[CentOS] iSCSI disk preperation

Mon Feb 7 21:29:35 UTC 2011
Jason Brown <jason.brown at millbrookprinting.com>

On 02/07/2011 03:57 PM, Ross Walker wrote:
> On Mon, Feb 7, 2011 at 3:41 PM, Jason Brown
> <jason.brown at millbrookprinting.com> wrote:
>> On 02/07/2011 03:26 PM, Ross Walker wrote:
>>> On Mon, Feb 7, 2011 at 1:56 PM, Jason Brown
>>> <jason.brown at millbrookprinting.com> wrote:
>>>> I am currently going through the process of installing/configuring an
>>>> iSCSI target and cannot find a good write up on how to prepare the disks
>>>> on the server.  I would like to mirror the two disks and present them to
>>>> the client.  Mirroring isn't the question, its how I go about it is the
>>>> problem.  When I partitioned the two drives and mirrored them together,
>>>> then presented them to the client, it showed to the client as a disk out
>>>> no partion on it.  Should I partition the drive again and then lay the
>>>> file system down on top of that?  Or should I delete the partitions on
>>>> the target server and just have sda and sdb mirrored, then when the
>>>> client attaches the disk, then partion it (/dev/sdc1) and write the file
>>>> system.
>>> Whatever you export, the whole disk, partition or logical volume, the
>>> initiator will see as a whole disk.
>>> So if you mirror sdaX and sdbX and export md0 the initiator will see a
>>> disk the size and contents of sdaX/sdbX.
>>> Just create the filesystem on the disk on the initiator and use it there.
>>> REMEMBER: iSCSI isn't a way for multiple initiators to share the same
>>> disk (though they can using specialized clustering file systems), it
>>> is a way for multiple initiators to share the same disk subsystem.
>>> You can't access the file system from both the target-side and
>>> initiator-side at once or it will corrupt the file system. If that's
>>> what you want then you want NFS or Samba and not iSCSI.
>> Well my first question would be, do you really need to partition the
>> disks on the target or can you just RAID them together (ie sdb/sdc and
>> not sdb1/sdc1)?  Then create your md0 based off of the two drives.  Once
>> that is done, export the md0 in /etc/tgt/targets.conf to present to the
>> clients.
> You don't need to partition the disks on the target if you want to
> export the whole disks. I just don't recommend it because exporting
> whole disks isn't the most economical use of the disks. The whole idea
> of iSCSI is you can create one huge RAID array on the target and all
> the initiators can then all benefit from it.
> If you have 13 disks, say they're 500GB SATA disks. If you create a
> RAID50 out of 2 6 disk RAID5s (stripe the LVs in LVM for management
> ease instead of nested mdraid devices), you would get 10x the READ
> IOPS of your mirror, and the same amount or better write IOPS then the
> mirror, double the write IOPS then the single RAID5 and a tad better
> read IOPS then your single RAID5. Not to mention a lot more storage
> potential for the two servers, or a third server, or a fourth
> server...
>> Second question.  This does not need to be a clustered file system as
>> only one server will need access to it at a time however, if server A
>> failed, could you create a new server and present it to server B and the
>> new server would have access to the files or would it show as an
>> unpartitioned drive?
> You can definitely allow two different hosts access to the target,
> just not simultaneously (unless it's a clustered file system). The
> second host can log in to the target, and have the disk at ready to
> mount after the first target is offline or "fenced", just don't mount
> it while it's mounted on the first target or zap! There goes your file
> system!
> -Ross
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I never thought about it that way; very interesting!

Thank you both for your suggestions it help out tremendously.