Comments on: Still bored…

By: storageguy201

storageguy201 — Fri, 30 May 2008 01:38:27 +0000

No experience with the smaller boxes but on my cx3-40 (clariion cache turned off and running IOZONE) I saw minimal difference. The LUNs I tested were 16 disk Meta (4 x 4+1 R5) and one 8 disk Meta. I figure a Meta Lun is the same as having a large raid group, only difference being the Metas provide added insurance against multiple drive failures.

With two camps one that says the more the number of spindles the better the overall performance and the other camp that says the parity based RAIDs will suffer from large RAID groups I expected to see a big difference – though the host buffer cache may have something to do with the skewd results.

By: SanGod

SanGod — Thu, 29 May 2008 20:16:37 +0000

Yes – but unless you have literally TONS of cache (which is how the Symmetrix gets away with having performance numbers almost identical between R5 and R1) your back-end speed directly affects the speed at which the cache is de-staged to disk. And when you’re talking about a system like a CX300, which really does have minimal cache onboard, you will see the performance hit.

By: storageguy201

storageguy201 — Thu, 29 May 2008 17:19:58 +0000

Ah, ok that’s what I thought you guys were referring to. That penalty is there for any type of parity RAID. While this is usually bad in the software based RAIDs as you pointed out with the Clariions, Symms and NetApps most of the writes are happening in the cache, then they’re coalesced and laid down on the disks in even stripes as much as possible.

This is also where NetApp is a bit different and more efficient. Since it owns the filesystem too it handles things a bit better. It keeps a bitmap of the the blocks in it’s memory and unless the filesystem is heavily fragmented it’ll try to lay down full stripes as much as possible. Even for re-writes it doesn’t modify the existing blocks, it writes a new stripe on the free blocks then adds the old blocks to it’s free-block list. Therefore it’s important to have 10-15% free space on the filesystem for the NetApps. However, as they get full it loses this advantage.

By: SanGod

SanGod — Thu, 29 May 2008 03:26:03 +0000

Because – and this is true with NetApp as well – when you stripe that wide – 14+1 or 13+2 in a 15 drive chassis, every write has to be proceeded by x-1 reads in order to compute the parity. Now granted most arrays do this parity calculation in cache long before it hits the disks, And systems with a real predictive read-ahead, like the Symm, will just pull the calculations from cache instead of having to go back to disk for data that is already there.

15 drive raid groups do offer the benefit of screaming-fast reads….Since the storage array (provided it has the CPU cycles and programming to do it) can, in the case of long, sequential, reads, pre-position more drives to allow the controller to stream data off the disks. It’s *WONDERFUL* for archival data and other data that is read 80% more often than it is written to.

By: storageguy201

storageguy201 — Wed, 28 May 2008 07:00:48 +0000

Interesting post and comments. I come from the NetApp world where 16 drive raid groups are recommended (RAID 6 though). So I’m curious why there would be a performance hit from having a large raid group such as a 15+1 vs. four 4+1 in terms of I/O performance as stated by Andrew above.

I understand about the performance degradation during rebuild as well as the risk of another drive failure during the process.

BTW I now look after a Clariion and a DMX and the clariion seems to be easier to grasp and understand. The DMX is a black box so far outside of the SMC

By: SanGod

SanGod — Mon, 05 May 2008 00:51:55 +0000

I have enough trouble keeping my 4 Dell 2650′s cool in my little 20×40 office/computer room. I think throwing a DMX into the mix would be fatal. (not to mention my wife would divorce me.)

By: williamwbishop

williamwbishop — Mon, 05 May 2008 00:08:50 +0000

Oddly enough, I have a DMX that I’m running a rather large VI3 buildout on….It is a nice toy–But stressful. You guys know what I mean I’m sure.

By: kaneda

kaneda — Sun, 04 May 2008 21:54:02 +0000

Don’t we all wish someone would give us nice shiny toys to play with

Myself, I’d like both a DMX and a VI3 setup to go with it…

By: SanGod

SanGod — Sun, 04 May 2008 02:16:38 +0000

Oh – and the last thing going back to the internal disks on the Clariion would do—it would make RaidGroup0=RaidGroup1 in size…

The most annoying thing is to go about creating a MetaLun and find you’re short on space in RG0 due to the vault drives.

By: SanGod

SanGod — Sun, 04 May 2008 02:15:18 +0000

Actually it’s funny you mention the SSD disk on a Clariion. They used to use a 2.5″ laptop drive on the Clariion SP. (I believe the 4700 and previous) and that’s where all of the clariion code was stored. The FC5300 was the first one to use the “Vault” drive (although it was only the first drive in the array at the time) but I think that was only because the raid controller on the 5300 was a low-profile card that fit into the slot usually occupied by an LCC card.

The problem with over-engineering the Clariion, is that the busses aren’t the bottlenecks on a Clariion. When it comes down to it all communications are serial because there is still only one internal processor (regardless of the number of cores that processor might have) and bus. That means that all IO, by design, is serial.

Now I won’t even get into the fact that it’s running Windows Embedded…..