On Wed, Jan 5, 2011 at 4:03 PM, Leslie Rhorer [off-list ref] wrote:

quoted

quoted

1) The Linux/MySQL stuff is a 3 drive RAID1. (sda/sdb/sdc)

       Well, RAID1 certainly offers the most robust solution, especially
with more than 1 mirror.

<SNIP>

quoted

quoted

3) There is a second RAID1 (sda/sdb/sdc) used for backups of the
RAID0. The RAID0 is backed up to RAID1 nightly. If the RAID0 fails
then I lose 1 day's work.

       Well, first of all, unless you meant to say sda1/sdb1/sdc1, etc,
then this can't be different from the #1 RAID array above.  Assuming you

are

quoted

actually using partitions, then I don't really see the value of two

separate

quoted

arrays.  Why not just one RAID1 array?

Sorry I wasn't very clear.

In #1 the RAID1 is my main Linux system on sda3/sdb3/sdc3.
In #3 the RAID1 is purely for backups on /swa6/sdb6/sdc6 and is used
only for the backup of the RAID0 data. It is not normally mounted
except when doing backups. I wanted the extra protection that if
something went wrong with the basic Linux box the backup partition
would not normally be mounted and the data hopefully a bit safer.

quoted

       Also, I personally would reverse the backup strategy.  I would

put

quoted

the Windows VM on the (single) main RAID array and back up the data to a
single 1T hard drive.

I do this today backing up the existing RAID1 partitions to an
external eSATA 1TB drive.

	What I meant was, you currently have a 2 disk RAID0.  Why not buy a
larger disk and move the 2 drives currently tied up in the RAID0 to a 5 disk
RAID6 array with no partitions?  The function currently provided by the 2nd
RAID1 can be taken over by the single drive, and the function of the RAID0
can be taken over by a directory on the main array.  This makes better
functional use of the space and may provide performance benefits, not to
mention being much simpler.  I'm a fan of having a separate, small disk for
booting.  My servers both have a pair of small drives each partitioned into
three sections.  Each of the partition pairs are in turn assembled into a
RAID array for a total of 3 mounts:

md1 - a tiny /boot
md2 - a small /
md3 - swap.  

quoted

quoted

   OK - the problems I have with this arrangement are:

1) I used the older v0.9 metadata.

       This may be necessary if you are booting from the array.  The
limitations of the 0.90 superblock may never impact you.  That's a small
system with only a few drives.

quoted

2) The RAIDs are assembled by the kernel automatically. I do not use
an initrd. (Because I don't know how/newer have)

       How is that a problem?  An initrd, or lack thereof, won't prevent
you from disabling the automatic assembly of one or more arrays, unless

once

quoted

again you boot from the array in question.  Most modern distros default

to

quoted

using an initrd.  What distro are you using?

The machines are Gentoo and an initrd/initramfs is up to the builder.
The new RAID6/superblock-1.2  boot uses one. The RAID1/superblock-0.9
does not.

	OK, well, either way, any array can have auto-assembly disabled.  If
you go with a single RAID6 plus a simple drive, it won't be required,
though.

quoted

quoted

3) I think with 5 disks I could get better performance  than I
currently get , with similar or better safety using maybe RAID5 or
RAID6.

       No, RAID1 is as safe as it gets.  RAID0 allows for better
performance, but if you make the RAID0 into your backup solution, the
performance won't matter much.

If I'm wrong about RAID6 please correct me as this understanding is
why I chose it.

1) A 5-drive RAID6 can survive losing 2 disks and still return good data.

	Yes.

2) A 5-drive RAID6 reads data as nearly fast as a 3-drive RAID0.

	Or faster.  Right now your 3 drive arrays are RAID1, though, aren't
they?  RAID6 should read *MUCH* faster than RAID1

If those two aren't true then my choice of RAID6 doesn't improve my
system as I hoped.

3) My current 3-drive RAID1 can lose 2 disks and still return good
data making #1 equivalent to #3

	Well, only in terms of survivability and only for the configuration
you mention.  An N drive RAID1 array can lose N-1 drives.  Any RAID6 array
can lose 2 drives.

4) #2 would be faster than my current 2-drive RAID0 and wouldn't have
the risk of a single drive loss.

	Yes.
 

--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html