Re: Building a file server - advice please

In comp.os.linux.setup Alan Adams <alan.adams@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:
> In message <11rqaf3lqdgm4ba@xxxxxxxxxxxxxxxxxx>
> Jean-David Beyer <jdbeyer@xxxxxxxxxxx> wrote:

>> Nico Kadel-Garcia wrote (in part):
>>
>> > That's what RAID5 does. That way, if one drive fails, you can keep going for
>> > a while until you can replace the drive. And with 4 drives, your odds of a
>> > single drive failing go way, way, way up.
>> >
>> Do I misunderstand you, or did you say it ambiguously?
>>
>> With 5 drives, the odds of a single drive failing _remain the same_; i.e.,
>> connecting up 5 drives in a RAID5 system does not affect the Mean Time To
>> Failure of a drive in any way (unless the box is improperly cooled or the
>> power supply overloaded). The perceived failure rate of a single drive may
>> go up slightly because of the failure rate of the RAID controller, but that
>> should be very small since the controller has no moving parts, so the
>> failure rate of the connectors and cables probably dominates this.

Controller might burn away, but it happens seldom, disk on the
other hand simply fall, it's not a question if, only when.

>> The odds of the entire system failing presumably go down, which is an
>> important reason to use a RAID system. I do not know the details of one
>> level of RAID from another, but if it is true that RAID 5 can tolerate one
>> drive failing and still work, the odds of a system failure with 5 drives in
>> there would be 1/5 those of using a single drive.

Yep, RAID 5 is the cheapest, allowing for little overhead, the
more disk you have the fewer loss, in exchange for slower
operation. It's unlikely more then one disk falls at the same
time and you can run hw/sw raid uninterrupted if you have hotplug
trays available.

> As originally designed, and as currently implemented in Hardware
> controllers, Raid allows for redundancy (Except Raid 0 which is simply
> adding disks together to get a big one - that INCREASES the risk of failure,
> as a failure on either disk trashes the data on both.)

> The concept is to "duplicate" part or all of the data on additional disks.
> With Raid 3 and 4, it's done by calculating a parity value and storing that.
> With Raid 5 the entire disc is duplicated. Raid 5 is also called mirroring.

Not really, RAID 5 stores parity information, *not* redundant
data. Data can be reconstructed from the parity information.

> Hardware controllers generally can have an additional spare disk configured
> so that when a disk in any of the raid arrays fails, the spare can be
> automagically used to replace it. This means you still have your original
> degree of redundancy, and now have to replace an offline disk when
> convenient.

Softraid can use spare disks although.

> Software raid emulates some of these functions, but doesn't give such a good
> degree of control.

What kind of control doesn't give it to you?

> Hardware raid presents each raid array to the host as one disk, or in some
> cases, can partition the array and present that as a disk. In either case
> the host operating system is ignorant of the raid system, and thus needs no
> knowledge of how to manage it. (There will usually be a utility program to
> do the management, but that is independent of the operating system, and may
> run on separate hardware from the host.)

The OS needs to have a driver to utilize a hardware raid
controller at all.

[..]

--
Michael Heiming (X-PGP-Sig > GPG-Key ID: EDD27B94)
mail: echo zvpunry@xxxxxxxxxx | perl -pe 'y/a-z/n-za-m/'
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.

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