Heeter

Hey all,


I am reading about different RAID setups to work with a FILE/WEB/EMAIL/FTP server. I am still left wondering which would best the best redundancy/performance setup of my gear.

I am using a Promise FastTrak TX2000 PCI RAID card with 4 x 320gig HD

As of now, I am using this card with 2x120 and 2x160 mirrors, with the 2x120 partitioned into two. So I have a C:,D:,and an E: drive listed in my computer setup.

I am thinking that there must be a better way to use my card instead of a simple 2 mirrored setup. Data redundancy is of upmost importance, in case of HD failure. I am wondering if it worthy of going RAID0+1

I have run out of space, as well I would like to redeploy as a Linux setup instead of a Windows Server setup.


Thanks for your input,

Heeter

fitz

If your card can do a proper RAID 1+0 (NOT RAID 0+1! There is a difference and it DOES matter!) that would be ideal from a performance and redundancy standpoint. Obviously, you lose half the drive space making it not the most economical choice.

RAID 0+1 is taking two disks, making it a stripe set and then mirroring the stripe set to a second pair of drives.
RAID 1+0 creates two mirror sets and strips across the two mirror sets.

From a redundancy/rebuild perspective, in a 0/1 solution, if a drive fails, one of your stripe sets have failed leaving you with one remaining stripe set (2 working disks). When you replace the drive, you have to wait for the whole 640GB stripe set (2x320GB) to rebuild before you are back online with all 4 disks.

In a 1+0 setup, if a drive fails, you still have the remaining drive in the mirror working and pulling data along with the two other drives (3 working disks). When you replace the failed disk, you only have to rebuild a mirror onto the single 320GB disk.

edit: all that being said, even if you can't do a RAID1+0, a RAID 0+1 will provide better performance than a RAID1/RAID5/etc.. but not really the same amount of data protection.. ie: they will only tolerate single drive failure.
In theory, if you have a good RAID card, a RAID1+0 can sustain a two drive failure depending on which drives fail. If a drive fails in each mirror set, the other drives in the mirror will still work and your array will still work.

If you want another RAID level to sustain multiple drive failures, you would be looking at a RAID6.

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Heeter

Awesome!

Thanks for the heads up on this.

RAID1+0 it is then.....




Thanks again,

Heeter

fitz

your controller doesn't do RAID 1+0.. only RAID0+1

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Heeter

Thanks fitz,

I just realized this when I downloaded the drivers earlier


Heeter

Steevo

RAID 5 gets my vote. Multiple stripes across drives, so any drive can fail and you are still going strong. Or a Raid 5 of three with a hot spare.

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Dark Atheist

seeing as we are asking about raid - i have 4 disks, now when i enter the config part it only gives me 2 options, mirror/stripe (2x2) or performance (stripe) is it always best to go for the mirror option (as near everything on the raid will be on a dvd somewhere) or perfomance - i know the only dif is how much space you have

fitz

It really depends on your needs.. He asked for the best performance/redundancy options which is RAID 10 (or RAID 0+1 if you must) over RAID 5 anyday. If you need a general purpose storage that maximizes the amount of storage, RAID 5 is your best option as RAID 10 provides only 50% capacity over the drives vs RAID 5 would offer n-1 capacity of the total raw storage.

RAID5 does stripe across multiple drives with a parity block thrown into the stripe that gets distributed across all the drives in the array. However because of the need to calculate parity info, the write performance of a RAID 5 array will NOT be as good as a RAID 1 or RAID 10 array as it requires more IO and overhead to calculate and write the parity information. A write operation requires 4 I/O operations per block of data it is writing to the disk as it needs to read the old parity block, read the old data block, then write the new data block, and write the new parity block onto the drives. In a sequential read operation, RAID 5 can meet (and in some cases exceed a RAID 10 array). But, how often is your drive doing nothing but a sequential read other than in a benchmark test? In a production/real world scenario, your disks are mostly doing both read and write operations simultaneously as the OS writes out to paging files, servers up files, etc.. and these are mostly smaller bursts of random read/write operations as well.

When a drive fails on your RAID 5 array, the performance of the array (both read and write) suffers considerably. Rebuilds take longer as the rebuild has to write the data back to the drive as well as recalculate parity (remember what I said about write performance on a RAID 5 and the amount of I/O it requires?). So, the longer rebuild time leaves a much longer window for a second drive to fail. If a second drive fails before the array completes rebuilding, your whole array is lost. RAID 0+1 has this same double disk failure issue (2 drive failure results in the loss of the array). However rebuild time is usually faster as it is basically a straight data copy of the other disk in the mirror and no parity need for parity calculation. If you are lucky enough to go up to a RAID 10 array, you can sustain a 2 drive failure in some cases (depending which drives fail). The performance impact of a failed drive and/or rebuild is significantly less in a RAID 1/0 array when compared to a RAID 5 array as well.

You could also look at RAID 50, but that requires at least 6 drives and a better RAID controller that what he has. And again, if you are valuing redundancy over performance, I would go for RAID 6 which contains 2 blocks of parity.

You could certainly look at building a mega RAID 5 array or in the EMC world build a meta LUN with 30 drives into one mongo RAID 5 array that will beat the performance of a 4 drive RAID 10 array as your performance does increase as you add spindles, but a 4 drive RAID 5 will almost never match the performance (especially write performance) and redundancy of a 4 drive RAID 10 array.

Cache certainly helps write performance of RAID 5, but large amounts of battery backed cached are usually not available on most home/low-end SATA Raid controllers.

edit:
Please note: I'm not completely bashing RAID 5, we actually use it on about 95% of your systems because the performance need does not outweigh the need for space and the capacity hit we take on a RAID 10 array is not worth the performance we might gain since we do not need that added performance of a RAID 10 on most systems.

Again, it goes back to the original question, he wanted the best performance RAID setup with 4 drives on a lower end RAID controller. This would be a RAID 10 (or in Heeters case, RAID 0+1). For most people, a RAID 5 array would serve them fine (and serves us fine). But you should at least be aware of the advantages and disadvantages of why you choose which array type.

mirror/stripe is the same as a RAID1+0 or 0+1.. performance is a straight RAID 0 operation and will have no redundancy in the case of a drive failure, your array is toast and you pray you have a good backup.

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Dark Atheist

thanks fitz - to sum it up if i use raid0 make sure everything is on a disc somewhere - which was my original plan

Heeter

Thanks Fitz,

For summing that up too,

I am still setting up RAID0+1 like you mentioned before.

Heeter