Re: [Xen-users] Xen hypervisor and HDDs with 4K sectors

["J. Roeleveld" <joost@xxxxxxxxxxxx>]

On Tuesday, May 28, 2019 12:05:31 PM CEST Peter Milesson wrote:
> On 2019-05-28 10:40, J. Roeleveld wrote:
> > Peter,
> > 
> > On zondag 26 mei 2019 19:31:22 CEST Peter Milesson wrote:
> >> On 2019-05-26 15:28, Simon Hobson wrote:
> >>>> I want to use the most modern hardware possible, and in line with that,
> >>>> I
> >>>> would like to have the option to use HDD (or SSD) drives with 4K
> >>>> sectors
> >>>> through a hardware RAID controller.
> > 
> > I have since switched to using ZFS for the storage layer, but on my
> > previous setup, I was using Xen, LVM and 4K sector disks with a hardware
> > raid controller. Never experienced any issues related to the disks being
> > 4K.
> > 
> > As recent tools all tend to default to aligning on 4K regardless, I didn't
> > even encounter any bad performance either.
> > 
> >>>> I have been searching information about Xen and support for drives with
> >>>> 4K sectors, but I haven't seen much information on the subject. I would
> >>>> be sincerely "dissappointed" if I purchase a new server for 10000+ USD,
> >>>> only to discover it to be utter useless.>
> > 
> > I buy them in Euros, but would also be annoyed if the server would be less
> > then 1000 USD or EUR.
> > 
> >>> It shouldn't matter.
> >>> As long as the hardware is supported in Xen/Dom0, and you partition on
> >>> 4k
> >>> boundaries, then the guests should be isolated from the underlying
> >>> details. Obviously it would be a good idea to also partition any virtual
> >>> drives on 4k boundaries (if partitioning in the guests).
> > 
> > This was my experience as well, but would like to add that partitioning
> > tools tend to default to 4K in any case.
> > 
> >> Hi Simon,
> >> 
> >> Thanks for your input. That's the point. I have seen a few posts that
> >> the advanced disk format using 4K sectors is not compatible with Xen.
> > 
> > Can you provide links? As I am curious on this.
> > 
> > --
> > Joost
> 
> Hi Joost,
> 
> Thanks for your input.

You're welcome.

> Links below. It's mostly about XenServer, but I
> assume it applies to Xen also.

Not necessarily, XenServer is OS+Xen.
I use Gentoo with Xen.

> There doesn't seem to be many links much
> later than 2016. It either implies that almost everything is 4K today,
> or the server manufacturers still clinging to 512 bytes, with the 512e
> disks some kind of strange animal in the jungle. I read the manual for
> HPE Smart Array Administrator, and 512e drives where mentioned in a way,
> that just muddles things even more.

I haven't used HPA SAA myself, but if that doesn't support 4K drives, you need 
to find something to replace it with.

> At least for desktop PCs, there seems to be a mix of HDDs available
> (512, 512e, and 4K). I migrated a SATA HDD to a SSD today, and CentOS
> reported the HDD to be of type 512e (512 bytes logical/4096 physical),
> while the (brand new 1TB SSD) was a pure 512 drive. So much for the
> advanced format. Anyway, the migration was successful.

My current server uses 512n SAS drives because I couldn't find decent 4kn 
drives. I prefer drives to be honest about what they report to the OS. On the 
previous system, I had no choice as the only affordable disks were all 512e 
(4K, but pretending to be 512) drives.

> https://community.spiceworks.com/topic/1457343-best-possible-way-to-configur
> e-raid-with-4kn-drives

Seems to be an issue with the hardware raid device, not the OS or Xen.

> https://discussions.citrix.com/topic/383676-is-xenserver-7-an-advanced-form
> at-512e-or-4kn-aware-os/

"No official support, but does seem to work"

> https://filers.blogspot.com/2018/12/citrix-xenserver-support-for-4k-sector.
> html

A single comment with no real value. If this were on an official (either Xen 
or Citrix) website, I would consider it. But when found on blogspot with no 
explanation, I would ignore it.





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