Re: a little confused about RAID ....

Giovanni Resta <g.restaxxx@xxxxxxxxxxxxxxxxxxx> writes:
In the research institute where I work, we decided to
buy a new machine to make (among other things) the backup
of the home's of the various people we have around.
After some hard disk crash last summer, we decided to
have a little of fault tolerance exploiting a RAID architecture based
on 3 disks of medium size (about 300 Gb each).

For backup data, depending on how you organize your backups, it may be
better not to use RAIDs. E.g., our backup servers are the only
servers we have where we do not use RAID. Instead, each disk
represents a set of backups, and we back up to these disks in an
interleaved fashion (e.g., one disk for Monday, one for Wednesday, one
for Friday). So if one or two of the disks fail, we still have usable
backups, just a little more sparsely. And when they are all working,
we have more space for backups (translating into a higher backup
frequency) than with a RAID.

Among the motherboards supporting latest cpu's (socket 775, I think)
can you suggest which will hopefully work well with Linux (we are
using the Suse 10) and with RAID ?
I may be wrong, but from what I read, it seems that raid can be
supported/provided either via hardware or via software. What should I
prefer ?

If you want to go with hardware RAID, you have to have a spare RAID
controller at all times, or you cannot access the data when the RAID
controller dies.

We have been using software RAID1 with Linux' md driver, and are quite
happy with it; we have had failing disks (Maxtor 6Y200 and 7Y250 are a
really bad idea; they tend to fail on power loss; and we now require
disks from at least two different manufacturers in each machine), and
survived.

Is the performance of the RAID roughly comparable to
that of the single disks ?

Writing a 2GB file to an ext3 file system software RAID1 (with a
Seagate and a WD 400GB SATA drive) costs:

real 0m45.488s
user 0m0.008s
sys 0m4.584s

giving a write bandwidth of about 45MB/s.

Reading that file (after umounting and mounting the file system to
clear the cache) gives:

2147483648 bytes (2.1 GB) copied, 38.5825 seconds, 55.7 MB/s

real 0m38.621s
user 0m0.700s
sys 0m2.864s

The CPU in the tests above is a Xeon 5160, BTW.

Is it advisable to install the system
on a fourth separate disk or it is ok to install it on the raid ?

I would advise putting the system on a separate RAID1 partition, but
you don't need a separate disk. If you want to afford another disk,
use it for backup data as well.

All in all what we want to put inside our new machine is
a cpu like the Intel Core 2 Duo E6600, 4GB of ram, a not-so-spectacular
graphic card, ethernet 100Mb, and 3 disks,
for example the Maxtor 300 Gb 16 MB cache Serial Ata 2.

Well, as for Maxtor, and having three of the same kind in a server,
see above.

I would also recommend using ECC RAM (this probably means a board with
an 975X chipset); you would not want to learn that a broken DIMM
corrupted all your backups when you need them.

- anton
--
M. Anton Ertl Some things have to be seen to be believed
anton@xxxxxxxxxxxxxxxxxxxxxxxxxx Most things have to be believed to be seen
http://www.complang.tuwien.ac.at/anton/home.html
.

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