----- Original Message ----- From: "Brian Tao" taob@risc.org To: "Bruce Sterling Woodcock" sirbruce@ix.netcom.com Cc: "Sam Schorr" sschorr@homestead-inc.com; "'Gilles TOURPE'" gilles@bnpcn.com; "toasters@mathworks. com" toasters@mathworks.com Sent: Monday, April 03, 2000 9:43 PM Subject: Re: EMC Celerra vs NetApp Filer
On Mon, 3 Apr 2000, Bruce Sterling Woodcock wrote:
What do you mean by this? Not enough total storage?
Might be a CPU limitation. Every single one of my Netapps willrun out of CPU cycles long before they run out of disk. A clustered pair of the F740's routinely burst over 10000 NFS ops/sec each, but they only need a couple shelves of 18GB drives between them. With an EMC Celerra, you can keep the same pool of drives, but plug in more front-end processors to soak up the NFS load. I believe they support up to 14 of those, sharing the same array of drives.
Strange, but I would think you could still get away with less disk per Netapp and just more Netapps and accomplish the same thing cheaper. But I guess EMC is giving you a good deal.
Hmm. Maybe I'm confused, but isn't RAID 1+0 *less* tolerant of disk failures, not more? Both can tolerate a single disk failure.
RAID 1+0 (striped mirrors) may tolerate multi-drive failures.
I don't see how? When one mirror loses a disk, the whole thing is lost; you switch over to the other mirror, which has n-1 disks as the RAID 4 array. So your initial chance of failure is 2n-1 and then n-1 for a second failure whereas RAID 4 is just n followed by n-1 for the second failure.
What I'd like to see is RAID 1+4 (mirrored data and parity drives, preferably with each half of the mirror on a separate FC-AL controller). I don't think any of the NAS vendors offer that yet, although you might be able to do that with Veritas. Anyone at Netapp working on that? :)
The big thing missing is a RAID 4 hardware controller. All the other vendors are relying on RAID hardware to do high-performance; that's why they don't offer RAID 4, and that's why Netapp will continue to be a better solution. :)
RAID 1+0 has essentially twice as many disks, so the chance of a single disk failure is twice as high. Once that happens, the chance of a second disk failure in the same array is about the same was the Netapp. (That's assuming one RAID group).
But since each stripe in a RAID 1+0 is on a mirrored pair ofdrives, you'd have to lose both drives for the RAID 0 part to fail. With N pairs of drives, the chance is only 1 in 2N-1 for the second failure to hit the other half of a broken mirror.
Oh, are the drives mirror images of each-other too? I didn't realize that. So if Drive 1 in Mirror A fails, breaking the mirror, and then Drive 2 in Mirror B fails, Mirror B will be smart enough to switch over to Drive 2 in Mirror A?
And what's the price difference for a Celerra, and a Symmetrix, with *3 times* as much disk as you need for the Netapp model? Can't you buy 2 Netapps for that price?
Well, thats's the kicker when you buy EMC. Not only do you pay apremium on hardware (yay for "RAID-S"), but the software and support as well. There are certain maintenance operations they will not allow you to perform yourself, and you must call in a field engineer. That goes against my philosophy of systems administration, but it works for some people.
Yes, I know about the support, but even so, is the support WORTH the extra you are paying for the hardware? What exactly is the price difference on what you evaluated?
Bruce