CentOS - Mar 2005 - Odd Question... MAX Ram Disk Size

Greetings,

Before I get slammed, I've looked all over the internet for the answer
to this (googled and more) and haven't been able to find what I'm
looking for.  Here's what I need.  I have a server that I use to hold
workstation images.  I use a Linux boot CD combined with some scripts I
wrote and some disk imaging software to allow me to image windows
workstations.  Anyway, when I roll out a new workstation Image, I may
roll it out to 20 or 30 computers at once, all using the same image file
(base build).  I want to put that image file on a RAMDISK on my server.
The image file is between 3 and 4 gig.  I have 6 gig in the server, so
plenty of memory.  I modified GRUB to increase the size of my ram
disk... First I started with a 200mb, that worked group, formatted, etc.
Then I tried 1gig - everything looks like it works but when I mount it,
it tells me the volume is not formatted or has invalid super blocks.
Perhaps I'm exceeding the allowable built-in ramdisk limit, but I
couldn't find any docs anywhere that would tell me what that limit was.


Does anyone know of an alternative ramdisk product, that does not load
at boot time, where I could do what I need (as described above)?  Or
does anyone know how I can get a 4-gig Ramdisk to work properly?

I'm running the new CentOS 4 on the server.

Thanks for any help.  I did spend 2 days doing Internet research with no
results, prior to posting here.

Thanks.
Scott

Scott Heisler wrote:
> I want to put that image file on a RAMDISK on my server.
> The image file is between 3 and 4 gig.  I have 6 gig in the server, so
> plenty of memory.  I modified GRUB to increase the size of my ram
> disk... First I started with a 200mb, that worked group, formatted, etc.
> Then I tried 1gig - everything looks like it works but when I mount it,
> it tells me the volume is not formatted or has invalid super blocks.
> Perhaps I'm exceeding the allowable built-in ramdisk limit, but I
> couldn't find any docs anywhere that would tell me what that limit was.
When you say ramdisk, do you mean ramdisk as in ramsdisk device 
(hardware) that keeps information accross system reboots and/or power 
cycles, or ramdisk as in file system that exists only in your server's 
RAM and is lost each time machine is rebooted?  Not clear from your 
question.  You mention having "enough RAM for it in server" (which
would
imply later), but you also say that you are looking for "device"
(which
would imply former).

If you are looking for self-contained device, there are many solid state 
disks (SSD) available on the market.  Basically, they look like disk, 
the "only" difference is instead of having magnetic plates, they store
information into internal RAM (either battery backed up, or of the 
non-volatile type).  System sees them as normal IDE/SATA/SCSI/FC drive 
(depending on the interface), and you don't need to have anything 
special to access them.  The speed is usually limited by interface used 
and type of memory used (obviously EEPROM based SSD will have much worse 
write times then DRAM with battery backup based SSD).

If you are looking for solution to use your server's memory as temporary 
RAM disk (you don't care information being lost when you reboot or power 
cycle), something like this works nicely:

# mount -t tmpfs -o size=8g none /ramdisk

That would create 8GB memory-based file system.  The memory file system 
uses is swappable.  So just make sure free RAM + free swap is larger 
than 8GB, and you should be fine.  Of course, you can create smaller or 
larger system too.  Basically it is the same thing as tmpfs on Solaris 
systems.  I kind of like to mount /tmp this way (of course, with much 
smaller size, usually 32-128MB, depending on server's needs).

Or you can place it in fstab to have it always available:

none   /ramdisk   tmpfs   size=8g   0   0

-- 
Aleksandar Milivojevic <amilivojevic at pbl.ca>    Pollard Banknote
Limited
Systems Administrator                           1499 Buffalo Place
Tel: (204) 474-2323 ext 276                     Winnipeg, MB  R3T 1L7

Thanks everyone for the great INFO... This was EXACTLY what I needed and
had a very hard time finding out.  The mount -t tmpfs -o size=1g none
/ramdisk worked perfectly.  I needed temporary ram space to use as disk
space.  Thanks!

Just to explain further, the "images" are not ISO's... These are
hard
drive images, like GHOST files (only using a Linux free application).
But to further explain, even if I mounted an ISO, there would still be
disk IO.. I'm trying to eliminate the disk I/O from the equation so I
can be downloading 20 or 25 machines at the same time.  Since the server
us a dual 2.0gig Xeon and I have gig all the way from the server to the
desktop.. This should scream!

Thanks again. 


Scott


-----Original Message-----
From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On
Behalf Of Craig White
Sent: Thursday, March 31, 2005 10:38 AM
To: centos at centos.org
Subject: Re: [CentOS] Odd Question... MAX Ram Disk Size

On Thu, 2005-03-31 at 09:59 -0600, Aleksandar Milivojevic
wrote:
> Scott Heisler wrote:
> > I want to put that image file on a RAMDISK on my server.
> > The image file is between 3 and 4 gig.  I have 6 gig in the server, 
> > so plenty of memory.  I modified GRUB to increase the size of my ram
> > disk... First I started with a 200mb, that worked group, formatted,
etc.
> > Then I tried 1gig - everything looks like it works but when I mount 
> > it, it tells me the volume is not formatted or has invalid super
blocks.
> > Perhaps I'm exceeding the allowable built-in ramdisk limit, but I 
> > couldn't find any docs anywhere that would tell me what that limit
was.
> 
> When you say ramdisk, do you mean ramdisk as in ramsdisk device
> (hardware) that keeps information accross system reboots and/or power 
> cycles, or ramdisk as in file system that exists only in your server's
> RAM and is lost each time machine is rebooted?  Not clear from your 
> question.  You mention having "enough RAM for it in server"
(which
> would imply later), but you also say that you are looking for
"device"
> (which would imply former).
> 
> If you are looking for self-contained device, there are many solid 
> state disks (SSD) available on the market.  Basically, they look like 
> disk, the "only" difference is instead of having magnetic plates,
they
> store information into internal RAM (either battery backed up, or of 
> the non-volatile type).  System sees them as normal IDE/SATA/SCSI/FC 
> drive (depending on the interface), and you don't need to have 
> anything special to access them.  The speed is usually limited by 
> interface used and type of memory used (obviously EEPROM based SSD 
> will have much worse write times then DRAM with battery backup based
SSD).
> 
> If you are looking for solution to use your server's memory as 
> temporary RAM disk (you don't care information being lost when you 
> reboot or power cycle), something like this works nicely:
> 
> # mount -t tmpfs -o size=8g none /ramdisk
> 
> That would create 8GB memory-based file system.  The memory file 
> system uses is swappable.  So just make sure free RAM + free swap is 
> larger than 8GB, and you should be fine.  Of course, you can create 
> smaller or larger system too.  Basically it is the same thing as tmpfs
> on Solaris systems.  I kind of like to mount /tmp this way (of course,
> with much smaller size, usually 32-128MB, depending on server's
needs).
> 
> Or you can place it in fstab to have it always available:
> 
> none   /ramdisk   tmpfs   size=8g   0   0
----
this is great info and I suppose I might find use for it someday but
wouldn't a loop mount from an iso file (the disk image) be better for
this purpose?

Craig

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