Re: kvm-83 write performance raw

[Paolo Pedaletti <paolo.pedaletti@gmail.com>]

Ciao Mark,

> RAID1 has *much* better write performance.  With striping RAIDs, alignment
> is important.  RAID controllers sometimes introduce hidden alignment
> offsets.  Excessive read-ahead is a waste of time with a lot of small
> random I/O, which is what I see mostly with guests on flat disk images.

ok, I can understand this
but on a big multimedia-file partition an "opportune" read-ahead could 
be useful (to set with blockdev)

> With LVM, it pays to make sure the LVs are aligned to the disk.  I prefer
> boundaries with multiples of at least 64-sectors, which makes the LVM
> overhead virtually disappear.  I align the guest filesystems too, when
> I can.

I use LVM extensively so can you explain how can you achieve alignments 
between lvm and filesistem? and how to check it?

I have found this interesting:
http://www.mail-archive.com/linux-raid@vger.kernel.org/msg09685.html
http://kerneltrap.org/mailarchive/linux-raid/2008/12/1/4272764
http://blog.endpoint.com/2008/09/filesystem-io-what-we-presented.html
http://lonesysadmin.net/2009/01/02/how-to-grow-linux-virtual-disks-in-vmware/
(useful even for kvm users :-)
http://orezpraw.com/blog/your-filesystem-starts-where
http://www.issociate.de/board/post/464221/stride_/_stripe_alignment_on_LVM_?.html
http://www.ocztechnologyforum.com/forum/showpost.php?p=335049&postcount=134
http://thunk.org/tytso/blog/2009/02/20/aligning-filesystems-to-an-ssds-erase-block-size/

I've post this links because:
1) I didn't know this alignment-problem
2) lvm is suggested as preferred/best solution instead qcow2 file-image
3) filesystem performance may not related to kvm driver
4) I still have to read those post and understand them :-)

thank you...

-- 
Paolo Pedaletti