[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [K12OSN] Need Help



Terrell Prudé Jr. wrote:
Write access is considerably slower on RAID5 and it tends to lock your
heads together even for reads.  I've always liked RAID1 for the simple
reason that if everything is broken except one disk you can still
recover the data it held.  Plus if you do it in software you don't
have to worry about having to match the controller to read on a
different machine.
However, RAID 1, by definition, is not scalable beyond two disks.

That's actually not true - linux md will happily mirror more if you want. I keep my backuppc archive on a 3-member set where one is an external firewire drive that is periodically added and removed after the sync completes. The rest of the time it is equally happy with two working members.

So
you're in a very similar situation (losing one disk and still going)
here as you are with RAID 5.

The difference is that a RAID1 will still run at full speed with one drive where RAID5 speed will drop drastically as it has to reconstruct everything from parity. But my first point was that even if everything in the machine melts except one drive with software raid you could connect that to about any controller and get the files it contained.


And as for RAID 5 locking the heads
together even for reads, that may well depend on your specific RAID
card. We haven't seen any evidence of that with our systems at work. However, it might well be true in some implementations, maybe to include
software RAID.

If you look at how it stores the data, I don't see how it can avoid having to make multiple heads seek to get it back. On raid1, reads can be completely independent.

That said, RAID 5 kicks RAID 1 in the delicate parts when it
comes to performance.  Again, we're back to, say, six or eight spindles
vs. two spindles; no contest.
That's not necessarily true.  If you configured those 8 drives in
RAID1 pairs, you'd have 4 independently seeking places that could be
writing at once and all 8 would be independent for reads.  The trick
is to arrange your data across the partitions so they are likely to be
used simultaneously.  These days you could just combine the RAID1 sets
into one LVM, though.

You're no longer talking about RAID 1, though.  You're talking about
RAID 10.

No, I'm talking about multiple partitions, mounted to distribute the load, like having /, /var, and /home as separate mirrored partitions, each filling a drive. Or in the LVM case, combined in a more flexible way than RAID10.

I've run many 14-disk SCSI RAID 5 setups,
and my God, they were quick!!  Yes, I'm assuming a real hardware RAID
card here; I generally don't recommend software RAID, no matter which
RAID level you use.
Software RAID1 works very nicely and does not add much overhead on
SCSI where there is not much CPU interaction anyway.  I probably
wouldn't do RAID5 in software.

Largely true with SCSI; I wasn't specific enough in that second
sentence.  Oh, how I wish all disk drive interfaces were SCSI--that
would solve plenty of problems!  But even with SCSI, we're back to the
scalability issue; RAID 1 cannot, by definition, scale beyond two
spindles, so if you want larger capacity, you've either got to buy two
*huge* drives or go with RAID 5.

Or use multiple mount points, or use LVM. With LVM you lose the ability to recover files from a single drive but don't have to worry so much about balancing the space usage yourself.

--
  Les Mikesell
   lesmikesell gmail com


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]