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== When Things Go Wrogn ==
== When Things Go Wrogn ==
Revision as of 21:05, 20 September 2016
This site is the Linux-raid kernel list community-managed reference for Linux software RAID as implemented in recent version 3 series and 2.6 kernels. It should replace many of the unmaintained and out-of-date documents out there such as the Software RAID HOWTO and the Linux RAID FAQ.
Where possible, information should be tagged with the minimum kernel/software version required to use the feature. Some of the information on these pages are unfortunately quite old, but we are in the process of updating the info (aren't we always...)
Linux RAID issues are discussed in the linux-raid mailing list to be found at http://vger.kernel.org/vger-lists.html#linux-raid
As of September 2016 Wol is updating it to mdadm 3.3 and the 4.x kernels. Please contact Wol or Nick if you want to help.
Where a page has been partially updated, but the updater lacks the knowledge to update all of it, please mark the old sections with "(2011)" in the section header to indicate it is old information.
There is an Overview section that is based on the RAID HowTo, covering the following:
- Why RAID?
- Hardware issues
- RAID setup
- Detecting, querying and testing
- Tweaking, tuning and troubleshooting
- Related tools
- Partitioning RAID / LVM on RAID
The document is sprinkled with references to the deprecated (since 2003) raidtools which are being gradually removed. Anything mentioning mkraid, raidtab or raidtools should be fixed.
When Things Go Wrogn
Don't panic, Mister Mannering!
RAID is very good at protecting your data. In fact, NEARLY ALL data lost as reported to the raid mailing list, is down to user error while attempting to recover a failed array.
In particular NEVER NEVER NEVER use "mdadm --create" on an already-existing array unless you are being guided by an expert. It is the single most effective way of turning a simple recovery exercise into a major forensic problem - it may not be quite as effective as "dd if=/dev/random of=/dev/sda", but it's pretty close ...
Frequently Asked Questions - FAQ
Here goes a collection of frequently asked questions.
A mdadm-faq is available.
Areas Of Interest
Proper hardware RAID systems are presented to linux as a block device and there's no coverage of them (yet) in this wiki.
BIOS / firmware RAID aka fake raid cards:
- offer a few performance benefits (like CPU, bus and RAM offloading), but may often be much slower than SW raid (link?)
- if the 'raid' card or motherboard dies then you often have to find an exact replacement and this can be tricky for older cards
- if drives move to other machines the data can't easily be read
- there is usually no monitoring or reporting on the array - if a problem occurs then it may not show up unless the machine is rebooted *and* someone is actually watching the BIOS boot screen (or until multiple errors occur and your data is lost)
- you are entrusting your data to unpatchable software written into a BIOS that has probably not been tested, has no support mechanism and almost no community.
- having seen how many bugs the kernel works around in various BIOSes it would be optimistic to think that the BIOS RAID has no bugs.
Given the point of RAID is usually to reduce risk it is fair to say that using fakeraid is a terrible idea and it's better to focus energy on either true HW raid or in-kernel SW raid .... but there is nothing stopping you :)
- Editing pages
- Wikipedia RAID including description of specific Linux RAID types
- Common RAID Disk Drive Format (DDF) standard specifying standard RAID levels from SNIA
- Kernel Newbies basic information about working on kernel
- The mathematics of RAID6
- FAQ about hardware/fake raid cards
- HW RAID support in Linux
- Fault-Tolerant RAID
- linux-raid mailing list archives
See Spam Blocks for the spam restrictions on this site.