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Old 10-27-2010, 08:01 AM   #1
Ulysses_
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Ramdisk that can do this?


Problem: if you run 3 vmware VM's simultaneously the definition of each has to specify 1/3 of the available memory, or whatever sums up to the available memory. But then such a VM run alone will miss 2/3 of the host memory.

Rather than change the definition of VM's again and again to take advantage of as much memory as possible, I wonder what happens if the following measures are taken:

Define VM's with 1/3 of available memory and with their swap spaces located on a ramdisk of the host, as NONPERSISTENT virtual drives.

Then you run a VM alone, and it has a swap file almost as fast as normal memory. But which occupies no space in the host's ramdisk initially because it is empty. After some use the swap file grows and the corresponding nonpersistent drive deltas grow on the host ramdisk.

1. Can a windows ramdisk grow in size?

2. Can a linux ramdisk grow in size? (tmpfs?)

3. When a ramdisk cannot grow any more because there is no more physical memory, can some of it automatically be allocated in the host swap space? Which ramdisk can do this?

Last edited by Ulysses_; 10-27-2010 at 08:11 AM.
 
Old 10-27-2010, 09:24 PM   #2
kbp
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The swap files don't grow, their size is static. It sounds like you're going to great lengths attempting to run more vm's than you have capacity for, I'm not sure it's worth the effort

cheers
 
Old 10-28-2010, 05:47 AM   #3
Ulysses_
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Thanks for the input, but the vmware representation of a swap drive does grow, if declared as nonpersistent. The VM sees a swap drive that is fixed in size, but vmware implements it otherwise if desired.
 
Old 10-28-2010, 08:20 AM   #4
syg00
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Hmmm - so you envisage taking a swapout in a guest to satisfy a hard page fault that will need to be resolved by (yet another) I/O.
The swapout will hit an in-memory hipervisor representation of the swap space. Hopefully. If not, the hipervisor will fault and allocate memory for the expansion of the (guest) swap. Then satisfy the guest request, and notify it.

Could happen a lot. This is more efficient how ?.

Note that tmpfs is not ramdisk - it was designed specifically because of limitations in (Linux) ramdisk.

(Caveat: I haven't used vmware in years; they have some smart folks that deal with guest memory "ballooning", but I'm not sure about this idea)
 
Old 10-28-2010, 12:22 PM   #5
Ulysses_
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Quote:
Originally Posted by syg00 View Post
The swapout will hit an in-memory hipervisor representation of the swap space. Hopefully. If not, the hipervisor will fault and allocate memory for the expansion of the (guest) swap.
Not exactly, the representation is just an empty file, which I want to place on a host ramdisk. Every fault in the guest, adds a little to this file. When the file occupies the entire ramdisk, we have a problem. No page-fault occurs in the hypervisor, it just freezes saying "out of disk space" or something.

Hoping to find a ramdisk that includes some physical disk space in it, so it starts off being as fast as ram and later becomes as fast as a disk if too much data is stored on it.

I have since found tmpfs that seems to use a mixture of ram and disk space. Seems to be just what I need, the place to store those vmware virtual drives that hold guest swap spaces.

But I don't understand the mounting of tmpfs. Why is it mounted at /

Last edited by Ulysses_; 10-28-2010 at 12:23 PM.
 
  


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