Re: some EVMS questions

On 3/17/2007 6:36 PM, Scott Kaelin wrote:
> On 3/17/07, Eric A. Hall <[email protected]> wrote:
>> Hi guys,
>>
>> I'm putting together a storage server for NAS and iSCSI purposes and have
>> some questions about EVMS please. I've got a box of Raptor 150 ADFD drives
>> and a 3Ware 9650SE board which will provide the basic back-end storage.
>> Now I'm looking at options for the partitioning to support the server
>> opsys, the NAS storage pool, and the multiple iSCSI volumes. It seems that
>> the easiest thing to do would be to just make a single massive RAID-6
>> volume out of the drives, expose that to EVMS, and then chop it into
>> segments for logical drives. Assuming that's the case:
> 
> If you're planning on using iscsi for high I/O applications putting
> all the drives in a single raid6 is a bad idea. Try not to have more
> than two active LUNs on a single array. If you use one large RAID6
> array and then export a bunch of LUNs you'll notice horrible I/O when
> multiple initiators begin to write/read from the disk, since each
> initiator is reading from significantly different areas of the disk.
> If you are exporting only one LUN and using the rest for a fileserver,
> NFS/Samba, then disregard what I just said.

That's interesting. I guess it depends on what is considered "high I/O".
I'm just planning to boot VMware guest opsys and application files (and
maybe some other systems too but not much), so I don't think this is a
factor here, but it's still an interesting point. Thanks.

>> I understand that EVMS will let me dynamically resize segments, but how
>> transparent is this? I assume it will work for the storage and system root
>> XFS partitions, and I also assume that it will not work for the iSCSI
>> targets (since the partition and filesystem information for those will be
>> managed by the requester, not the server).
> 
> EVMS will let you resize the segments of LUNS as well. The trick is to
> export LVM/LVM2 regions rather than full fledged partitions. I use a
> windows initiator and when I run out of space on my exported LUN, I
> stop the target software, go in EVMS and expand that region, bring the
> target software back up and use the dynamic disk feature of windows.

Nice. So assuming that my iSCSI disks are going to be 20-80 GB (depending
on guest OS and application needs at the time), with an estimation of
around 40 GB average use... How should I structure the disk and LVM layout
for this to avoid problems later, after there has been a lot of resizing?
This seems to be where the idea of having oodles of small segments comes
into play, but I would prefer to avoid that.

> If you look around on the iscsi-target mail archive there is alot of
> information: http://www.nabble.com/iSCSI-Enterprise-Target-f4401.html
> about similar issues along with possible performance.

Thanks. I looked at some of the discussion there and elsewhere, and it
seemed that the best performance comes from having a very large RAID with
the largest number of active spindles. That may be due to the way that
people are benchmarking (single-user, effectively), but I will keep this
and the other points in mind.



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