[re-sending with the right email address, hoping the first email won't make it through..]
Fajar Priyanto wrote:
Hi all, Sorry for the OT. I've got an IBM N3300-A10 NAS. It runs Data Ontap 7.2.5.1. The problem is, from the docs it says that it only supports either RAID-DP
or RAID4.
What I want to achieve is Max Storage Capacity, so I change it from
RAID-DP to RAID4, but with RAID4, the maximum disk in a RAID Group decrease from 14 to 7. In the end, either using RAID-DP or RAID4, the capacity is the same.
It's the array's way of saying it's not really safe to operate a 14 disk RAID-4 group in the event of a drive failure. Also performance will suffer greatly with such a large data:parity drive ratio.
NetApp RAID DP (what you have): http://www.netapp.com/us/library/white-papers/wp_3298.html
What's wrong with RAID5, is there any technical limitation with RAID5?
Most onboard RAID controllers are absolute crap, RAID 5 isn't as easy to support as RAID1/RAID0, as extra processing time and code is needed for the parity stuff. I wouldn't use them for even RAID 1 though. If it's onboard, and I want RAID then I'll use software RAID. Unless it's a SCSI/SAS RAID controller often times those onboard RAID controllers are of decent quality.
RAID 5 as implemented by most disk vendors does have issues at least with large SATA disks -
Why RAID 5 stops working in 2009 - http://blogs.zdnet.com/storage/?p=162
A couple vendors that I know of - 3PAR(which I use) and Compellent don't suffer from this limitation because they virtualize the disks, I can't speak too much for Compellent's architecture but 3PAR doesn't RAID physical disks, they raid portions of each disk, when a disk fails, every disk in the system participates in rebuilding that failed set of arrays, which results in upwards of a 10x improvement in recovery time and 90%+ drop in system impact while the arrays are rebuilding. This virtualization also allows you to run multiple levels of RAID on the same spindles for different purposes.
The likelihood of suffering a double disk failure on such a system during a RAID rebuild is probably quite a bit less than suffering a triple disk failure on a RAID-6 system which runs RAID on whole physical disks. At full system load my array can recover from a 750GB drive failure in less than 3 hours. Our previous array took nearly 30 hours to recover from a 300GB(SATA) drive failure. If on my array the disk is only half utilized, then it will take half the time to rebuild, as only written data is rebuilt, no point in replicating portions of the disk that haven't been used, though other vendors do this because they don't have the architecture to support sub-disk rebuilds.
My own 200-disk array has more than 86,000 individual RAID arrays on it, most of them being RAID 5(5+1). Array management is automatic and transparent to the storage system I just tell it how big a volume I want and what type of data protection.
For performance reasons I'm migrating my RAID 5(5+1) to 3+1 next year after I add more space. I miscalculated the growth of the big driver of space on the current system so I had to migrate to 5+1(RAID level migrations have no service impact either on my array). On my system there is roughly only a 9% performance drop from RAID 1+0 to RAID 5+0, so in many cases running RAID 5 on my array is about the same as RAID 1 on other arrays, it's that fast.
RAID 6 has a pretty good performance hit vs RAID 5 due to the extra parity disk, on some arrays the performance hit is even greater as the array calculates parity twice, NetApp I think has as good a RAID 6 implementation as there is, though they can't get around writing the parity information to two different disks.
nate
On Thu, Sep 24, 2009 at 8:48 PM, nate centos@linuxpowered.net wrote:
It's the array's way of saying it's not really safe to operate a 14 disk RAID-4 group in the event of a drive failure. Also performance will suffer greatly with such a large data:parity drive ratio.
NetApp RAID DP (what you have): http://www.netapp.com/us/library/white-papers/wp_3298.html
Why RAID 5 stops working in 2009 - http://blogs.zdnet.com/storage/?p=162
Thanks so much everyone for the comments and insight. This is absolutely giving me very valuable new understanding on RAID. I've just got to say this is another proof that Centos mailing list is one of the most highly trusted list I've ever known. Salut!
Hi,
On Thu, Sep 24, 2009 at 08:48, nate centos@linuxpowered.net wrote:
RAID 6 has a pretty good performance hit vs RAID 5 due to the extra parity disk, on some arrays the performance hit is even greater as the array calculates parity twice, NetApp I think has as good a RAID 6 implementation as there is, though they can't get around writing the parity information to two different disks.
Actually, NetApp tries to attenuate that impact.
As the mirroring is coupled with the filesystem, NetApp uses an intelligent algorithm that tries to fill all the blocks in a whole stripe before writing to the disks, that way it only has to calculate the parity of data in RAM and can flush to all disks at the same time. That's why in most cases they do not have the "read before write" impact of RAID-5 and RAID-6. That's also why they have RAID-4 instead of RAID-5, they say that the parity disk will not be a hot spot as in most cases all the disks will be written at the same time. Well, at least that's what they say...
I believe ZFS implements similar ideas. I guess that's one of the patents for which NetApp tried to sue Sun a couple of years ago when ZFS first came out.
HTH, Filipe
Filipe Brandenburger wrote:
Hi,
On Thu, Sep 24, 2009 at 08:48, nate centos@linuxpowered.net wrote:
RAID 6 has a pretty good performance hit vs RAID 5 due to the extra parity disk, on some arrays the performance hit is even greater as the array calculates parity twice, NetApp I think has as good a RAID 6 implementation as there is, though they can't get around writing the parity information to two different disks.
Actually, NetApp tries to attenuate that impact.
As the mirroring is coupled with the filesystem, NetApp uses an intelligent algorithm that tries to fill all the blocks in a whole stripe before writing to the disks, that way it only has to calculate the parity of data in RAM and can flush to all disks at the same time. That's why in most cases they do not have the "read before write" impact of RAID-5 and RAID-6. That's also why they have RAID-4 instead of RAID-5, they say that the parity disk will not be a hot spot as in most cases all the disks will be written at the same time. Well, at least that's what they say...
I believe ZFS implements similar ideas. I guess that's one of the patents for which NetApp tried to sue Sun a couple of years ago when ZFS first came out.
Not quite. ZFS does things the other way around. "RAID-Z is a data/parity scheme like RAID-5, but it uses dynamic stripe width. " From your description, NetApp massages data to fit the stripe. ZFS's raidz massages the stripe to fit the data.
-
Christopher Chan -
Fajar Priyanto -
Filipe Brandenburger -
nate