Re: File server questions.

m. Th. wrote:

> Hi,
>
> We saw many benchmarks in which SATA drivers doesn't perform better than
> SATA II drivers (with NCQ or not). "Who's to blame?" (AFAIK, the disks
> cannot yet fill not the SATA max 1.5GBps neither the PCI-X bandwidth,
> but any comments are wellcome). A hardware RAID controller can get more
> speed from the disks?
>
> We think to upgrade the disks for our file server. Do you think that it
> matters to buy disks
> 1. SATA II
> 2. SATA II with NCQ
> 3. or stay with SATA (we have a 3ware 9550SX SATA II controller on a
> PCI-X bus with two Maxtor DiamondMax 10 in RAID 1).
>
> Do you think that RAID 5 is better than RAID 1 considering that in the
> case of RAID 5 the controller becomes a Point of failure? Or is very
> unlikely that the controller can go broken?
> RAID 5 is *really* faster than RAID 1?

First, while controller failure is not unheard of, they are in general as
reliable as the motherboard, video board, and other components of the
machine. If you want a "belt and suspenders" approach use RAID 5+1--two
RAID-5 arrays on separate host adapters, with one of the arrays a mirror of
the other.

If your concern is getting the system back up after a controller failure,
keep a spare controller on hand, but understand that if it failed it may
have taken one mor more of the disks in the array with it.

RAID is not a substitute for backup.

Second, RAID 5 generally offers improved _read_ performance over RAID 1, but
it in general sacrifices _write_ performance to get that--there is a parity
calculation involved in every write and unless your host adapter has a very
efficient hardware engine to perform that parity calculation there can be a
significant performance impact.

> In RAID 1 we have the advantage that immediately we can put the disk on
> other computer to save the most actual data. For RAID5 we don't know a
> program that can do this on another computer if the controller fails.

This is a dangerous plan regardless of _what_ RAID level you're using. The
purposes of RAID are to improve performance and reduce downtime due to disk
failure by allowing a disk to be replaced with the system in operation.
RAID is _not_ a substitute for good backups.

If the system fails to the point that you need to physically move drives to
a different machine in order to get it back up, the _plan_ should be to
restore the most recent backup--if you can move the drive and get back up
that way quickly yeah, do it, but don't _count_ on being able to do it.

If your requirements are such that you can't afford to lose the data
recorded since previous night's backup then you really should be looking at
clustering--the kind of system where losing a whole server doesn't lose
data. Even there though you need to maintain good backups, because such
systems remain vulnerable to _software_ failures, malware, etc.

>
> TIA,
>
> m. Th.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)
.

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