Mirrored configuration vs RAID5

[tonty]

We currently have a mirrored configuration in our IBM i570 (V5R4M0) but are very tight on space. Would it be easy enough to "switch" to RAID so we gain a whole load of storage?

We also have several prgrams that are extremely I/O intensive so I am wary of a performance decrease... is RAID slower than mirror?

Regards,
Tony.

 

[goombawaho]

I can see that you need to do some research. You have to have a minimum of three drives for any type of RAID other than mirroring (not counting RAID 0 which gives no redundancy).

The first stop is RAID 5 and if you had the three drives, you would still have the same storage capacity that you do now due to the way RAID 5 takes one drive away for striping. Only when you get into the higher number of disks on a RAID5 array is it more efficient than mirroring. In other words, with 6 disks, you get the capacity of 5 as opposed to only 3 with mirroring.

But the bottom line right now is that you have no options without buying more hard drives. A lot of people mirror (RAID 1) th Operating System on a server and then use RAID 5 for the data storage because the data is more precious than the OS, but you still want some redundancy in the OS.

As for performance, that question has been discussed here before. Check out some links.

http://www.bytepile.com/raid_class.php#10

and google "RAID io performance"

 

[tonty]

We have 30 mirrored drives (60 in total) so there's no issue making up parity sets for RAID. It seems that RAID5 is likely offer worse performance than mirroring (4 IOs for every logical write using RAID5 compared to 2 using RAID1) so that may not be the route I want to take.

Thanks for taking the time to reply.

Regards,
Tony.

 

[p5wizard]

Well yes, if your DB-server has to do the IOs itself, then there is a write penalty going from mirroring to RAID5. If however the RAID5 capability is inside a SAN-server and that SAN-server has a big enough write cache, then your DB-server won't feel the write penalty as it becomes the SAN-server's responsibility to de-stage the IOs from its cache to the drives in the RAID5 ranks.

Same goes for having RAID-adapter cards with their own write-cache in the DB-server. However mostly those type of adapter's write cache is limited in size. Then it all comes down to how write-intensive your DB is and how many of those adapter cards you have to distribute the RAID5 ranks on. Plus, I don't know if those type of cards are available for i5 servers.


HTH,

p5wizard

 

[xmsre]

It's more than just cache, you have to consider the workload as well. If this is small block IO (4K to 16K) writes across a large dataset, then your write cache will be largely ineffective add you will see the full write penalty. If this is 64K IOs in a sequential write pattern or spread over a small subset of your data, then write cache can be very effective, reducing the write penalty to 3 or 2.

 

[technome]

First if you go from a raid 1 to a three drive raid 5, your capacity will INCREASE two fold over a simple raid 1 mirror.
raid 1 results in 50% of the capacity of the two drives
eg 2 100Gig gives you 100Gig capacity
Raid 5 results in n-1 of the capacity of the 3 drives.
eg 3 100Gig drives give you 200 Gig capacity.

Migration from a raid 1 to a 3 drive raid 5 is handled by most raid adapters easily, but does take some time, as the parity needs to be created. Making a 4 drive raid 5 from a 3 drive raid 5 is more involved.

Raid 5 is misaligned, on the newest Lsilogic based adapters with a 4 disk raid 5, reads are a good bit higher then raid 1, I/O are slightly lower overall than raid 1 (with write cache enabled for both raid 1 and 5)...and this is with benchmark programs which stress the disks/cache. Without over utilizing the disk sub system, the write penalty is further negated. Since most server are approx 80% reads, raid 5 comes out well ahead overall.

Yes, raid 1 will beat a 3 drive raid 5 in ALL areas. Personally I would not use 3 drive raid 5s.. you need 4 drives (or more)for performance.


........................................
Chernobyl disaster..a must see pictorial

http://www.kiddofspeed.com/default.htm