Re: Best OS for Postgres 8.2 - Mailing list pgsql-performance

On Tue, 8 May 2007, �~]~N彦 Ian Li wrote:

> In #postgresql on freenode, somebody ever mentioned that ZFS from Solaris
> helps a lot to the performance of pgsql, so dose anyone have information
> about that?

the filesystem you use will affect the performance of postgres
significantly. I've heard a lot of claims for ZFS, unfortunantly many of
them from people who have prooven that they didn't know what they were
talking about by the end of their first or second e-mails.

much of the hype for ZFS is it's volume management capabilities and admin
tools. Linux has most (if not all) of the volume management capabilities,
it just seperates them from the filesystems so that any filesystem can use
them, and as a result you use one tool to setup your RAID, one to setup
snapshots, and a third to format your filesystems where ZFS does this in
one userspace tool.

once you seperate the volume management piece out, the actual performance
question is a lot harder to answer. there are a lot of people who say that
it's far faster then the alternate filesystems on Solaris, but I haven't
seen any good comparisons between it and Linux filesystems.

On Linux you have the choice of several filesystems, and the perfomance
will vary wildly depending on your workload. I personally tend to favor
ext2 (for small filesystems where the application is ensuring data
integrity) or XFS (for large filesystems)

I personally don't trust reiserfs, jfs seems to be a tools for
transitioning from AIX more then anything else, and ext3 seems to have all
the scaling issues of ext2 plus the overhead (and bottleneck) of
journaling.

one issue with journaling filesystems, if you journal the data as well as
the metadata you end up with a very reliable setup, however it means that
all your data needs to be written twice, oncce to the journal, and once to
the final location. the write to the journal can be slightly faster then a
normal write to the final location (the journal is a sequential write to
an existing file), however the need to write twice can effectivly cut your
disk I/O bandwidth in half when doing heavy writes. worse, when you end up
writing mor ethen will fit in the journal (128M is the max for ext3) the
entire system then needs to stall while the journal gets cleared to make
space for the additional writes.

if you don't journal your data then you avoid the problems above, but in a
crash you may find that you lost data, even though the filesystem is
'intact' according to fsck.

David Lang

> Steve Atkins wrote:
>>
>>  On May 7, 2007, at 2:55 PM, David Levy wrote:
>>
>> >  Hi,
>> >
>> >  I am about to order a new server for my Postgres cluster. I will
>> >  probably get a Dual Xeon Quad Core instead of my current Dual Xeon.
>> >  Which OS would you recommend to optimize Postgres behaviour (i/o
>> >  access, multithreading, etc) ?
>> >
>> >  I am hesitating between Fedora Core 6, CentOS and Debian. Can anyone
>> >  help with this ?
>>
>>  Well, all three you mention are much the same, just with a different
>>  badge on the box, as far as performance is concerned. They're all
>>  going to be a moderately recent Linux kernel, with your choice
>>  of filesystems, so any choice between them is going to be driven
>>  more by available staff and support or personal preference.
>>
>>  I'd probably go CentOS 5 over Fedora  just because Fedora doesn't
>>  get supported for very long - more of an issue with a dedicated
>>  database box with a long lifespan than your typical desktop or
>>  interchangeable webserver.
>>
>>  I might also look at Solaris 10, though. I've yet to play with it much,
>>  but it
>>  seems nice, and I suspect it might manage 8 cores better than current
>>  Linux setups.
>>
>>  Cheers,
>>    Steve
>>
>>
>>
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>
> Regards
>
> Ian
>
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