Re: SSD + RAID - Mailing list pgsql-performance

From Anton Rommerskirchen
Subject Re: SSD + RAID
Date
Msg-id 200911191901.14300.atr@atrsoft.de
Whole thread Raw
In response to Re: SSD + RAID  (Craig Ringer <craig@postnewspapers.com.au>)
Responses Re: SSD + RAID  (Brad Nicholson <bnichols@ca.afilias.info>)
List pgsql-performance
Am Donnerstag, 19. November 2009 13:29:56 schrieb Craig Ringer:
> On 19/11/2009 12:22 PM, Scott Carey wrote:
> > 3:  Have PG wait a half second (configurable) after the checkpoint
> > fsync() completes before deleting/ overwriting any WAL segments.  This
> > would be a trivial "feature" to add to a postgres release, I think.
>
> How does that help? It doesn't provide any guarantee that the data has
> hit main storage - it could lurk in SDD cache for hours.
>
> > 4: Yet another solution:  The drives DO adhere to write barriers
> > properly. A filesystem that used these in the process of fsync() would be
> > fine too. So XFS without LVM or MD (or the newer versions of those that
> > don't ignore barriers) would work too.
>
> *if* the WAL is also on the SSD.
>
> If the WAL is on a separate drive, the write barriers do you no good,
> because they won't ensure that the data hits the main drive storage
> before the WAL recycling hits the WAL disk storage. The two drives
> operate independently and the write barriers don't interact.
>
> You'd need some kind of inter-drive write barrier.
>
> --
> Craig Ringer


Hello !

as i understand this:
ssd performace is great, but caching is the problem.

questions:

1. what about conventional disks with 32/64 mb cache ? how do they handle the
plug test if their caches are on ?

2. what about using seperated power supply for the disks ? it it possible to
write back the cache after switching the sata to another machine controller ?

3. what about making a statement about a lacking enterprise feature (aka
emergency battery equipped ssd) and submitting this to the producers ?

I found that one of them (OCZ) seems to handle suggestions of customers (see
write speed discussins on vertex fro example)

and another (intel) seems to handle serious problems with his disks in
rewriting and sometimes redesigning his products - if you tell them and
market dictades to react (see degeneration of performace before 1.11
firmware).

perhaps its time to act and not only to complain about the fact.

(btw: found funny bonnie++ for my intel 160 gb postville and my samsung pb22
after using the sam for now approx. 3 months+ ... my conclusion: NOT all SSD
are equal ...)

best regards

anton

--

ATRSoft GmbH
Bivetsweg 12
D 41542 Dormagen
Deutschland
Tel .: +49(0)2182 8339951
Mobil: +49(0)172 3490817

Geschäftsführer Anton Rommerskirchen

Köln HRB 44927
STNR 122/5701 - 2030
USTID DE213791450

pgsql-performance by date:

Previous
From: Scott Marlowe
Date:
Subject: Re: SSD + RAID
Next
From: Craig James
Date:
Subject: FSM - per database or per installation?