On Sat, Sep 13, 2008 at 5:26 PM, <david@lang.hm> wrote:
> On Fri, 12 Sep 2008, Merlin Moncure wrote:
>>
>> While this is correct, if heavy writing is sustained, especially on
>> large databases, you will eventually outrun the write cache on the
>> controller and things will start to degrade towards the slow case. So
>> it's fairer to say that caching raid controllers burst up to several
>> thousand per second, with a sustained write rate somewhat better than
>> write-through but much worse than the burst rate.
>>
>> How fast things degrade from the burst rate depends on certain
>> factors...how big the database is relative to the o/s read cache in
>> the controller write cache, and how random the i/o is generally. One
>> thing raid controllers are great at is smoothing bursty i/o during
>> checkpoints for example.
>>
>> Unfortunately when you outrun cache on raid controllers the behavior
>> is not always very pleasant...in at least one case I've experienced
>> (perc 5/i) when the cache fills up the card decides to clear it before
>> continuing. This means that if fsync is on, you get unpredictable
>> random freezing pauses while the cache is clearing.
>
> although for postgres the thing that you are doing the fsync on is the WAL
> log file. that is a single (usually) contiguous file. As such it is very
> efficiant to write large chunks of it. so while you will degrade from the
> battery-only mode, the fact that the controller can flush many requests
> worth of writes out to the WAL log at once while you fill the cache with
> them one at a time is still a significant win.
The heap files have to be synced as well during checkpoints, etc.
merlin
