RAID VFS Module SoC Project - Requirements Doc

[Peter Stamfest]

Hello Ming,

On Thu, 1 Jun 2006, Ming Wong wrote:

> On 01/06/06, Ming Wong <mingwong111 at gmail.com> wrote:
>> 
>> 
>> Hi Peter,
>> 
>> Everytime I read about this RAID vfs module I wonder: What is the use
>> > case? If you need RAID functionality: use a real RAID - either HW or SW.
>> >
>> > What am I missing?
>> 
>> 
>> 
>> The good thing about this project is, repartitioning of the existing disks
>> is not required. You can simply use the particular paths/directories to
>> simulate a raid environment.
>> 
>> System admin can enable/disable data redundancy anytime easily at his/her
>> own discretion through the samba configuration. Also, swapping between
>> disks, which sets a particular disk to be the primary disk, can be done 
>> with
>> minimum effort.
>> 
>> So why bother setting up a SW RAID (more work), when all you need to do

SW RAID most of the time is real easy to do (If you know how). Eg. RAID 1:

1. Buy a new disk and hook it up
2. Create a degraded RAID 1 device from it
3. Copy the data from the old disk to the new (degraded RAID)
4. Unmount the old disk, remount the RAID-based FS
5. Hot-add the old disk to the RAID and have it resync.

This (minus the actual resyncing/copying) takes no more than 15 Minutes.

OTOH, a RAID VFS always has to be prepared to find differences between the 
RAID dirs, because some local user messes around with one of the copies.

>> is, create a share on samba whilst having a redundant mechanism in place.
>> :-)

While laziness is one of the virtues of a *real* sysadmin, it should not 
be encouraged. ;-)

And to really make this fly, you would always have to assure some 
properties of the RAID directories:

  * Free space should always be equal on both RAID Filesystems (and not
    just when samba starts or the RAID VFS gets set up). This basically
    means that it only makes sense for Filesystems used exclusively for
    that particular samba share.
  * Disallow access to the RAID directories behind the back of samba
  * Recheck/Rebuild the RAID after every hard crash of the machine - how do
    you find out about such a crash from within samba? Do you implement
    some sort of RAID superblock? Where do you put it? In a file everybody
    (or at least the sysadmin) can mess around with?


And: What do you do if a "RAID" disk *really* fails? The operating systems 
I know do not always deal gracefully with *that*. A process (samba) might 
then just wait for ages or just crash. Or the entire system crashes or at 
least requires a reboot to make sure it works again. This is just not what 
an OS gets prepared for. Real RAID is designed for this. So I think this 
might give a false sense of security: While the data may be safe due to 
the second copy, the system likely won't be any more reliable/stable. 
Basically it means: You get the data security properties of RAID-1 with 
the failure tolerance of a RAID-0. Might be called "RAID-0 minus 1" 
(RAID0-1)  ;-))

Now I just doubt that this makes the setup/maintenance any easier. And if 
you tell a sysadmin to use the RAID VFS module you have to teach him/her 
ahout these properties. Better save the time and teach the sysadmin real 
RAID properties.

>> 
>> cheers,
>> Ming
>> 
>
>
> One thing I'd like to add is, I should admit that HW RAID is portable whilst
> SW is extremely non-portable.

I disagree. As long as you stay with one software platform, SW RAID is 
more portable than HW RAID (eg. in linux you can even take out IDE disks, 
put them into USB-IDE cases and attach them to a linux system that does no 
longer have enough IDE controllers. Not nice, but it *works*). And have 
you ever been in a situation where a HW RAID controller fails, just to 
find out that there is no replacement anywhere in the market?

But I disgress.

> But with the implementation happening here, it has no concept of O/S
> specific partitioning issues. So this RAID vfs module is going to be
> portable.

How often have you moved a disk from one operating system to another? 
While this happens, it certainly is not common.

>From what I heard so far, this RAID vfs module for me is a curiosity.

But who am I to judge this. I also do not want to discourage its 
development, I just do not see any use for it in any of my installations 
due to the beforementioned strangenesses.

peter