your raid controller.
>
> This leaves table spaces as a mere administrative feature, many (most)
> installations will happily live without that.
>
> Regards,
> Andreas

Sorry, I just can't leave this one alone. Having multiple spindles i.e.
separate data paths to separate disks and disk systems makes a big
difference. Take this simple program:

>>>>>> testio.c >>>>>>>>>>>>>>>>>>>>>>>>>>
#include <unistd.h>
#include <stdio.h>
/* gcc testio.c -o testio */
int main(int argc, char **argv)
{
        int i;
        int blocks;
        FILE *files[16];
        int block[512];
        int foo[512];
        int nblocks = atoi(argv[1]);

        printf("Using %d 2K blocks, total file size %d\n",
              nblocks, sizeof(block)*nblocks);
        for(i=2; i < argc; i++)
                files[i]=fopen(argv[i], "w+b");
        for(blocks=0; blocks < nblocks; blocks++)
                for(i=2; i < argc; i++)
                        fwrite(block, 1, sizeof(block), files[i]);
        for(i=2; i < argc; i++)
                fseek(files[i], 0, SEEK_SET);
        for(blocks=0; blocks < nblocks; blocks++)
                for(i=2; i < argc; i++)
                        fread(foo, 1, sizeof(foo), files[i]);
}
<<<<<<<<<<<<<<<<<<<<

The purpose of the program is to write out files, and read them back in.
It is crude, obviously, and not a good general test, but it does show the
effect of which I am writing.

On my test system, I have a bunch of disks, but I'll use "/home" and
"/vol01" as examples: vol01 is ext2 and home is reiserfs, and both are IBM
ultra SCSI 10K RPM disks.

[root(at)node1 markw]# time ./testio 100000 /home/tmp/test.dat
Using 100000 2K blocks, total file size 204800000

real    0m6.790s
user    0m0.290s
sys     0m4.120s
[root(at)node1 markw]# time ./testio 100000 /vol01/tmp/test.dat
Using 100000 2K blocks, total file size 204800000

real    0m7.274s
user    0m0.210s
sys     0m1.940s

As you can see, they are fairly well matched +- filesystem issues. Now,
lets run the program across two disks:

[root(at)node1 markw]# time ./testio 100000 /vol01/tmp/test.dat
/home/tmp/test.dat
Using 100000 2K blocks, total file size 204800000

real    0m12.012s
user    0m0.610s
sys     0m6.820s

As you can see, it looks like almost double the time, and you might be
looking at this as proof that you are right. Actually, it is mostly an
artifact of program.

Now, look ate the results if I write two files to the same volume:

[root(at)node1 markw]# time ./testio 100000 /vol01/tmp/test.dat
/vol01/tmp/test.dat1
Using 100000 2K blocks, total file size 204800000

real    0m19.298s
user    0m0.680s
sys     0m3.990s

As you can see, it takes a LOT longer. That's because the disk has to do a
lot more work, the hardware data path has to carry twice as much data, and
you have to manage multiple accesses to a single object, the disk.

It doesn't take much imagination to see what this would mean for pg_xlog.

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Subject: Re: tablespaces and DB administration
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