RE: [Xen-users] Linux software RAID1 (Dom0 or domU)?

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Ryan Burke
> Sent: 01 February 2007 16:47
> To: Liang Yang
> Cc: Marduk; Petersson, Mats; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Linux software RAID1 (Dom0 or domU)?
> 
> > Mats,
> >
> > The problem when you use guest domain to create MD-RAID 
> array is that the
> > array will only be visible to that guest domain. If we 
> create array in
> > domain0, we can make it visible to all guest domains.
> >
> > I think this is also the benefit to use domain0 to manage 
> MD-RAID array.
> >
> > Regards,
> >
> > Liang
> >
> > ----- Original Message -----
> > From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
> > To: "Marduk" <xen@xxxxxxxxxxxxxxxxxxxxxx>; 
> <xen-users@xxxxxxxxxxxxxxxxxxx>
> > Sent: Thursday, February 01, 2007 8:38 AM
> > Subject: RE: [Xen-users] Linux software RAID1 (Dom0 or domU)?
> >
> >
> >
> >
> >> -----Original Message-----
> >> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> >> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Marduk
> >> Sent: 01 February 2007 15:29
> >> To: xen-users@xxxxxxxxxxxxxxxxxxx
> >> Subject: [Xen-users] Linux software RAID1 (Dom0 or domU)?
> >>
> >> Just wondering if anyone has had any experience with this.
> >>
> >> I'm thinking about setting up a 2-drive RAID1 set using 
> Linux software
> >> RAID.  The question is, as far as performance is 
> concerned, which is
> >> better:
> >>
> >>       * Let dom0 handle the RAID and pass the volume to domU or
> >>       * Pass both drives to domU and let it handle the RAID.
> >
> > I would have thought that the performance penalty for software raid
> > would be about equal - however, there is a difference 
> between loading
> > Dom0 and DomU if you have more than one guest-domain - Dom0 
> will be used
> > by all domains, so any extra load on Dom0 will affect all domains,
> > whilst extra load in DomU only affects that DomU. 
> Particulary, latency
> > for each domain will be higher if Dom0 is handling the raid.
> >
> > That's my thoughts on the subject, and be aware that I've never run
> > software raid on any system ever!
> >
> > --
> > Mats
> >>
> >> thanks in advance,
> >> -m
> >>
[snip list gunk]
> 
> Wouldn't there be a performance impact if the RAID was in a 
> DomU? Every
> I/O request would have to go through a hypercall to the Dom0 
> to perform
> the I/O task and then send the data back. And since this is 
> software RAID
> it isn't a single I/O request, it is *potentially* (depending 
> on the RAID
> level) an I/O request per drive in the RAID. While the Dom0 
> is not running
> the actual RAID algorithms (mdadm stuff, or raidtools for the 
> older crowd)
> it is still handling alot of I/O and the associated overhead for the
> hypercalls for each request. I understand what Mats is saying about
> putting it in a DomU will not starve ALL the other DomU's as 
> if it is in
> Dom0, but it really isn't saving the Dom0 much work.
> 
> Since Xen is currently prone to I/O wait issues with alot of DomU's I
> would strongly suggest getting a hardware RAID card unless 
> the server has
> alot of processing power and not alot of I/O intesive DomU's on it.
> 
> Just my thoughts.

Very good points (I did say that I had never run software raid!)

The increase in I/O requests from DomU to Dom0 will be adding to the
Dom0 load if DomU is doing the software raid, that is correct. Of
course, that's less load in Dom0 than also doing the software raid
functionality itself, but it even further increases the IO latency,
compared to doing two requests within Dom0. 

I agree that hardware raid is the leanest option (cpu-wise, maybe not
for the bank). 

--
Mats
> 
> Ryan
> 
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