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137

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 ...


84

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) Virus' Software bugs that wipe out data ..


73

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


64

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 ...


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 ...


40

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 ...


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 ...


36

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

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...


34

Remember it like this:


31

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


31

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 ...


30

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 ...


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 ...


27

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 ...


26

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. There are ...


26

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 ...


25

!!!!! 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.


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 ...


23

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 ...


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-5 is a fault-tolerance solution, not a data-integrity solution. Remember that RAID stands for Redundant Array of Inexpensive Disks. Disks are the atomic unit of redundancy -- RAID doesn't really care about data. You buy solutions that employ filesystems like WAFL or ZFS to address data redundancy and integrity. The RAID controller (hardware or ...


21

RAID controllers did not (and could not) synchronize disk spindles, but it was an option on some drives. Given a set of identical drives with spindle sync connectors you could ensure a set of disks were all synchronized. I happened to own some Seagate Elite 3 (ancient, obsolete SCSI-2 drives) which I remembered having such a connector so I found the ...


21

More likely than not your provider is using Hard Drives that are not meant to be used in RAID. Normal consumer SATA drives fall into this category. The likely problem is that the drive started experiencing Uncorrectable Read Errors (UREs). When this happens in a consumer drive, the drive sits there and retries the read operation (usually for 30-60 seconds) ...


20

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 ...


20

It depends on your controller. If it supports hot-swap, then yes. If not, then you might blow the controller and kill the whole array. If you do take a drive out of the array (either while running or powered off) you will have a full rebuild to do once you put it back in which will take a while and degrade performance while it happens. Testing your RAID ...


20

Transactional databases RAID-5 is relatively slow to write as the controller needs to load in enough data to recalculate the parity on a write. Write operations will incur at least four disk operations: Reading in the parity block Reading in the old block (assuming it is not already in cache) to XOR the value with the parity block. Writing the new parity ...



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