On Tue, Sep 09, 2014 at 01:01:09PM +0100, Peter Robinson wrote: > >> >> on http://comments.gmane.org/gmane.linux.redhat.fedora.devel/199113 > >> >> *you* complain about systemd-readahead - guess what - if a virtual > >> >> machine is detected it is skipped > >> > > >> > And why is it a good idea to skip it on a virtual machine? > >> > >> guess what happens if you fire up 20 guests at the same > >> time prefetch a lot of data from a shared storage - if > >> the data is not cached at the host you overload disks > > > > How is that different from if you have a room full of physical > > machines using a single SAN? > > It would depend on how many of those are boot from SAN or local > storage for boot, in the case of boot from SAN they tend to also have > dedicated LUNs where as in most cases VMs are on the same LUN. It can > have similar issues but from experience there tends to be situations > that mitigate the issue with physical machines. Sure, we can mitigate the problem in both cases. I run all my VMs on separate iSCSI LUNs these days after seeing performance problems with sharing single LUNs. My point is that "network of physical machines" and "virtual machines on a physical host" are not really that different. They're not even that different when you look at them architecturally -- modern PCs have CPUs that communicate internally and with peripherals using networks; they have separate banks of memory arranged in NUMA nodes. It's common to locate storage and compute in separate places, with layers of caching in between. The difference, if any, is really just one of distances and outward appearance. Therefore I am suspicious of any Fedora package that ships with ConditionVirtualization in its unit file. It's likely covering over a bug in the package. What it should be doing is detecting the specific features it needs and using them, or exiting if they are not available. "It doesn't work virtualized" probably means it doesn't work on physical machines either, in some scenario or other. It maybe, might possibly be added by an operator after they've carefully analyzed some performance problem. At the same time, this issue is not some huge big deal we all need to worry about. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v -- devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxxx https://admin.fedoraproject.org/mailman/listinfo/devel Fedora Code of Conduct: http://fedoraproject.org/code-of-conduct
