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Message-ID: <33c6269f05072612113bd68434@mail.gmail.com>
Date: Tue, 26 Jul 2005 15:11:42 -0400
From: Alex Turner <armtuk@gmail.com>
Reply-To: Alex Turner <armtuk@gmail.com>
To: John A Meinel <john@arbash-meinel.com>
Subject: Re: Cheap RAM disk?
Cc: Postgresql Performance <pgsql-performance@postgresql.org>
In-Reply-To: <42E6661D.3060304@arbash-meinel.com>
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References: <42E6661D.3060304@arbash-meinel.com>

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@arbash-meinel.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=3D2480
>=20
> 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=
.
>=20
> 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.
>=20
> 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).
>=20
> 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.
>=20
> 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).
>=20
> But it seems like this might be a decent way to improve insert
> performance, without setting fsync=3Dfalse.
>=20
> 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.
>=20
> 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.
>=20
> 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)
>=20
> John
> =3D:->
>=20
>=20
>=20
>