Storwize V7000 Easy Tier: SATA RAID10 Vs SAS RAID6

When IBM released it’s SPC-1 Easy Tier benchmark on DS8000 earlier this year, it was done with SATA RAID10 and SSD RAID10, so when we announced Storwize V7000 with Easy Tier for the midrange, the natural assumption was to pair SATA RAID10 and SSD RAID10 again. But it seems to me that 600GB SAS RAID6 + SSD might be a better combination than 2TB SATA  RAID10 + SSD.

Let’s examine the relative pricing, and the performance as modeled in Intellimagic’s Disk Magic (licensed by IBM for marketing use and made available to IBM Business Partners) for the following example. We will consider this configuration:

• Storwize V7000 head unit plus expansion tray
• 44 x 600GB 10KRPM SAS configured as 2 hot spares, plus 3 arrays of 12+2 RAID6.
• 3 x 300GB SSDs configured as 1 hot spare, plus 1 mirrored pair.
• That’s 20 TB (18 TiB) allowing about 7% for internal overheads
• No capacity contribution from SSD since all extents start life on the magnetic tier
• By the way, be careful if you plan to use Capacity Magic for calculating capacity and drive counts as it tends to overdo hot spares for SSDs

And compare it to this configuration:

• Storwize V7000 head unit plus two expansion trays
• 24 x 2000GB SATA configured as 2 hot spares, plus 11 mirrored pairs.
• 3 x 300GB SSDs configured as 1 hot spare, plus 1 mirrored pair.
• That’s also about 20TB (18 TiB) according to my calculations
• No capacity contribution from SSD since all extents start life on the magnetic tier

So firstly let us compare the relative purchase price of the two configurations (based on list price, and having it on good authority that any discounts applied would be applied evenly to both configs). Surely SATA will be cheaper, right? But no it isn’t…

• SAS config cost approx 100,000 groats
• SATA config cost approx 100,000 groats

I’m not disclosing details of the groat to dollar exchange rate, but I think it’s clear that the decision to use either SATA RAID10 or SAS RAID6 is not a price decision.

The reasons that SATA isn’t cheaper include:

• SATA is only available in a 3.5″ format which requires a 12-bay disk tray (rather than the 2.5″ 24-bay)
• Storwize V7000 has significant license charges per disk tray
• The SATA configs I used did not have an existing 2.5″ tray to house the SSDs, so even though there were only a few SSDs they required a whole 2.5″ tray to be added and licensed.
• Note that more trays also equals higher 7×24 maintenance upgrade costs if standard warranty (next business day) is not for you
• Note also that if you are using remote replication, that also requires licensing per tray
• And note finally and very importantly that Storwize V7000 is very well-priced overall – all we are talking about here is how the structure of the pricing affects your choice of configuration

So now let us compare performance, using Disk Magic. I will measure to the nearest 100 iops and work to a max I/O latency of 20 ms (MS Exchange jetstress requires sub 20ms response for example, so it’s a reasonable cut-off to choose).

First I try the OLTP workload pre-set, which is based on 50/50 read/write with 10KB I/O size and with 28% of reads being sequential. I also enable Easy Tier based on light skewing (i.e. assuming I/Os are reasonably spread across the data on the disk, rather than heavily localized to a few extents)

• SAS config peaks at 5,800 iops (disk bound)
• SATA config peaks at 5,100 iops (disk bound)

On that workload it’s clearly better to use SAS RAID6 rather than SATA RAID10. But my experience is that most customers run read/write ratios more like 70/30.

As an aside, one of the other workload choices in Disk Magic is SPC-1 which shows as a 40/60 read/write ratio which I can’t help feeling would favour Netapp who have some very cool write technology (WAFL and write journaling) but not so strong on reads. But I digress…

Anyway the second time I’ll manually set the I/O size to 10KB and use the basic default in Disk Magic for 70/30 and only 15% of reads being sequential.

• SAS config peaks at 4,800 iops (disk bound)
• SATA config peaks at 4,200 iops (disk bound)

So it does seem that the overall results are fairly consistent – 10KRPM SAS RAID6 will give you 10-15% better performance than SATA RAID10 for the same money. I tried a few other combinations, with 2 mirrored pairs of SSDs for example, higher skew on Easy Tier etc, and the relative cost and performance ratio stayed about the same.

Overall I think I’d rather have SAS RAID6 than SATA RAID10. Of course you could use SAS RAID5 but I’m not a fan of RAID5 (maybe it’s that Netapp influence coming through again).

If I was going to buy a Storwize V7000 I think I would want to embrace Easy Tier for most if not all of the pools on the system. Like my last blog post said, embrace the architecture, don’t try to meld it to your old way of thinking – Storwize V7000 is not just a box of disks – it’s much smarter than that.