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DL380 G6 and RAID performance

Julius Verne
Occasional Contributor

DL380 G6 and RAID performance

Hi there,

I'm looking to run Citrix XenServer on a DL380 G6 which is fully loaded with 8 300GB drives. I am debating if I should create one large RAID-5 container across all of the drives, or if I should create two separate RAID-5 arrays: one on each channel.

I'm going to be running VM's on this and am just wondering if the performance of doing it one way or the other would be really noticeable or negligible.

Thanks experts,

Julian
12 REPLIES
Cederberg
Honored Contributor

Re: DL380 G6 and RAID performance

Hi.

I'm no expert but i'm under the impression that the bigger the raid 5 the more I/O it can handle.

This is how i would set up the raid.

2 disks on diffrent channels in Raid1+0 for OS.

6 disks in RAID5 (3 on each channel) for VM's

but it seems like a waste of space with 300gb disks in a mirror for the OS. And as i said i'm no expert with raid and I/O. this is just my thoughts.

Check out Citrix best practices.
Michael Steele_2
Honored Contributor

Re: DL380 G6 and RAID performance

Hi

a) you don't stripe the O/S. The above should be changed to O/S raid 1 (* mirroring only *)

b) You'll getter faster performance with raid1 than raid5
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Michael Steele_2
Honored Contributor

Re: DL380 G6 and RAID performance

c) If you have firmware raid option then you should use it.

d) Don't combine O/S LVM raid with firmware raid
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wobbe
Respected Contributor

Re: DL380 G6 and RAID performance

I would used raid 1+0.
Michael Steele_2
Honored Contributor

Re: DL380 G6 and RAID performance

Hi

The problems with raid 10 is in its administration and debatable performance inhencement. Not all get better performance because of the file system overhead where the primary disk constally goes back and forth forever.

Additionally, when extending any lvol in raid 10 is impossible without first creating another, bigger lvol first. This is because every disk in striped in round robin order eaually, ro any additional disk won't be eaual.

Look into getting firmware smart array controllers with built in raid options.
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Michael Steele_2
Honored Contributor

Re: DL380 G6 and RAID performance

Note: Firmware raid is always faster than O/S raid.
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u84823
Regular Advisor

Re: DL380 G6 and RAID performance

Your initial thoughts are correct - use as many disks as possible in a single raid-5 set. If using SATA drives you will certainly want hot spare, probably less important if using higher spec SAS drives.

The volume will do (with 15k drives) roughly 170 IOPS per disk, so an 8-drive RAID-5 volume will perform ~1200 IOPS (1 disk overhead). For virtualisation the overriding requirement is maximising concurrent IO throughput to deal with contending loads.

Splitted out a mirror for the OS really is completely pointless and a waste of drive bays. By doing so the performance is by definition reduced by c.340 IOPS, nearly 30%. Provided the box has sufficient physical RAM - which for ESX(i) or Hyper-V is probably a given - there is next to no hypervisor level IO anyway. For example, ESXi can be run from the slowest USB flash drive no bother.

One final thing - make sure your array controller has the battery-backed write cache module installed. Performance with Hyper-V or ESX will be absolutely awful without it.

HTH
Julius Verne
Occasional Contributor

Re: DL380 G6 and RAID performance

We have a the integrated RAID controller on this DL380 G6. It's high-end firmware based. I've been setting up these servers as one large RAID-5 container: 8 drives in RAID-5.

But when I look closer, I notice the controller has two channels to the drive bays. In my mind, I would think performance would suffer when striping across two channels to 8 disks. I would think performance would be better if I allocate 4 drives to each channel, as that is what it appears to be designed to do.

Do you understand my question now? Or does it really not make a difference striping across all the disks across the two channels?

Thanks.

Michael Steele_2
Honored Contributor

Re: DL380 G6 and RAID performance

More channels better performance
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marcus1234
Honored Contributor

Re: DL380 G6 and RAID performance



the os should be on mirror, why easier to manage

if OS and data is on same array,and oS fails predictive or otherwise has domino effect on rest of the logical array ......


raid 5 for the rest witha hot spare assigned when you create the arrray ,

special note if you add a hot spare after the raids are created , some times it is not "really hot" and the server needs a reboot to initiate it , which means if you are not present to reboot, the system will be really slow for that time and run in degraded status painfully sloooow...

also ensure you have 512 cache and battery too on controller it makes a big performance differance

you can set read write cache in HP ACU array config utility so reads or writes can be set faster, this is a dynamic option so server does not require restarts etc..
this is set within the os

IN ACU array config utility you can also set rebuild priority,its just in case a disk fails default is medium priority, high makes it slow for users that are logged on so should be used in none production hours...

tip before going in production pull a disk to check the hot one jumps in and have the rebuild priority to high , this will check cache and the battery on controller too..


have fun points appreciated :)
marcus1234
Honored Contributor

Re: DL380 G6 and RAID performance

forgot to mention each array on its own channel , much better performance,
especially,
when a disk rebuid is in progress or if a disk is having a predictive failture, and re.. requesting , read writes

Tim Nelson
Honored Contributor

Re: DL380 G6 and RAID performance

my 2 cents.

If you are really concerned about performance then migrate to a san based storage array.

You are only going to get so much out of an internal sas controller using 300GB drives.

If you are stuck with what you have then a mirrored pair for the host OS. and raid5 the rest to present to your VMs.

Again, you are limited to what you are going to get out of this configuration.