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Re: Making the most of memory?

From: Chris Browne <cbbrowne(at)acm(dot)org>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Making the most of memory?
Date: 2008-01-24 16:09:08
Message-ID: 60ve5jkxy3.fsf@dba2.int.libertyrms.com (view raw or flat)
Thread:
Lists: pgsql-performance
scott(dot)marlowe(at)gmail(dot)com ("Scott Marlowe") writes:
> On Jan 23, 2008 1:57 PM, Guy Rouillier <guyr-ml1(at)burntmail(dot)com> wrote:
>> Scott Marlowe wrote:
>> > I assume you're talking about solid state drives?  They have their
>> > uses, but for most use cases, having plenty of RAM in your server will
>> > be a better way to spend your money.  For certain high throughput,
>> > relatively small databases (i.e. transactional work) the SSD can be
>> > quite useful.
>>
>> Unless somebody has changes some physics recently, I'm not understanding
>> the recent discussions of SSD in the general press.  Flash has a limited
>> number of writes before it becomes unreliable.  On good quality consumer
>
> Actually, I was referring to all SSD systems, some of which are based
> on flash memory, some on DRAM, sometimes backed by hard drives.
>
> There's always a use case for a given piece of tech.

Yeah, I could see an SSD making use of a mixture of technologies...
 - Obviously, it needs a pile of RAM.
 - Then, have a battery that can keep the RAM backed up for [a while].
 - If power goes out, then contents of RAM get copied out to the "flash"
   memory.

In this context, "flash" has several merits over disk drives.
Notably, the absence of moving mechanical parts means:
  - Hopefully lower power consumption than a disk drive
  - Less fragility than a disk drive
  - Quite likely the "flash" will be smaller than a disk drive

The fact that the number of writes may be limited should only be an
important factor if power goes out *INCREDIBLY* frequently, as data
only gets written upon power loss.

The combination of RAM + battery + flash looks like a real winner,
when they are combined using a protocol that takes advantage of their
strengths, and which doesn't rest on their weaknesses.
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