Re: Planning a new server - help needed

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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 there.

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@xxxxxxxxxxxxx http://www.gregsmith.com Baltimore, MD

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

[Postgresql General]     [Postgresql PHP]     [PHP Users]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Yosemite]

  Powered by Linux