RAID Hardware Controller -- cache reasons ?

I am looking at a Dell PowerEdge T630
server with PERC H330 which has
0 cache memory.

What reasons would you recommend
upgrading this to a PERC ....
which has cache memory ?
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andyalderSaggar maker's framemakerCommented:
For performance and enabling the higher RAID levels I would add the cache. Your choice though if you want to use it as JBOD then cache is just a waste of money. What RAID level are you planning to use?

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finance_teacherAuthor Commented:

OK for no cache ?
Well, all caches are setup so that Thing A can continue without waiting for Thing B to read or write something. So in this case the RAID card can acknowledge the write was done and let whatever was writing continue on, and then write the cached information to the disk when it wants to without holding up the process.  I think they may also be used in read ahead scenarios where the RAID controller assumes what you might need next and loads it into cache memory for quicker access ahead of time. As you know disk reads/writes are a lot slower than memory reads/writes.
So as mentioned it is for performance reasons. If you use a RAID cache you must be sure the battery is good in case of a power failure for RAID levels that do striping so that you want lose everything.
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It will be a lot slower without the cache. It depends upon what performance you are willing to live with/without versus safety of knowing it has already been written to disk.  If the drives are larger than 1TB and you care about the data and having to rebuild, I just lost a RAID5 array with 4TB drives and will not go back with it. I am switching to RAID 10 for better redundancy. In my case it took so long to rebuild a drive that another failed in the mean time and that was it for RAID5.  RAID10 should be a lot faster. The only expense is a lot more drives for less space.  As drive size increases RAID5 as a recommendation goes out the door for most people today.
andyalderSaggar maker's framemakerCommented:
Raid 5 write performance without cache is really bad. For each block it writes it has to read the data and parity disks and then write the data and parity disks - 4 physical I/Os for one logical I/O. If you were to write a whole stripe that would require 4 logical I/Os so 16 physical I/Os. Introduce some cache and it doesn't have to do all those parity reads and writes to disk because it can process them in cache so just ends up doing 4 physical writes instead of 16. says "*See Help Me Choose for R5/R50
performance with no controller cache" against the controller although I have no idea where this "help me choose" is - maybe it pops up when you specify the parts for the machine on the configuration page. Nope, don't see it there I think you'll have to ask Dell for a link to this "help me choose".

You are also exposed to the RAID 5 write hole should you get power loss without battery/flash backed cache. Assume the controller has read the data and parity, done the XOR and written the data and then power dies. It hasn't updated the parity since it lost power before that disk got updated. You can still read that data OK when the power comes back on since you don't normally read parity but then one of the disks fails, it reconstructs the data on the replacement disk using the parity that didn't get updated when you had the power cut so now you have corrupt data. This is called the RAID 5 write hole although it does apply to RAID 1/10 as well to a lesser extent. You can run for months without the parity/mirror getting updated until the fateful day that a disk fails. Background scrubbing can detect the error but it can't correct it since it does not know what disk didn't get updated when power died, for all it knows parity may be good and data may have not been updated.
finance_teacherAuthor Commented:
I get your point about CACHE, but this Dell PowerEdge T630 is just a FRONT-END fileserver, the backend is a Dell EqualLogic SAN.  I assume if I have CACHE on the SAN I do NOT need to worry about having CACHE on the FRONT-END fileserver which is basically just a "pass-through" ?
andyalderSaggar maker's framemakerCommented:
If it's just a NAS head why do you want to use RAID 5? You only need the OS on it so a couple of disks in RAID 1 will do.
If you have a Dell EqualLogic SAN, it would be wise to consult your vendor (DELL) about this.
Whatever is managing the RAID5, should have cache and battery to be safe and perform best.

Here's an article on them:
I would think the RAID functionality would be built into the SAN itself, but again, I'd contact the vendor.  If I understand correctly, the point of a SAN is to not have drives in the servers, or even RAID controllers.  However you do probably need some controller that talks to the iSCSI. I don't know what sort of card that is.  The Equalogics should already have cache onboard.
Gerald ConnollyCommented:
In your initial post your query was about a PERC RAID controller and its Cache - that is a completely different animal from using iSCSI on an Equalogic SAN.

I think of the RAID Controller Cache as speeding things UP rather than the negatives of "it will be slower without Cache".

READ Cache can speed things up by reading more from disk than you requested on the premise that you will read the adjoining data next, doesn't always work well opc's due to the nature if the data stored and the file system.
WRITE Cache will speed up OS and Application writes as the controller will say the write is complete as soon as the data is in cache rather than having to wait for the stuff to make its pedestrian way onto disk. BUT beware Corruption is inevitable in your Data and/or your file system if the write cache is not protected from power fail (such as with a battery)
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