[Dovecot] XFS vs EXT4 for mail storage

[Stan Hoeppner]

On 5/2/2013 8:12 AM, lst_hoe02 at 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.

http://youtu.be/FegjLbCnoBw

> 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