diff options
author | madduck <madduck@3cfab66f-1918-0410-86b3-c06b76f9a464> | 2006-09-16 09:40:53 +0000 |
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committer | madduck <madduck@3cfab66f-1918-0410-86b3-c06b76f9a464> | 2006-09-16 09:40:53 +0000 |
commit | 9459a36cf1f50444463811d2807fe3352b52e56d (patch) | |
tree | c8ac0a6b60bb1f2df6c6e209fc6befd09c7a4d51 /debian | |
parent | ac6676d8d0148507bf7ad80d62ae8a07ac5e228d (diff) |
FAQ updates
Diffstat (limited to 'debian')
-rw-r--r-- | debian/FAQ | 71 |
1 files changed, 63 insertions, 8 deletions
@@ -5,15 +5,16 @@ Also see /usr/share/doc/mdadm/README.recipes.gz 0. What does MD stand for? ~~~~~~~~~~~~~~~~~~~~~~~~~~ -MD is an abbreviation for "multiple device". The Linux MD implementation -implements various strategies for combining multiple physical devices into -single logical ones. The most common use case is commonly known as "Software -RAID". Linux supports RAID levels 1, 5, 6, and 10, as well as the -"pseudo-redundant" RAID level 0. In addition, the MD implementation covers -linear and multipath configurations. + MD is an abbreviation for "multiple device" (also often called "multi- + disk"). The Linux MD implementation implements various strategies for + combining multiple physical devices into single logical ones. The most + common use case is commonly known as "Software RAID". Linux supports RAID + levels 1, 4, 5, 6, and 10, as well as the "pseudo-redundant" RAID level 0. + In addition, the MD implementation covers linear and multipath + configurations. -Most people refer to MD as RAID. Since the original name of the RAID -configuration software is "md"adm, I chose to use MD consistently instead. + Most people refer to MD as RAID. Since the original name of the RAID + configuration software is "md"adm, I chose to use MD consistently instead. 1. How do I overwrite ("zero") the superblock? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -98,6 +99,60 @@ configuration software is "md"adm, I chose to use MD consistently instead. I know this all sounds inconsistent and upstream has some work to do. We're on it. +4. Which RAID level should I use? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + Please read /usr/share/doc/mdadm/RAID5_versus_RAID10.txt.gz . + + Many people seem to prefer RAID4/5/6 because it makes more efficient use of + space. If you have disks of size X, then in order to get 2X of usable space, + you need e.g. 3 disks with RAID5, but 4 if you use RAID10 or RAID1+0. + + This gain in usable space comes at a price: performance; RAID1/10 can be up + to four times faster than RAID4/5/6. + + At the same time, however, RAID4/5/6 provide somewhat better redundancy in + the event of two failing disks. In a RAID10 configuration, if one disk is + already dead, the RAID can only survive if any of the two disks in the other + RAID1 array fails, but not if the second disk in the degraded RADI1 array + fails. A RAID6 across four disks can cope with any two disks failing. + + If you can afford the extra disks (storage *is* cheap these days), I suggest + RAID1/10 over RAID4/5/6. If you don't care about performance but need as + much space as possible, go with RAID4/5/6, but make sure to have backups. + Heck, make sure to have backups whatever you do. + +5. What is the difference between RAID1+0 and RAID10? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + RAID1+0 is a form of RAID in which a RAID0 is striped across two RAID1 + arrays. To assemble it, you create two RAID1 arrays and then create a RAID0 + array with the two md arrays. + + The Linux kernel provides the RAID10 level to do pretty much exactly the + same for you, but with greater flexibility (and somewhat improved + performance). While RAID1+0 makes sense with 4 disks, RAID10 can be + configured to work with only 3 disks. Also, RAID10 has a little less + overhead than RAID1+0, which has data pass the md layer twice. + + I prefer RAID10 over RAID1+0. + +6. (One of) my RAID arrays is busy and cannot be stopped. What gives? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + It is perfectly normal for mdadm to report the array with the root + filesystem to be busy on shutdown. The reason for this is that the root + filesystem must be mounted to be able to stop the array (or otherwise + /sbin/mdadm does not exist), but to stop the array, the root filesystem + cannot be mounted. Catch 22. The kernel actually stops the array just before + halting, so it's all well. + + If mdadm cannot stop other arrays on your system, check that these arrays + aren't used anymore. Common causes for busy/locked arrays are: + + * LVM + * dm-crypt + * EVMS + + Check that none of these are using the md arrays before trying to stop them. + -- martin f. krafft <madduck@debian.org> Wed, 02 Aug 2006 16:38:29 +0100 $Id$ |