[Discuss] LVM vs File system file for KVM Virtual Machines?

[blu at nedharvey.com (Edward Ned Harvey)]

> From: discuss-bounces+blu=nedharvey.com at blu.org [mailto:discuss-
> bounces+blu=nedharvey.com at blu.org] On Behalf Of
> 
> there is caching in the file system layer that we
> wouldn't get with LVM, 

Think about it like this:

On reads:  The guest OS will read some file.  If it's a partial file, or
it's fragmented, then the guest OS is smart enough to know how much to
readahead, and which sectors.  Unfortunately, the host OS doesn't know any
of that, and the host OS might be doing readahead for useless sectors.  But
the host OS is only going to do readahead when/if the disks were otherwise
going to be idle.  So there's no benefit and no harm, regardless of whether
you have LVM or file backing.

On async writes:  The guest OS will be buffering these anyway.  The guest OS
writes them to "disk."  The host OS now buffers them.  They're queued up and
going to disk, in the background, everything is smooth.  No benefit and no
harm either way, whether you have LVM or file backing.

On sync writes:  The guest OS writes something to "disk" and issues the
flush command to the "device."  The host OS then interprets this, and issues
the actual flush command to the actual device.  It returns exactly the same
as the guest would have thought.  No benefit and no harm either way, whether
you have LVM or file backing.

I say it's neutral either way.