DL380 G6 and RAID performance

[Julius Verne]

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

[Cederberg]

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]

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

[Michael Steele_2]

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

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

[wobbe]

I would used raid 1+0.

[Michael Steele_2]

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.

[Michael Steele_2]

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

[u84823]

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]

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]

More channels better performance