[Gluster-devel] GlusterFS Example

[Matt Paine]

>>
>>option replicate *.tmp:1,*.html:4,*.php:4,*.db:1,*:2
>>
>>In this case any .tmp or .db files will not get replicated (will sit on
>>the first afr brick only), all html and php files will be mirrored
>>across the first four bricks of afr, and all other files will be
>>mirrorred across the first two bricks. (note all on one line, note comma
>>delimited, note also no spaces before or after the comma).
> 
> 
> Right, but is it possible to just tell the config you want to store 2
> copies of everything without telling each system where the redundant
> copies should be stored? I.E. I am looking for something like RAID 6,
> where I have say 8 bricks with two copies of every file. If I just enter
> *:2 then the spare copies will just be on the first two bricks and since
> each 8 brick has the same amount of storage I will run out of space on the
> first two before the others vs the redundant copies distributed over the 8
> bricks.


Absolutly, except its there is a bit more work to do. AFR works by 
starting at the first brick and any replicating goes to the second, 
third etc as described above. But these are normal gluster bricks, so 
there is nothing stopping you from creating an AFR for the first brick, 
and AFR for the second, and just a posix brick for the rest (if thats 
what you want of course). Let me try to explain.....


Server one has one brick: S1
Server two has one brick: S2
Server three has one brick: S3
Server four has one brick: S4
Server five has one brick: S5

Client unifies S1 and S2 (call him U1)
Client unifies S3 and S4 (call him U2)
Client then AFR's together U1 and U2 and S5

In this example, if I save a file and its *:3, it will appear....
* on U1 (either S1 or S2 depending on scheduler)
* on U2 (either S3 or S4 depending on sched.)
* on S5

If I save a file as only *:1, it will appear only on U1 (either S1 or S2 
depending on scheduler).

Ad nausium.


Of coures there is nothing stopping you from unifying three bricks, or 
even unifying an afr to afr.


i.e. (might need a mono spaced font to see correctly...)

                         A5
             +-----------------------+
             |                       |
             U1                      U2
      +-------------+           +---------+
      |             |           |         |
      A1            A2          A3        A4
  +---+---+   +---+---+---+   +---+   +---+---+
  |   |   |   |   |   |   |   |   |   |   |   |
S01 S02 S03 S04 S05 S06 S07 S08 S09 S10 S11 S12


Where Sxx = Server bricks
        Ax = AFR brick
        Ux = Unify brick



So in this configuration (which you have already worked out i'm sure) 
that if you save something *:2, then it will appear in both U1 and U2, 
which means (depending on the spec from a[1-4], assume *:2) it will 
appear in either A1 or A2 (because of the unify), AND it will also 
appear in either A3 or A4. etc etc etc.....

I think i've laboured the point far enough :)




> 
> 
>>>4) In my application the server is also the client, in my current config
>>>when one out of 3 servers is done I can no longer write, even tho I am
>>>telling my config to write to the local brick. Is there any way to fix
>>>this? If a note is down I understand how I would not have access to the
>>>files, but I don't want to take down the full cluster.
>>
>>I'm note sure sorry. Maybe one of the devs can clarify that. I know
>>there was an issue with an earlier relase to do with AFR. When the first
>>brick was down then afr was no longer available.
> 
> 
> Ah... That could be it.

Which version of glusterfs are you using? tla, pre ?

(that issue has been fixed for a little while now, so if your using pre6 
you shoulnd't have come across it)





Matt.