ramdisks [a solution]

[Mike Wright]

Sharpe, Sam J wrote:
> 2009/4/15 Mike Wright <mike.wright at mailinator.com>:
>> Oh, great pool of collective wisdom, oracle of all, please be so kind as to
>> share your knowledge with me.
>>
>> I seek knowledge of ramdisks.
>>
>> 1) In /dev there are 16 ramdisks of 16M each.  Are these free for the
>> current user or are there other system processes that require them?
> 
> Yes they should be free.
> 
>> If they are free to use do I need them to exist if I have no use for them?
> 
> They don't exist (they aren't using any RAM) if you aren't using them...
> 
>> 2) Where are these created?  I've seen documents that say one may add kernel
>> options in grub/lilo to set their size but that implies that they all have
>> that same size in common.  Where is their quantity determined?
> 
> The kernel creates them. Here's a primer that's still pretty relevant:
> http://www.vanemery.com/Linux/Ramdisk/ramdisk.html

Been there...

> 
> In particular if you look at /boot/config-`uname -r` which is the
> config your running kernel was created with, you will see:
> CONFIG_BLK_DEV_RAM=y
> CONFIG_BLK_DEV_RAM_COUNT=16
> CONFIG_BLK_DEV_RAM_SIZE=16384
> 
> So without a kernel reconfig, you can't change them, but see my answer
> above about them not using any space.

Wow.  Now that was very informative.  I didn't think to look in the 
kernel configs.

> 
>> 3) Must they be created during the boot process?
> 
> Depends what you mean. Yes those ones must be created during the boot
> process because the kernel is configured like that, but extra ones can
> be created outside the boot process as well.
> 
>> Is there a way to override all of that and create my own layout?  For my
>> application I'd rather have two ramdisks, one 10M and the other 30M, plus
>> any other(s) that may be required by the o/s.
> 
> Now that, I don't know the answer to because I've never wanted two ram
> disks of different sizes. I've set the ramdisk_size kernel parameter
> to something bigger and only used one disk but never two of different
> sizes. You'll need some other collective wisdom ;o)
> 

I came up with a solution that uses LVM and it works a *charm*, albeit, 
probably because of large block sizes relative to the 16M size of the 
ramdisks, seems inefficient (costs 25% each).  Maybe I can tune these 
and get better.

pvcreate /dev/ram15
pvcreate /dev/ram14
pvcreate /dev/ram13
pvcreate /dev/ram12

vgcreate my36Mdisk /dev/ram15 /dev/ram14 /dev/ram13
vgcreate my12Mdisk /dev/ram/12

lvcreate -n myRamDisk1 -L 36M my36Mdisk
lvcreate -n myRamDisk2 -L 12M my12Mdisk

mkfs.....


... and I have my ramdisks and my app screams!!!!


Sam, thanks for the help :D