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Cheap RAM disk?

From: John A Meinel <john(at)arbash-meinel(dot)com>
To: Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Cheap RAM disk?
Date: 2005-07-26 16:34:37
Message-ID: 42E6661D.3060304@arbash-meinel.com (view raw or flat)
Thread:
Lists: pgsql-performance
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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