Reply to: RAID6 Vs RAID5 + HotSpare?

[Paddy]

Think I would have gone for simple mirror, cron'd detach, backup the detached mirror, reattach if only a file/print, raid 5 really is for slow writes, fast reads for most DB's and LDAP in my view - not file and print. (assuming Unix-based OS for cron etc)

I might be biased but I'd always go Sun/IBM/HP for this sort of thing, I've seen Dell fall over spectacularly before, more with SAN's but still an über-fük....

Sun etc, really, one throat to choke, my mantra never let me down. Dell are for PC's and lappies...

And u can ring Sun Support 24/7...

I would seconed that. That gets my vote with both hands.

[doodab]

But what if any advantages are there running RAID6 over a RAID5 + hot spare?

RAID5 seems to gives better R/W throughput over RAID6 and the hotspare means that the array needs to fail three disks before it falls over (assuming that the hot spare takes over)

With RAID 5 + hot spare you will lose the array if a second disk failure occurs while the array is rebuilding onto the hot spare.

You might also want to think about how performance degrades after a drive failure & during the subsequent rebuild.

[AtW]

But what if any advantages are there running RAID6 over a RAID5 + hot spare?

In all honesty I can't say for sure - even though we do use RAID6 on one of the boxes that really needs it. I think the main advantage of RAID6 is that if something fails and then rebuild starts then the system can be resistant to another failure during rebuild.

I think there are some advantages, personally I was attracted to the fact that on 2U box with 4 disks put into RAID6 it was ok for us to lose 2 of them.

[wxman]

RAID6 (apparently) involves much heavier CPU calculations than previous RAIDs, which means a lot of early cards that kind of supported it did not have enough CPU power to handle calculations with adequate speed.

Lesson: buy a card known to be able to handle RAID6 calculations...

HTH

But what if any advantages are there running RAID6 over a RAID5 + hot spare?

RAID5 seems to gives better R/W throughput over RAID6 and the hotspare means that the array needs to fail three disks before it falls over (assuming that the hot spare takes over)

[AtW]

Lesson learned – Pick RAID5 + hot spare over RAID6

RAID6 (apparently) involves much heavier CPU calculations than previous RAIDs, which means a lot of early cards that kind of supported it did not have enough CPU power to handle calculations with adequate speed.

Lesson: buy a card known to be able to handle RAID6 calculations...

HTH

[wxman]

Just a small update on this….

Dell replaced the backplane to the disk array (was proven to be faulty) and now the RAID array seems to be performing OK.

I did some benchmark testing (passmark) on the array and found RAID6 to be slower on both read and write when compared to RAID5

Array now configured as RAID5 + 1 hotspare (6 disks in total) – performance is now OK.

272 Mb/s Read
309 Mb/s Write

The server is the main clients file and print server replacing an old IBM server which benchmarks at a very poor (RAID5)

60 Mb/s read
72 MB/s write

Lesson learned – Pick RAID5 + hot spare over RAID6

[omega]

Can you move the RAID card to a different bus (slot) inside the server or are there no others choices ? and then re-test ? - not all buses have the same internal bandwidth in some servers.

Generally you should enable the Read Ahead caching, but the performance resulting from this is very dependent on the profile of the data, i.e are the files typically large or small, are they stored sequentially on the disk ? or very randomly ?

What type of data are you using for testing ? is this 'live' data from the original file server or is it test data that you have generated ?

Also are there any other settings on in the RAID card BIOS which may have an effect when fine tuning performance ?

Try to keep the data on the drives defragmented if possible in all cases.

A couple of other points...

I would stick with RAID 5, the read performance under optimal conditions should be better than RAID 1. RAID 1 is obviously very wastefull in terms of storage capacity.

Unless you really need the resilience provided by two parity disks stick with RAID 5. With RAID 6 you are increasing the overhead of having to write parity information to two disks, therefore slight slowing write performance.

[AtW]

SATA storage is not the same - if you use "consumer" level SATA HDDs then they are often intentionally crippled by manufacturers to make "enterprise" users buy into their much more expensive "enterprise" disks: crippling is lack of quick timeout to recover from bad sectors, this may cause RAID card to think whole disk dropped out.

[stek]

And, in my experience in real life, only SCSI or FC-AL cuts the mustard in the enterprise, we sold some SATA storage to Uni of Wales in Aberystwyth and it was totaltoolip, IMHO nothing beats the sound of local SCSI (or FC-AL!) battering the disk, while the OS doesn't even't notice....

[AtW]

SSDs are not good if you write a lot on them - they are perfect however for mainly read only databases.

[PAH]

If you don't need masses of disk storage I'd look into SSD, particularly the Intel X25-E or OCZ Vertex drives (current market leaders performance wise afaik), especially if read performance is more important than write.

SSD access times and IOPS are untouchable by mechanical drives regardless of the raid config and also cuts out all the ballaches associated with RAID, though you can RAID SSDs too if you want to go extreme.

Many other benefits of SSD over mechanical too: lower power usage (every watt counts in a rackmount server), cooler, silent (loving the OCZ Vertex in my PC ), more reliable (no moving parts), smaller (2.5").

Only downside at the moment is lack of storage capacity and price per GB. They (or a similar solid state alternative) are the future, but why wait.

http://www.infoq.com/news/2009/12/myspace-ssd

[AtW]

PERC is Dells own brand RAID contoller

That's white label - "own brand" means tulip, who actually created it? My bet would be on cheap low end RAID supplier ...

[stek]

And if disk3 failed why didn't the hot-swap engage?

[stek]

stek

Problem I have is that this server is business critical - therefore I went for an RAID 5 + hot spare or even RAID6 array - both gave duff performance.

However all the above is a mute point - disk 3 has failed in the array AGAIN RIGHT NOW

FYI Dell replaced disk 3 which went suspect today under warrently.

As of 22:15 Friday night Dell UK warrenty support is not "Open" again until Monday 09:00am WTF

Clients Del Poweredge server R610 (£4K in price) is therefore DOA and my only recomandation to the client is to postpone this weekends server install until the hardware is proven.

FYI Client has ALWAYS purchased IBM branded servers - But I recomended Dell at 2/3 the price of IAB as the first of many server refreshes. (I have 20+ servers to replace / virutal)

Client's I.T. muppets (if is is not IBM then it is not worth shagging attitude) are loving this

Think I would have gone for simple mirror, cron'd detach, backup the detached mirror, reattach if only a file/print, raid 5 really is for slow writes, fast reads for most DB's and LDAP in my view - not file and print. (assuming Unix-based OS for cron etc)

I might be biased but I'd always go Sun/IBM/HP for this sort of thing, I've seen Dell fall over spectacularly before, more with SAN's but still an über-fük....

Sun etc, really, one throat to choke, my mantra never let me down. Dell are for PC's and lappies...

And u can ring Sun Support 24/7...

[wxman]

stek

Problem I have is that this server is business critical - therefore I went for an RAID 5 + hot spare or even RAID6 array - both gave duff performance.

However all the above is a mute point - disk 3 has failed in the array AGAIN RIGHT NOW

FYI Dell replaced disk 3 which went suspect today under warrently.

As of 22:15 Friday night Dell UK warrenty support is not "Open" again until Monday 09:00am WTF

Clients Del Poweredge server R610 (£4K in price) is therefore DOA and my only recomandation to the client is to postpone this weekends server install until the hardware is proven.

FYI Client has ALWAYS purchased IBM branded servers - But I recomended Dell at 2/3 the price of IAB as the first of many server refreshes. (I have 20+ servers to replace / virutal)

Client's I.T. muppets (if is is not IBM then it is not worth shagging attitude) are loving this