RE: [Xen-users] Xen SAN Questions

["Tait Clarridge" <Tait.Clarridge@xxxxxxxxxxxx>]

Hi Ricardo,

I will do some testing with XFS and report some numbers of performance using 
both native and DRBD.

And to answer some of your responses...


El Martes 27 Enero 2009, Tait Clarridge escribió:
>> Hello Everyone,

>Hi, I'm no expert but I'm in the same path as you, so let's try to help each 
>other... and get help from others as we go :)

Sounds good man, like I said above I can provide you with some of the figures 
for the tests I am doing. :)

>> First of all, I need to know if this is good practice because I can see a
>> looming clusterf**k if both machines are running VMs from the same shared
>> storage location.

>Well, it shouldn't happen if you're using GFS or another cluster aware 
>filesystem.

True, but from what I have seen, the two primary DRBD nodes (that were 
implemented so I could write actively to both) suffered some brutal 
split-brains and terrible slowness and aren't the performance I need.. so 
effectively I think "clusters" are out of the picture.

>> Second, I ran a test on two identical servers with DRBD and GFS in a
>> Primary/Primary cluster setup and the performance numbers were appalling
>> compared to local ext3 storage, for example:

>Yes, cluster filesystem have lower performance than non-cluster filesystems, 
>due to the former performing lokcs on files/dirs.
>Add DRBD replication on top of that and performance will be lower.

So maybe I should use DRBD in a non-clustered setup?

>> And more of the same where basically it can range from being 4x to however
>> many times slower reading was. I can only assume that this would be a
>> garbage setup for Xen VM storage and was wondering if anyone could point me
>> to a solution that may be more promising. We currently are running out of
>> space on our NetApp (that does snapshots for backups) for VMs not to
>> mention the I/O available for multiple VMs on a single NetApp directory is
>> already dangerously low.
>>
>> Anyone have thoughts as to what might solve my problems?

>Have you tried any GFS optimizations? e.g. use noatime and nodiratime, disable 
>gfs quotas, etc. The first two should improve reading performance.

I used both noatime and nodiratime but I recently destroyed the GFS setup in 
preparation for testing DRBD+XFS+NFS exporting

>> I am thinking a few things:
>>
>> - Experiment with DRBD again with another Filesystem (XFS?) and have it
>> re-exported as NFS to both machines (so they can both bring up VMs from the
>> "pool")

>I guess NFS could work, unless you have too many machines using it (Linux's 
>NFS sucks)

There would only be two machines using it with a 3rd if there was some planned 
maintenance.

>> - Export one of the machines as iSCSI and software raid it on a primary (not
>> really what I want but might work) 

>This one sound interesting.

I will let you know if the software RAID approach works better than DRBD, but I 
doubt it will as someone else posted in another thread that since DRBD is 
essentially optimized for networking stuff. 

>> - Write a custom script that will backup the VM storage directories to a 3rd
>> server (don't really have the budget for a redundant backup server) using
>> something like rsync 
>>
>> And finally, what kind of redundant server to server storage do most people
>> use here?

>From what I'been reading on the list, most people uses some form of DRBD + AoE 
>or iSCSI.

>Check the thread with subject "disk backend performance" from November 27, 
>2008. There started a very nice discussion involving Thomas Halinka and  
>Stefan de Konink about AoE vs. iSCSI (thank you both!).

>Also, the thread with subject "lenny amd64 and xen" will be of your interest, 
>on November 27 Thomas started a description of his self-build SAN which is 
>very insightful.

Alright, I will check those out after I finish some more testing.

Thanks a lot for the reply, if I find anything I will report it back here.

All the best,
Tait Clarridge


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