I have some questions regarding a Dell 2650 configuration that I am considering, and I have yet to find someone at Dell that can answer them. Since I know several people here are running Dell servers, I was hoping some of you could enlighten me.

I am considering purchasing a PowerEdge 2650, 2x2.8GHz, 4GB RAM, with five 73 GB SCSI 320 HDDs, and I want to configure it as RAID 10 with four disks, having one hot spare. I am going to request a PERC4-DC RAID controller (LSI card, dual channels, 320MB/sec, 128 MB cache) instead of the standard PERC3-Di which is an Adaptec 160 controller that has a bad reputation for performance.

However, I am unsure if the PERC4-DC controller supports RAID 10 proper, or just RAID Level 1-Concatenated like the PERC4-Di spec says at http://docs.us.dell.com/docs/software/smarrman/marb32/ch8_perc.htm (you may have to log in to access that page). Specifically, it says, "RAID 10 support on [the PERC4-Di] controller is implemented as RAID Level 1-Concatenated." Since the PERC4-Di is the integrated/onboard version of the PERC4-DC, I suspect that its RAID support is the same. Does anyone know the details on this or where it is documented?

If I opted for software RAID instead, I think hot swapping would be more trouble, but would Dell's RAID monitoring still work full-featured? Are there other drawbacks to running software RAID vs hardware RAID with the PERC3/PERC4 controllers?

Also, the server has an option for a 5 Bay (2+3) hot plug SCSI split backplane. As I recall, channels are transparent to the RAID array so using a split backplane with a four disk RAID 10 configuration should be fine. Is this correct? Will the hot spare be available to both channels in the event of a disk crash?

The config you are talking about would only give you about 140 gig of space for over $6000. If you want speed and redundancy you should fill up a cabinet with 18gig drives.

I personally switched from a Dell array to the Apple xRaid. It's cheaper, has more storage, fast enough and gets the controller into the drive array where it belongs.

Plus when you discover your 32bit machine is to slow the xRaid will work on your new database server

Dell distinguishes between RAID 10 and RAID-1 Concatenated:

The RAID Advisory Board considers RAID 10 to be an implementation of RAID level 1. RAID 10 combines mirrored drives (RAID 1) with data striping (RAID 0). With RAID 10, data is striped across multiple drives. The set of striped drives is then mirrored onto another set of drives. RAID 10 can be considered a mirror of stripes. NOTE: This RAID level is used only with PERC 2, PERC 2/Si, PERC 3/Si, and PERC 3/Di controllers.

• Groups n disks as one large virtual disk with a capacity of (n/2) disks.
• Mirror images of the data are striped across sets of disk drives. This level provides redundancy through mirroring.
• When a disk fails, the virtual disk is still functional. The data will be read from the surviving mirrored disk.
• Improved read performance and write performance.
• Redundancy for protection of data.

RAID-10 on PERC 2/SC, 2/DC, 3/SC, 3/DCL, 3/DC, 3/QC, 4/Di, and CERC ATA100/4ch controllers is implemented as RAID Level 1-Concatenated. RAID-1 Concatenated is a RAID-1 array that spans across more than a single pair of array disks. This combines the advantages of concatenation with the redundancy of RAID-1. No striping is involved in this RAID type. Also, RAID-1 Concatenated can be implemented on hardware that supports only RAID-1 by creating multiple RAID-1 virtual disks, upgrading the virtual disks to dynamic disks, and then using spanning to concatenate all of the RAID-1 virtual disks into one large dynamic volume. In a concatenation (spanned volume), when an array disk in a concatenated or spanned volume fails, the entire volume becomes unavailable.

So it appears that RAID 1 concatenated differs from RAID 10 in that there is no striping and if one disk fails in a volume, the entire volume becomes unavailable.

AFAIK "RAID 10" can be done in two different ways, 1,0 or 0,1. Either your mirror first and then stripe (10), or you stripe first and then mirror (01). Presumably 10 is better than 01.

James, Dell's RAID 10 info sounds confused to me. Actually, it reads as if it was written by a secretary after taking notes on something she didn't actually understand.

From the above it is not really clear just what the hell their "RAID Level 1-Concatenated" is, but they presumably mean that it is atually RAID 10 done in 1,0 fashion, which would be good.

But their info above is all confused, they are mixing together discussion of hardware and software RAID without ever explicitly saying so, the whole sentence about "In a concatenation ... the entire volume becomes unavailable." is misleading and only partially accurate, etc.

James, why limit yourself to only 4+1 disks? Do you have severe space constraints for the server? I.e., it it being co-located somewhere and has to fit in 1 or 2 U?

Because if not, the traditional "small number of very fast SCSI disks" might still be the way to go, but a larger number of cheaper disks might actually work much better. Unfortunately, I have never seen a good study addressing that question.

You might try asking the guys at Net Express. I know people who've bought servers from them, and have heard that they're very knowledgeable.

Hm, even partially complete scaling laws for representative current technology would be very handy, but there are lots of potential variables to take into account, at least:

• Number of individual disks: 2 to N, where N is perhaps 20 or so.
• Physical characteristics of the disk platters: Rotational speed, seek time.
• Drive communications bus: SCSI (which version), IDE (which version), SATA. (Support for tagged command queueing vs. not, etc. etc.)
• RAID type: 1, 10, 5, or combinations there of.
• RAID controller: Different hardware models, Linux software RAID, etc.
• Dollar costs of all of the above.
And that's just a start, really. Ideally, the max N number of disks should be high enough that the various price/performance curves have stabilized and the answers wouldn't change much as you add even more disks - if there is any such N.

I think the big huge proprietary arrays for video streaming and the like are basically RAID 5 with a large number of IDE disks plus, perhaps, a big chunk of battery backed RAM (AKA, a "solid state disk") used as a cache, but I've no idea whether anyone uses that sort of stuff for an RDBMS. It should be useful for one but I have no data...

All of which is probably irrelevant to you, James - fortunately for you, your problem is much more specific. Have you benchmarked those million product nightly inserts on existing hardware you have lying around? It might be plenty fast enough even on your low-end desktop...

Also, for that sort of bulk sequential load, the key is probably to make sure that the database tables for those bulk loaded products are off on their own disk volume somewhere with nothing else on that disk. As long as you keep everything else off that specialized volume, RAID 1 on just two disks would probably do just fine. (Then spend more of your memory on lots of RAM, as Barry suggests.)

Just how complicated are your "1 million products"? If it's just stuffing a 1 million or so srows into an RDBMS that should be no big deal. But if it's much more complicated than that, your software and business rules for loading in those 1 million products could easily be the dominant factor, far more important than hardware performance differences.

BTW: Since LSI makes all of the current PERC4 cards, I called LSI yesterday and asked them about the PERC4-DC's support for RAID 10. The tech I spoke with said that the PERC4-DC is an LSI MegaRAID 320-2, and it has full support for RAID 10 as does the PERC4-Di card so Dell's documentation is apparently wrong.

Also, Matt Domsch, a Dell lead software engineer, confirmed that hot the PERC4-DC will support a global hot spare using specific software from LSI, but you have to manually configure this to do so. Since hot spares are per-controller, not per channel, it's fine to have a hot spare on the 3 side in a 2+3 split channel configuration. If you lose a disk on the 2-side, it'll rebuild onto the spare such that you'll really be running with one channel with one disk plus a bad disk, and the second channel with three disks.

For the record, I think Dell's docs, as quoted above, are technically "correct", in that they don't seem to actually contain any factual errors per se. It's just that they're so poorly written that it's almost impossible to draw any correct conclusions from them!

Clearly you did the right thing by tracking down the real info from the manufacturer.

The swapping is a result of the code building up data structures in memory.

Yes, move PG off the journaling filesystem.

VACUUM FULL reclaims empty pages after compacting data so, yes, it does shuffle stuff around.

I went perusing the Net for PostgreSQL file system recommendations on Linux, and I found that the SGI contributed XFS and IBM contributed JFS look like the top choices out of the journaling file systems. XFS is known for it's good support for large files, and some PostgreSQL benchmarks have JFS performing better than ext2. It is still unclear to me if there is a "best" FS for PostgreSQL.

"misleading and only partially accurate"
Just a note on Dell...
Dell's definition of raid 10 is NOT the industries standard, the jerks at Dell find it better to keep customers totally confused; confused mushrooms are good for business.
Dell's definition of raid 10 is raid 0+1, which NO one should use, as you can only loose one drive, the second is total array loss. The newer Percs are LSIlogic, and are quit capable of doing industry defined raid 10 (mirrored and striped) or raid 0+1 (stripes mirrored). Anyone confused should go to lsilogic.com and follow their raid 10 setup..
http://lsionline2.lsil.com/esupport/esupportlsi/consumer/esupport.asp?id=cff7d451-f111-4eb1-9b19-de71291b4b9f&resource=&number=1&isExternal=0&nShowFacts=&nShowCause=&nShowChange=&nShowAddInfo=&activepage=statement.asp&bForceMatch=False&strCurrentSymptom=&searchtype=normal&searchclass=QuickSearch&bnewsession=false&selecttype=match

Well, here it is. Dell's idea of RAID10 is creating two or more mirrored sets, and then concatenating them. What this "concatenation" means that it spans across those mirrored sets, as opposed to striping them (which is what RAID10 is), so it's not really a RAID10. You don't get the performance benefits of RAID10 from it, as it accesses the spanned volumes one at a time, as opposed to accessing them all at the same time in a proper striping set.

However, it was possible (although not very intuitive) to create proper RAID10 is you used HBA's BIOS, as opposed to Dell Array Manager GUI, in Perc3/DC. I'm not sure if PERC4/DC allows to do it in a BIOS as well. If anyone has PERC4/DC and 4 spare drives, I'd appreciate the feedback.

The way you did this in PERC3/DC was to create 2 or more mirrored sets first, and then choose those volumes to create a RAID0 volume.