[mythtv-users] RAID5 ATA IDE hardware card recommendations
formythbackend machine
Yan-Fa Li
yanfali at best.com
Wed Dec 8 20:47:36 UTC 2004
Being cheap, I still think that I would rather not use the onboard RAID
implementation, even of a well respected card like 3ware, because of the
lack of low-level access to the actual drive which impacts recovery and
performance.
In my experience, most problems with RAID tend to be:
a. sectors going bad - transient
b. drives going bad - fatal
Sectors going bad, pretty much always requires taking the drive out of
service and running the vendor provided sector reallocation tool. I'm
hoping one day you'll be able to initiate this from SMART commands
directly but as of now this is not the case.
Drives going bad is self explanatory.
In my experience, once a drive has been added to a Hardware RAID set,
you can't access it directly. This makes using the Hard Drive vendor
recovery tools impossible, since the drive is hidden from low-level
access. This requires you to either remove it from the set to perform
repair tasks or take it out of the system and put it into a standalone
box where the repair can be performed.
At least with Soft RAID solutions, you have the additional flexibility
of being able to run the HD vendor tool directly on the drive.
Additionally Soft RAID can use physical disk partitioning on the drive
to speed up recovery and guarentee performance.
Here's something I've been playing with recently after getting the idea
off of other Soft RAID users. I set up a two 160GB drives. Then I
partitioned each of them into 4 x 40GB partitions. I made each pair of
partitions into RAID1 sets. I then LVM'd all the RAID1 sets into a
single partition and formatted them using XFS.
So, as you may ask, why the crazy partitioning scheme ? Speeding up
resync time. The biggest problem with large RAID sets is that it takes
a longer and longer to resync. Because the majority of problems with
RAID arrays are related to sector failure, large RAID1 sets are
suceptible to being knocked out with only a few bad sectors. This means
excessive resync time and unnecessary wear and tear on the drive.
Bottom line: I'd rather resync a 40GB partition than a 160GB one.
As a side benefit there now are some interesting things you can do
related to performance. This is an old trick streaming video guys used
to use when disks had much lower performance. Disk performance is
related to how far you are from the spindle.
Think of each of the RAID sets as a zone, with Zone 1, being closest to
the spindle and having a higher areal density and Zone 4 the outer most
part of the disk being the slowest. If one wanted, one could use the
different zones of the disk for different jobs. For example, it would
make sense to put TV buffering on the fastest part of the drive, so you
could use Zone 1 for TV buffering. Zone 4 however is all about storage
and one could use it for long term file or video storage, or even not at
all if performance were critical.
Yan
Jurgen Kramer wrote:
> On Tue, 2004-12-07 at 16:51 -0500, Andrew Plumb wrote:
>
>>Hi Everyone,
>>
>>For those who do use hardware RAID5 cards for ATA IDE drives, which
>>cards have you (not) had success with, for use in a mythtvbackend
>>machine? Or in any Linux-based machine for that matter?
>>
>>I'm pondering picking up something like a Promise FastTrak SX4000.
>>
>>Thanks!
>>
>>Andrew.
>
> I can recommend the 3ware 9000 series (SATA), it is a bit expansive but
> is otherwise rock solid and has good Linux support.
>
> Jurgen
>
>
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