Tom Haapanen wrote:
I am wondering whether there are any comparative
efficiencies in memory utilization. Will the jails share the disk cache,
for example, or does each jail allocate its own? Will other kernel
structures (and code!) be shared across jails, or allocated multiple times?
And what about userland applications, like httpd, for example? (I suspect
userland would not be able to benefit, but that's just a guess.)
It is extremely efficient. It is as if the host system sees a single
system with the processes of all the jails running.
* the jails share disk cache
* jails don't have any reserved memory so any unused memory returns to
the free pool of the kernel, available for disk cache
* there is a single kernel shared across all jails
* userland code can also be shared across jails *if* you run the code
from the same set of on-disk binaries (which is not the way most people
set up jails)
* since there is only a single kernel all network and disk I/O from the
jails goes at the same speed as the host
However, VMware installations are now able to achieve the same level of
efficiency using the following measures:
* "transparent page sharing" stores only a single copy of shared memory
pages
* free and unused memory in each guest can be used by installing the
vmmemctl driver (part of VMware-tools)
* disk cache can be "shared" by using shared storage (NAS)
* paravirtualisation drivers for disk and networking in the guest can
achieve full host speed & efficiency
- Andrew
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