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Re: Planning a new server - help needed

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: PFC <lists(at)peufeu(dot)com>
Cc: James Mansion <james(at)mansionfamily(dot)plus(dot)com>, Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance(at)postgresql(dot)org, Tony Nagy <tony(at)shopzeus(dot)com>
Subject: Re: Planning a new server - help needed
Date: 2008-03-30 04:45:36
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Sat, 29 Mar 2008, PFC wrote:

>> Why do you claim that 'More platters also means slower seeks
>> and generally slower performance.'?
> 	More platters -> more heads -> heavier head assembly -> slower seek 
> time

I recall seeing many designs with more platters that have slower seek 
times in benchmarks, and I always presumed this was the reason.  That's 
the basis for that comment.  I'll disclaim that section a bit.

> Actually, now that 8.3 can sync to disk every second instead of at every 
> commit, I wonder, did someone do some enlightening benchmarks ?

I've seen some really heavy workloads where using async commit helped 
group commits in a larger batches usefully, but I personally haven't found 
it to be all that useful if you're already got a caching controller to 
accelerate writes on the kinds of hardware most people have.  It's a great 
solution for situations without a usable write cache though.

> Also, there is a thing called write barriers, which supposedly could be 
> used to implement fsync-like behaviour without the penalty, if the disk, 
> the OS, the controller, and the filesystem support it (that's a lot of 
> ifs)...

The database can't use fsync-like behavior for the things it calls fsync 
for; it needs the full semantics.  You're either doing the full operation, 
or you're cheating and it doesn't do what it's supposed to.  Write 
barriers aren't any improvement over a good direct I/O sync write setup 
for the WAL.  There may be some limited value to that approach for the 
database writes at checkpoint time, but again there's a real fsync coming 
at the end of that and it's not satisfied until everything is on disk (or 
in a good disk controller cache).

> Gigabyte should revamp their i-RAM to use ECC RAM of a larger 
> capacity... and longer lasting battery backup...

I saw a rumor somewhere that they were close to having a new version of 
that using DDR2 ready, which would make it pretty easy to have 8GB on 

> I wonder, how many write cycles those Flash drives can take before 
> reliability becomes a problem...

The newer SSD drives with good write leveling should last at least as long 
as you'd expect a mechanical drive to, even in a WAL application.  Lesser 
grades of flash used as disk could be a problem though.

* Greg Smith gsmith(at)gregsmith(dot)com Baltimore, MD

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