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

["Ryan Burke" <burke@xxxxxxxxxxxxxxxxx>]

> 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
>>
>>
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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.

Ryan

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