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160

RAID: Why and When RAID stands for Redundant Array of Independent Disks (some are taught "Inexpensive" to indicate that they are "normal" disks; historically there were internally redundant disks which were very expensive; since those are no longer available the acronym has adapted). At the most general level, a RAID is a group of disks that act on the ...


105

RAID guards against one kind of hardware failure. There's lots of failure modes that it doesn't guard against. File corruption Human error (deleting files by mistake) Catastrophic damage (someone dumps water onto the server) Viruses and other malware Software bugs that wipe out data Hardware problems that wipe out data or cause hardware damage (controller ...


83

Q: Why is RAID not a backup? A: Because the whole purpose of a RAID is to make sure that nothing in the world can interrupt that accidental rm -rf / (or DELTREE /X C:\), not even yanking the power chord in panic. Q: But whats the difference between redundancy and a backup? A: If you accidentally overwrite your PhD thesis with garbage, redundancy ...


76

I use kexec-reboot on all of my production systems. It works well, allowing me to bypass the long POST time on HP ProLiant servers and reduce the boot cycle from 5 minutes to 45 seconds. https://github.com/error10/kexec-reboot


59

In short: if using a low-end RAID card (without cache), do yourself a favor and switch to software RAID. If using a mid-to-high-end card (with BBU or NVRAM), then hardware is often (but not always! see below) a good choice. Long answer: when computing power was limited, hardware RAID cards had the significantly advantage to offload parity/syndrome ...


50

Your analysis is fine -- to a point -- in that it absolutely will make things faster. You still have to account for a couple of other issues though: Not everyone can afford enough memory; when you have multiple terabytes of data, you have to put it on disk some time. If you don't have much data, anything is fast enough. Write performance for your ...


42

Also RAID ONE MILLION!!!! 128 Disks so reads would be fast, horrible writes but very reliable I'd imagine, oh and you'd get 1/128th the available space, so not great from a budgetary perspective. Don't do this with flash drives, I tried and set fire to the atmosphere...


42

No, it's not. What happens when your filesystem or RAID volume gets corrupted? Or your server gets set on fire? Or someone accidentally formats the wrong array? You lose all your data and the not-real-backups you thought you had. That's why real backups are on a completely different system than the data you're backing up - because backups protect ...


38

If your array is md0 then echo "idle" > /sys/block/md0/md/sync_action 'idle' will stop an active resync/recovery etc. There is no guarantee that another resync/recovery may not be automatically started again, though some event will be needed to trigger this. http://www.mjmwired.net/kernel/Documentation/md.txt#477


37

I prefer software RAID. Software RAID has the big advantage of not being tied to a particular set of hardware. For example, I've had controller and/or mainboard failures which result in loss of the array. Today's CPUs are plenty fast enough to handle parity on RAID-5 variants. I've also never had any issue with bus saturation from multiple concurrent ...


37

Your options are: Restoring from backups. You do have backups, don't you? RAID is not a backup. Professional data recovery It's possible, though very expensive and not guaranteed, that a professional recovery service will be able to recover your data. Accepting your data loss and learning from the experience. As noted in the comments, large SATA ...


35

You have a double disk failure. This means your data is gone, and you will have to restore from a backup. This is why we aren't supposed to use raid 5 on large disks. You want to set up your raid so you always have the ability to withstand two disk failures, especially with large slow disks.


35

You actually part answered this in your question. The lowest form of RAID is RAID 1. RAID 0 was added well after RAID was defined (can't find reference to a date for this though) The 0 in RAID 0 is used to signify that actually it isn't considered redundant. Think of it as more True/False where 0 is False.


35

Remember it like this:


31

The market for RAID controllers is fairly much consolidated these days. Three broad brush heuristics can be applied: Price Take a look at the pricing for genuine RAID cards from Areca, 3Ware, Adaptec and LSI. Anything that is much, much cheaper than these controllers is a 'fake RAID'. Remember, if it's too good to be true it probably isn't. Manufacturer ...


31

Generally no, this is not a good thing to do. Let your underlying storage do the RAID and don't add software RAID unless you have a compelling edge case, and even then you probably should reconsider your design. It will increase overhead, decrease performance, and not add a whole lot of benefit. Software RAID has its place. That place isn't on top of ...


29

Your current setup is like this: | / | /var | /usr | /home | -------------------------- | LVM Volume | -------------------------- | RAID Volume | -------------------------- | Disk 1 | Disk 2 | Disk 3 | It's a much simpler setup with more flexibility. You can use all of the disks in the RAID volume and slice and dice them ...


28

It has to do with the order that the operations are performed in, and it only appies to arrays that are 6 disks or larger (if you have 4 disks, they're both pretty much the same). RAID 1+0 (10): Disks 1 + 2, 3 + 4, 5 + 6 are mirrored to create a RAID-1 array, and a RAID 0 array is created ontop of the arrays. RAID 0+1 (01): Disk 1 + 2 + 3 are striped to ...


27

!!!!! ONE !!!!! Do one at a time, seriously dude, don't think of doing this ANY other way ok. Anything else will test your full system restoration skills.


27

Generally I'm pretty sure the answer is no (in fact I know of no controller that does this). Doing such a synchronization would be incredibly difficult - vibration, temperature, natural power supply fluctuation, etc. all have small effects on the platter rotational speed (and if you want to be REALLY picky, the size of a sector). You would need to ...


26

Yes, defrag does still make sense for RAID. While it's true that the layout the OS sees isn't the same as the physical layout, it's monotonic, ie the virtual sectors are in the same order on the disk as they are on the array, it's just they are scattered across disks. Also, the RAID controller will use predictive caching (if it has it) based on an ...


25

There's an interesting intersection of server design, disk technology and economics here: Also see: Why are Large Form Factor (LFF) disks still fairly prevelant? The move toward dense rackmount and small form-factor servers. E.g. you don't see many tower offerings anymore from the major manufacturers, whereas the denser product lines enjoy more frequent ...


24

A few items to help clarify SAS technology... SATA drives can connect to SAS ports. SAS drives cannot connect to SATA ports. Server-class hardware typically uses an embedded RAID controller or a separate RAID controller PCIe device. Most RAID controllers and SAS HBAs will use SAS connections (multilane or 4-lane SAS ports). Internally, these systems will ...


24

Redundancy protects you against your hardware failing. It does not protect against user error, nor against malicious activity (e.g., crackers getting into your system). See: Why Mirroring is Not a Backup Solution for a hard-earned lesson.


24

LVM is actually quite heavily used. Basically, LVM sits above the hardware (driver) layer. It doesn't add any redundancy or increased reliability (it relies on the underlying storage system to handle reliability). Instead, it provides a lot of added flexibility and additional features. LVM should never see a disk disappear or fail, because the disk ...


24

Biggest 'Doh!' of the week I reckon - sorry dude. The drives themselves won't be physically broken, this is simply that you've killed the array by removing a second disk before the first one had rebuilt - I'm >90% sure your array is toast. Basically you shouldn't have removed them at all while live, if you absolutely had to you should have waited for the ...


23

I've wrestled with this question for a while. There are a number of factors determining how many disks should go into a RAID5 array. I don't know the HP 2012i, so here is my generic advice for RAID5: Non-recoverable read error rate: When a non-recoverable read error occurs, that read fails. For a healthy RAID5 array this is no problem since the missed read ...


22

I prefer HW raid, 'cause if you have to pull good disks out of a dead machine you're not limited to the OS configuration of the raid "array". You do keep backups of your RAID controllers config, don't you? So just load that up on a donor machine, slot in the drives (in the right order! You did label your drives before your pulled them right?) and ...


22

One worthwhile location to check out is StorageReview.com's Comparison of RAID Levels But focused on the answer: LEVEL | CAPACITY | STORAGE | FAILURE | RDM READ | RDM WRITE | SEQ READ | SEQ WRITE | 0 | S * N | 100% | 0 | **** | **** | **** | **** | 1 | S | 50% | 1 | *** | *** | ** | ...


21

RAID 10 is usually recommended since the I/O is so random. Here's an example. The calculations are a bit simplified, but pretty representative. Let's say you have a 6 drive array and your drives can do 100 I/Os per second (IOPS). If you have 100% reads, all six drives will be used and you'll have about 600 IOPS for both RAID 10 and RAID 5. The worst case ...



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