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Re: [K12OSN] backend raid 1, raid 5 or none of the above?



john wrote:
> Hi all,
>
> I am waffling on a question that I hope you all will be able to help
> me come to a resolution about. Should my backend file storage system
> be software raid 1 or software raid 5 or non of these? OS is Linux of
> course. My LTSP server resides on a different machine entirely.
>
> I've already setup a RAID5 system with 3 active disks and one hotswap
> by following a very good howto
> http://www.debian-administration.org/articles/512
>
> My final setup uses /dev/md0 in a raid 1 config using 10 Gb to host
> the OS and mbr
> and /dev/md1 in a 1 tb raid 5 to host the file system which has been
> partitioned with separate /home, /tmp , /usr, /var
>
> However the more I think about it the more I am worried that I am
> setting myself for pain in the future. I am thinking about dumping the
> whole setup and going to  raid 1 for the following reasons:
>
> 1. My reading tells me that RAID 1 has a longer MTBF
> http://en.wikipedia.org/wiki/RAID_1#RAID_1
>
> 2. RAID 1 offers redundancy similar to raid5 but also relative
> simplicity because I can clone one of the disks and keep a copy around
> in case of problems
> 3. RAID 1 offers comparable disk read times when compared to RAID 5
>
>
> On the other hand:
>
> RAID 5 offers large volume sizes which we are looking for
> RAID 5 offers comparable disk read times when compared to RAID 1
> RAID 5 doens't offer comparable MTBF rates as RAID as you factor in extra disk
> RAID 5 may be more complex to administer or rebuild than a "simple" mirror
>
> Perhaps if I am such a coward I should go with a single 500 gig disk,
> and make a backup image with G4U or a hardware disk cloner (which we
> have) and keep it on hand and do a incremental backup of student files
> every night (which we intend to do anyway). That way, although
> students might not have access to their resources for a day, at least
> I would have a path to recovery that I already understand.
>
> As a side note we have frequent power outages do to weather, isolation
> etc, and I haven't looked into setting up linux to talk to UPS's. Is a
> raided system more intolerant of having the plug pulled than a normal
> Linux box running ext3?
>
>
> waffle, waffle, waffle :-)
>
> I hope folks will set me straight!
>
> Thanks,
>
> John

If I were you, I'd go with hardware RAID 5 with, say, an LSI Logic
MegaRAID card or something similar.  I imagine that the ability to
replace a failed disk quickly--preferably on the fly--is important,
since you're considering something like RAID.  Others may differ, but I
find that, for me, hardware RAID 5 offers the best balance of capacity
per disk, speed, quick recovery from a failed disk, and expense.

Actually, my own RAID (I built one recently) is a RAID 5 with six SATA
disks and an LSI MegaRAID 150-6.  It works quite nicely indeed and
didn't break the bank.

If you need something *very* reliable, then RAID 50 would be your bet. 
If you need balls-out speed and good reliability, RAID 10 is your bet.

--TP


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