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Re: [K12OSN] Linux "Software RAID"



On Sat, Aug 9, 2008 at 10:00 PM, Almquist Burke
<balmquist mindfirestudios com> wrote:
>
> On Aug 9, 2008, at 4:05 PM, Terrell Prude' Jr. wrote:
>>
>> I use RAID 5 in 14-disk arrays at work.  We find the speed excellent, and
>> the savings in disk space is enormous.  Say you have fourteen 500GB disks,
>> be they SATA or SCSI.  With RAID 10, your storage would be 14 disks * 500GB
>> / 2, which is 3.5TB.  With RAID 5, you'd have (14 disks - 1 for parity) *
>> 500GB = 13 * 500GB = 6.5TB.  I'll take the extra 3TB any day!
>>
>> --TP
>>
>
> This is true, although in an array that big you might consider RAID 6
> instead. RAID 5 cannot handle more that one disk failure at once. Whereas a
> RAID 10 with 14 disks could lose one disk in each pair and still not fail
> (up to 7 in this case). But like you said, the downside is lost disk space.
> It's just that disk space is so cheep these days, especially with LVM,  that
> I don't worry much about giving it up. A pair of 6 disk Raid 10 arrays in
> LVM would be pretty outstanding I think.

My users don't really use a lot of disk space. I can get by with just
about 1TB of disk without any issues.  I have found that adding more
disks to my RAID 5 increases performance until I reach the throughput
of the controller itself?  I know there is a lot more that goes into
this than just that, but for my small arrays (5 SATA drives, 250GB
each in a RAID 5) it seems to be true.  I also tend not to use LVM and
instead use ext3.  I've never actually needed to extend an array where
I didn't have the time to add a drive and recover (via rsync) from my
backup system.  In a worst case scenario, I have actually had to
switch /home to the backup array temporarily when my main fileserver
flipped out. Then, it was just a matter of letting users run on the
backup system while rsync rebuilt the main fileserver and switching
/home back the next morning after a 'second' rsync to pick up any last
minute files that changed. I also have another near-line hard disk
backup (single enclosure 1TB box). Can never be too paranoid about
data loss. I take the adage "that there are two types of people in the
world: those that have lost data and those that are going to" very
much to heart.

Sincerely,
Dave Hopkins


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