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Re: Cheap RAM disk?

From: Chris Browne <cbbrowne(at)acm(dot)org>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Cheap RAM disk?
Date: 2005-07-26 16:51:14
Message-ID: 60u0ih1pfh.fsf@dba2.int.libertyrms.com (view raw or flat)
Thread:
Lists: pgsql-performance
john(at)arbash-meinel(dot)com (John A Meinel) writes:
> I saw a review of a relatively inexpensive RAM disk over at
> anandtech.com, the Gigabyte i-RAM
> http://www.anandtech.com/storage/showdoc.aspx?i=2480

And the review shows that it's not *all* that valuable for many of the
cases they looked at.

> Basically, it is a PCI card, which takes standard DDR RAM, and has a
> SATA port on it, so that to the system, it looks like a normal SATA
> drive.
>
> The card costs about $100-150, and you fill it with your own ram, so
> for a 4GB (max size) disk, it costs around $500. Looking for solid
> state storage devices, the cheapest I found was around $5k for 2GB.
>
> Gigabyte claims that the battery backup can last up to 16h, which
> seems decent, if not really long (the $5k solution has a built-in
> harddrive so that if the power goes out, it uses the battery power to
> copy the ramdisk onto the harddrive for more permanent storage).
>
> Anyway, would something like this be reasonable as a drive for storing
> pg_xlog? With 4GB you could have as many as 256 checkpoint segments.
>
> I'm a little leary as it is definitely a version 1.0 product (it is
> still using an FPGA as the controller, so they were obviously pushing
> to get the card into production).

What disappoints me is that nobody has tried the CF/RAM answer; rather
than putting a hard drive on the board, you put on some form of flash
device (CompactFlash or such), where if power fails, it pushes data
onto the CF.  That ought to be cheaper (both in terms of hardware cost
and power consumption) than using a hard disk.

> But it seems like this might be a decent way to improve insert
> performance, without setting fsync=false.

That's the case which might prove Ludicrously Quicker than any of the
sample cases in the review.

> Probably it should see some serious testing (as in power spikes/pulled
> plugs, etc). I know the article made some claim that if you actually
> pull out the card it goes into "high consumption mode" which is
> somehow greater than if you leave it in the slot with the power
> off. Which to me seems like a lot of bull, and really means the 16h is
> only under best-case circumstances. But even 1-2h is sufficient to
> handle a simple power outage.

Certainly.

> Anyway, I thought I would mention it to the list, to see if anyone
> else has heard of it, or has any thoughts on the matter. I'm sure
> there are some people who are using more expensive ram disks, maybe
> they have some ideas about what this device is missing. (other than
> costing about 1/10th the price)

Well, if it hits a "2.0" version, it may get interesting...
-- 
(format nil "~S(at)~S" "cbbrowne" "acm.org")
http://www.ntlug.org/~cbbrowne/sap.html
Rules of the Evil Overlord #78.  "I will not tell my Legions of Terror
"And he must  be taken alive!" The command will be:  ``And try to take
him alive if it is reasonably practical.''"
<http://www.eviloverlord.com/>

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