On 5/2/2013 8:12 AM, lst_hoe02@kwsoft.de wrote:
IMHO if you say "VM" than the filesystem inside the guest doesn't matter that much.
Malarky.
The difference of ext4/xfs are mostly the knowledge and adjustability for special (high-end) hardware and the like. With a
XFS doesn't require "high end" hardware to demonstrate its advantages over EXT4. In his LCA 2012 presentation on XFS development, Dave Chinner showed data from IIRC a 12 disk RAID0 array, which is hardly high end. Watch the presentation and note the massive lead XFS has over EXT4 (and BTRFS) in most areas. The performance gap is quite staggering. You'll see the same performance, and differences, in a VM or on bare hardware.
Hypervisor providing some standard I/O channel and hiding/handling the hardware details itself, most of the differences are gone. With this in
Again, malarky. The parallel performance in XFS resides in multiple threads and memory structures, b+ trees, and how these are executed and manipulated, and via the on disk layout of AGs and how they're written to in parallel. Virtualization doesn't change nor limit any of this. The block device driver, not the filesystem, talks through the hypervisor to the hardware. No hypervisor imposes limits on XFS parallelism or performance, nor block device drivers. Some may be configured to prioritize IO amongst guests, but that's a different issue entirely.
Worthy of note here is that nearly all XFS testing performed by the developers today is done within virtual machines on filesystems that reside within sparse files atop another XFS filesystem--not directly on hardware. According to you, this double layer of virtualization, OS and filesystem, would further eliminate all meaningful performance differences between XFS and EXT4. Yet this is not the case at all because EXT4 doesn't yet handle sparse files very well, so the XFS lead increases.
mind your question should maybe more of "what filesystem is more Hypervisor friendly". For this i would suspect the simpler the better, so i would choose ext4.
Again, malarky. The hypervisor imposes no limits on filesystem performance, other than the CPU cycles, scheduling, and RAM overhead of the hypervisor itself. I.e. the same things imposed on all aspects of guest operation.
-- Stan