[linux-lvm] Mirroring a Drive for load-balancing AND failover
Jonathan E Brassow
jbrassow at redhat.com
Thu Jul 28 15:12:58 UTC 2005
Yes, cmirror seems like what you are looking for (along with
GNBD/iSCSI).
I am testing cmirror right now, it adds the ability to:
1) have a mirrored device in a cluster environment
2) do pvmoves on clustered volumes
Many of the pieces you need are in the form of patches currently, but
we're working on pulling them together... maybe a month to get the
kernel changes in + user space tools cleaned up.
If anyone is interested in helping me test this beast, I'd be thankful.
I've recently uncovered a data corruption issue (I think) in cluster
mirrors when simultaneously doing looping lvs's, pvmove, and dd's from
different machines. You need only a few machines and you can use gnbd
for shared storage. (Actually, gnbd is the _best_ device to use for
this testing, as you can forcibly remove a single device while it is in
use. This obviously makes gnbd ideal for single machine mirror
testing, as well.)
brassow
On Jul 27, 2005, at 3:10 PM, Fury wrote:
> I am now looking at iSCSI. The problem is exactly what Matt
> describes. Recovery isn't that big of a deal, when the other server
> comes up I can carefully set one of the devices as failed, remove it,
> add it again, and get them to sync. after a sync is complete on both
> servers, I can mount the drive on the server that went down and resume
> services.
>
> Maybe iSCSI will report things diferently to md (raid). I will try.
>
> I'm looking forward to cmirror, I hear it will be ready soon. I'm not
> completely sure it's what I need, maybe someone involved will see this
> and chime in.
>
> -Derek
>
> On 7/27/05, Matthew Gillen <me at mattgillen.net> wrote:
>> AJ Lewis wrote:
>>> On Wed, Jul 27, 2005 at 09:06:01AM -0400, Matthew Gillen wrote:
>>>
>>>> Fury wrote:
>>>>
>>>>> I've racked my brain on this one, so hopefully someone will be of
>>>>> some help.
>>>>>
>>>>> I'm trying to set up two servers which share a drive and do not
>>>>> have a
>>>>> Single Point of Failure. They are on a local network with each
>>>>> other.
>>>>> The best solution would be to have /dev/sda1 on one server mirrored
>>>>> with /dev/sda1 on the second server.
>>>>> ...
>>>>> A second solution was to use GFS/GNBD. I can export each drive to
>>>>> the
>>>>> other server, and do RAID 1 (on both servers) between the local
>>>>> /dev/sda1 and the remote gnbd device. I then format the raid
>>>>> device
>>>>> with GFS so both servers can mount it.
>>>>>
>>>>> Surprisingly, this last system works. Both systems can mount the
>>>>> drive and read-write to it. However, if either server in this
>>>>> configuration drops dead, the other server cannot deal with the
>>>>> dead
>>>>> gnbd device, and the raid device and mount point are no longer
>>>>> usable.
>>>>> I'm sure there are numerous other problems with this setup, also.
>>>>>
>>>>> So I'm looking for ideas. With two servers, how can I mirror a
>>>>> drive
>>>>> in real-time, and allow for failover?
>>>>
>>>> You might want to use something more like iSCSI + RAID:
>>>> http://linux-iscsi.sourceforge.net/
>>>
>>>
>>> How is that different than GNBD + RAID? The issue isn't the network
>>> transport, it's recovery of a RAID on two nodes simultaneously.
>> I don't think he was even worried about recovery, although you're
>> right
>> and that's another problem. I read that he couldn't access anything
>> after a failure of one server, which is what I was addressing.
>>
>> Honestly, I don't know how GNBD works. But if it makes makes the
>> remote
>> volume look local and doesn't report problems in a way that RAID
>> understands (or at all), I can see how things would hang (just like a
>> client system would hang if an NFS server for a mounted filesystem
>> went
>> down). I imagine (but I don't know from personal experience) that
>> iSCSI
>> (with the ConnFailTimeout=x sec) would report a failed write and RAID
>> knows how to handle that.
>>
>> But, like I said, I don't know for sure about any of this, since I
>> haven't tried it. However, the page:
>> http://linas.org/linux/raid.html
>> mentions iSCSI, so it appears that some people have gotten it to work.
>> --Matt
>>
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>>
>
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