[SOLVED] Why is RAID 5 not recommended ?

[rsinfo]

While going through Systems Requirements, I came across a line "RAID 5 is NOT recommended for 100+ accounts".

Could somebody throw some light on it. Is it dues to speed issues or something else ? Normally, RAID 5 is recommended where ever data integrity & safety is important but Zimbra is recommending something else !

If not RAID 5 then what ? RAID 1 ?

[rsinfo]

Got the answer in forum. Its due to speed issues - writes are slower than reads in RAID 5, but with modern fast true RAID controller card I don't think this should be an issue any more.

[phoenix]

Originally Posted by rsinfo

Got the answer in forum. Its due to speed issues - writes are slower than reads in RAID 5, but with modern fast true RAID controller card I don't think this should be an issue any more.

It will always be an issue compared to a RAID10 set-up, it's still not recommended to use RAID5 but feel free to use it if you want.

[rsinfo]

Originally Posted by phoenix

It will always be an issue compared to a RAID10 set-up, it's still not recommended to use RAID5 but feel free to use it if you want.

RAID 5 also solves one more problem - space. In RAID 5 you get n-1 disk capacity wheres as in RAID 10 you get n/2. This could have direct bearing on the solution you can goin for.

For my IBM x Series server, the largest hard disks available are 300GB. With 4 x 300 GB HDD, I can get 900 GB in RAID5 but only 600GB in RAID10. And I have to support approx. 250 users right now.

[phoenix]

No, the solution to HD space is to add more Drives (they're remarkable inexpensive) not potentially impact your performance by using RAID5. As I said, it's your choice.

[rsinfo]

Point taken.

What's the percentage difference of speed between RAID 5 and RAID 10 ? Has anyone benchmarked it in Zimbra ?

[phoenix]

You'll find plenty of reports if you do a google/yahoo search, most reports seem to suggest about 40% difference in speed. You'll need to look for reports that have a mix of files, zimbra uses MySQL and lots of small files for mail storage. Unfortunately benchmarks tend to concentrate on one type of file configuration.

[rsinfo]

Searched on google. Got a ton of hits & all of them more or less advocated RAID10 over RAID 5 due to write speed hit.

Here's one of them : [WBEL-users] RAID 5 vs. RAID 10

[OneLoveAmaru]

The reason raid 5 is slower writing is because it has to stripe data across all drives in the arrary and it's different data and extra data it's writing to the drives. Say you have 8 drives, it's writing different data PLUS parity bits to all drives. Raid 10 is faster because it only has to split the data between a few drives and it does not have to write any parity bits to the drives, thus increasing significantly the write speed.

Think of two people copying data by hand from a white board. One has to write by hand the data on the white board to 4 pieces of paper. With raid 5 using 4 drives, they have to write to 4 pieces of paper and EXTRA data between the lines in case you lose 1. With that extra data between the lines, you must be able to read the extra information to create any one of those pieces of paper again with the original data on it. Raid 10, the other person has to only split the data on the white board pretty much 50/50 to two different pieces of paper and then make 1 exact copy of each.

Who is going to finish first?? I'm going with raid 10. http://www.zimbra.com/forums/images/smilies/biggrin.gif

But I do still use raid 5 on a lot of our servers, because I like the extra space I get and the performance isn't a factor since it's not getting hit hard enough to matter.