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113

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 ...


77

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 ..


57

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 ...


38

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 ...


34

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 ...


33

Remember it like this:


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 ...


28

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...


27

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


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

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 ...


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 ...


24

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 ...


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 ...


22

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

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

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 ...


22

!!!!! 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.


21

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 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

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.


20

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 ...


19

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 ...


19

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 ...


19

On Linux, you could use LVM to gather several hard drives (PV) into one Volume Group (VG) and partition it with the Logical Volumes (LV) you wish to share through samba. See this link for more info.


19

The idea of most RAID levels is to provide better reliability or speed for arrays of disks using a combination of the below methods: striping - splits data speed-efficiently across two or more disks +speed mirroring - copies the data onto two or more disks -capacity +reliability parity - a separate disk(s) to verify the data on the other disks is correct ...


19

Potential Issues I have a couple points of issue with using SSDs for production databases at the present time The majority of database transactions on a the majority of websites are reads not writes. As Dave Markle said, you maximize this performance with RAM first. SSDs are new to the mainstream and enterprise markets and no admin worth his salt is going ...


18

You probably got a bad batch. I am nervous about deploying arrays built from disks from the same batch for that reason -- they are likely to have a similar life-span, which makes getting replacements potentially very exciting when one fails. It isn't impossible that there is some design defect with the drives, that's definitely happened before; however ...



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