Linux Raid

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This site is the Linux-raid kernel list community-managed reference for Linux software RAID as implemented in recent version 4 kernels and earlier. It should replace many of the unmaintained and out-of-date documents out there such as the Software RAID HOWTO and the Linux RAID FAQ.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

Contents

Help wanted

This site was created by David Greaves and Nick Yeates. But life moved on and having tried to provide up-to-date info, the info became out of date again. Keld Simonsen updated a lot of the information, and made good ratings for Google.

As of September 2016 Wol is updating it to mdadm 3.3 and the 4.x kernels. Please contact Wol, Keld 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.

Overview

Overview (2011)

This section has been partially updated and is now being rewritten instead.

There is an Overview section that is based on the RAID HowTo, covering the following:

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 Mainwaring!

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

Hardware RAID

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 :)

External links


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