On Fri, 13 Aug 2004, Ninti Systems wrote:
> Sory about having to re-post, but that last email wasn't too well laid out ...
>
> I'm looking at a method of creating maximum redundancy using software
> RAID with four equal disks. I know that the most common advice is to do
> RAID 5 if there are four disks and redundancy is required.
So why not use RAID5 ?
> Still, redundancy really is more important to me than performance
> (within reason of course). I know that a four disk RAID1 array
> (including swap) built out of primary and secondary masters/slaves would
> not perform well.
It's not neccessarily performance thats the issue here - I've had personal
experience of a drive failing in a master/slave cable set and its failure
preventing access to the other drive on the same cable. Fortunately for
me, it wasn't part of a RAID set, so I didn't lose the data (on the good
drive), but if it had been, the I might have lost the lot...
A Promise card has 2 ports, so for 4 drives, one drive on each of the
motherboard ports and one on each of the Promise cards will work well.
Cabling is a hassle though - you'll have 4 flat cables to deal with inside
the case, but it's not an insurmountable problem.
As for performance, RAID1 won't give you much more than the speed of a
single drive and it's half the write speed, and for reading, RAID5 might
have an advantage, depending on your data. I've never seen RAID5 slower
than a single drive though. If you really want performance, you'll need to
go to a RAID1+0 combination and have the hardware to match.
> So I'm wondering if anyone has any comments on the following scenario,
> or has tried it. Let's assume partitioning is /boot, swap and /(root):
>
> 1. Put /boot on a four disk RAID1 array across all disks (dev/md0).
> 2. Put swap on a two disk RAID1 array (primary and secondary masters)
> (/dev/md1).
> 3. Put /(root) on a two disk RAID1 array (primary and secondary masters)
> (/dev/md2).
>
> So I end up with something like this (all RAID autodetect type):
>
> /dev/md0 - /dev/hda1, /dev/hdb1, /dev/hdc1, /dev/hdd1
> /dev/md2 - /dev/hda2, /dev/hdc2
> /dev/md2 - /dev/hda3, /dev/hdc3
>
> /dev/md0 - /boot
> /dev/md2 - swap
> /dev/md2 - /
>
> 4. Configure the remaining /dev/hdb2 and /dev/hdd2 partitions as normal swap
> partitions.
You've just lot the point of having RAID here - your machine will crash if
one of these swap partitions develops a bad sector.
> 5. Configure and format the remaining /dev/hdb3 and /dev/hdd3 partitions as
> normal ext2 partitions.
> 6. Run rsync daily to mirror /(root) on /dev/md2 to both /dev/hdb3 and
> /dev/hdd3.
Again, it'll have problems if one of the non-RAIDed partitions develops a
fault.
> I'm hoping that this would create something like a four disk RAID1 array
> with the performance of a two disk RAID1 array, and that in theory up to
> 3 of 4 disks could fail but a usable system would still be bootable
> (even if it may be the case that the system may only be as up to date as
> the last rsync process). I realise that depending on which disk(s)
> failed, I may have to fiddle lilo and/or fstab to boot a running system.
>
> Does this idea have wheels, or am I overlooking some fatal flaw?
It's not neccessarily fatal, as your important data is on a RAID
partition, but you can make life a lot easier for yourself...
Heres a scenario that I use myself. Firstly I'm not a fan of a separate
/boot partition - thats all to do with me being a boring old fart and new
hardware not needing it... So..
If you partition all 4 disks identically it'll save you headaches later
when you need to replace one.
So I'd do it like this:
6 partitions: The first used as root, 2nd as swap, 3rd as /usr, 4th as
/var 5th as your data and 6th as the backup for the data. You can combine
root and /var and even /usr if you like to make it simpler. There are pros
and cons for each way. (as well as holy wars)
Combine the root partitions together with RAID1 and 2 hot-spares. All
others in RAID5. (including swap, yes, I know, not efficient, but if you
are swapping heavilly you are runing sub-optimally in the first place -
buy more memory!)
You can still do the daily backup via rsync from one RAID5 to another.
Here is a live example of one of my servers: (This has 4 x 150GB SCSI
drives in a Dell rack-mount box)
gordonh @ pixel: df -h -t ext3
Filesystem Size Used Avail Use% Mounted on
/dev/md0 235M 38M 185M 17% /
/dev/md2 2.8G 2.4G 268M 91% /usr
/dev/md3 5.6G 726M 4.6G 14% /var
/dev/md4 195G 164G 21G 89% /mounts/local0
/dev/md5 196G 165G 22G 89% /mounts/local0.yesterday
gordonh @ pixel: cat /proc/swaps
Filename Type Size Used Priority
/dev/md1 partition 2097136 60888 -1
Heres another with IDE drives and a promise card:
gordonh @ blue: df -h -t ext3
Filesystem Size Used Avail Use% Mounted on
/dev/md0 235M 33M 191M 15% /
/dev/md2 5.6G 994M 4.3G 19% /usr
/dev/md3 1.4G 178M 1.1G 14% /var
/dev/md4 165G 125G 32G 80% /mounts/jdrive
/dev/md6 165G 125G 32G 80% /mounts/jdrive.yesterday
An extract from /proc/mdstat:
md6 : active raid5 hdi7[3] hde7[1] hdc7[2] hda7[0]
176080128 blocks level 5, 64k chunk, algorithm 0 [4/4] [UUUU]
(There is no md5 in this system - I made a typo when I created it and
never bothered to fix it as it wasn't that important)
Gordon
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