See section 4.1 Deployment Considerations in the Flash Pool Technical Report (TR-4070). Publicly accessible here:
http://www.netapp.com/us/media/tr-4070.pdf
I had similar questions a while ago. It seems that RAID-DP for cache RAID group is the best practice however the TR goes on to call out a couple of exceptions with regard to the FAS2200 series arrays. Straight from section 4.1 of the TR:
"When using Data ONTAP 8.2 or a subsequent release, the RAID policies for the SSD RAID group and HDD RAID groups in a Flash Pool aggregate are independent. That means an SSD RAID group could be RAID 4 protected, while the HDD RAID groups in the same Flash Pool aggregate use RAID-DP protection. *Nevertheless, the added protection of RAID-DP makes it a best practice to use RAID-DP for SSD RAID groups as well.* An uncorrectable error in an SSD RAID group that is configured with RAID 4 and has experienced the failure of one SSD results in the entire Flash Pool aggregate being taken offline. It could also cause a loss of data that is cached in the write cache. Therefore, NetApp recommends using RAID-DP protection for SSD RAID groups and HDD RAID groups."
"NetApp also recommends maintaining at least one spare SSD for each node (controller) that has a Flash Pool aggregate configured on it. *However, on a FAS2200 series system that is running a Data ONTAP 8.1.x release, it is acceptable to configure the SSD RAID group in the Flash Pool aggregate with RAIDDP and to operate without a hot spare SSD.*"
"*On a FAS2200 series system that is running Data ONTAP 8.2, the recommendation is to configure the SSD RAID group with RAID 4 protection and one hot spare SSD.* With this configuration, data is protected, and an immediate and fast RAID group rebuild occurs if a SSD fails. If RAID 4 protection is chosen for an SSD RAID group in a Flash Pool aggregate on any other FAS or V-Series system, at least one hot spare SSD should be maintained for each node that has a Flash Pool aggregate configured on it."
On Thu, Feb 19, 2015 at 11:57 AM, Vervloesem Wouter < wouter.vervloesem@neoria.be> wrote:
NetApp best practice documents (like the storage subsystem technical faq) indicate it is better to have raid_dp without spares than raid4 with a spare. It is correct that the rebuild times of SSD are rather small, but when using raid_dp you can lose two disks at the same time, raid4 only one.
Mvg, Wouter Vervloesem
Verstuurd vanaf mijn iPhone
Op 19-feb.-2015 om 18:52 heeft Jordan Slingerland < Jordan.Slingerland@independenthealth.com> het volgende geschreven:
I think you would be okay with RAID4, 2 data disk, 1 parity and 1 spare.
Due to the size and speed of SSD, rebuilds are much faster so this is a much more resilient configuration than it would be with a similar configuration and spinning drives. In addition, with the raid group being so small having only 1 spare and 1 parity disk is not such a big deal. I certainly would not recommend no spares as the fact that you can rebuild a failed disk to a hot spare in under a few hours is an important part of the resiliency.
--JMS
*From:* toasters-bounces@teaparty.net [ mailto:toasters-bounces@teaparty.net toasters-bounces@teaparty.net] *On Behalf Of *Basil *Sent:* Thursday, February 19, 2015 12:14 PM *To:* Edward Rolison *Cc:* toasters@teaparty.net *Subject:* Re: FAS2240-4 'cluster' with 4 SSDs
If you don't protect the SSDs and there's a failure, you'll lose data and corrupt the aggregate, I think. For flash pool, at least.
On Thu, Feb 19, 2015 at 11:38 AM, Edward Rolison ed.rolison@gmail.com wrote:
I'm not overly keen on the concept - these systems aren't particularly performance critical, so can do without. What I'm not clear on is just how weak a link a two drive/no spare RAID4 flashpool would actually be.
I mean, I don't like running without spares at all, but given I believe the flash pool is used like a cache (e.g. not a unique copy, but a replica) is that as much of a risk?
On 19 February 2015 at 16:32, Vervloesem Wouter < wouter.vervloesem@neoria.be> wrote:
I try not to use raid4 in FlashPools. Using raid4 may increase the amount of usable cache size, but this means your SSD raidgroup becomes the weakest link in the aggregate. I would assign all SSD's to one of the nodes and create one SSD raid_dp raigroup.
Mvg, Wouter Vervloesem
Neoria - Uptime Group Veldkant 35D B-2550 Kontich
Tel: +32 (0)3 451 23 82 Mailto: wouter.vervloesem@neoria.be Web: http://www.neoria.be
Op 19-feb.-2015, om 16:48 heeft Ed Morgan Ed.Morgan@ansgroup.co.uk
het volgende geschreven:
I’d do 1 head with a flashpool and one without.
From: Edward Rolison ed.rolison@gmail.com Date: Thursday, 19 February 2015 15:39 To: "toasters@teaparty.net" toasters@teaparty.net Subject: FAS2240-4 'cluster' with 4 SSDs
Can I sanity check something?
I've got a cluster in a box - in the form of a 2240-4. Single shelf, but
it's got 4 SSDs installed.
Which works as two per head.
Looking at it, I can't actually configure a hybrid aggregate, because I
have to match RAID types (RAID-DP on two drives doesn't work). That's on my current code rev of 8.1.3
Upgrading to 8.2 might be a solution, because then I could RAID4 my SSDs.
But I'm wondering - what happens, given a system with no spares, one of
the SSDs dies? (Or indeed, whether running with no spares is actually a good idea in the first place - I do have spares for my SAS drives, but not SSDs).
With that in mind - am I onto a bit of a loser here? And I should just
be looking to replace my SSDs with some drives to match what I've got already?
Or perhaps have one head with flashpool, 3 drives + spare and one
without?
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