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Re: Cheap RAM disk?

From: Alex Turner <armtuk(at)gmail(dot)com>
To: John A Meinel <john(at)arbash-meinel(dot)com>
Cc: Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Cheap RAM disk?
Date: 2005-07-26 19:11:42
Message-ID: 33c6269f05072612113bd68434@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Also seems pretty silly to put it on a regular SATA connection, when
all that can manage is 150MB/sec.  If you made it connection directly
to 66/64-bit PCI then it could actualy _use_ the speed of the RAM, not
to mention PCI-X.

Alex Turner
NetEconomist

On 7/26/05, John A Meinel <john(at)arbash-meinel(dot)com> wrote:
> 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
> 
> 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).
> 
> But it seems like this might be a decent way to improve insert
> performance, without setting fsync=false.
> 
> 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.
> 
> And if you had a UPS with detection of power failure, you could always
> sync the ramdisk to a local partition before the power goes out. Though
> you could do that with a normal in-memory ramdisk (tmpfs) without having
> to buy the card. Though it does give you up-to an extra 4GB of ram, for
> machines which have already maxed out their slots.
> 
> 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)
> 
> John
> =:->
> 
> 
> 
>

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