-----Original Message----- From: centos-bounces@centos.org [mailto:centos-bounces@centos.org] On Behalf Of Les Mikesell Sent: Sunday, March 04, 2007 12:51 PM To: CentOS mailing list Subject: Re: [CentOS] CentOS 4.4 lvm and drbd 0.8?
Ross S. W. Walker wrote:
If you were running a later kernel version of MD, it is conceivable that you could create a mirror with a remote storage drive over iscsi.
It would be up to you though to figure out how to fail-over to it and to limit the bandwidth MD takes to that remote mirror and releasize that it will always be fully synchronous and so performance may not be the best over a WAN.
You can also use a pair of vise grip plyers to do the job of an adjustable wrench, but it will probably strip the bolt in the process.
Unix has always been about combining tools that each do one job well. If we already have a tool (iscsi) that exports remote block devices well, following standards that would the actual storage to be on non-linux devices, and another tool (md raid) that mirrors block devices, why not combine them instead of inventing yet another special purpose tool? I realize that drbd and nbd were developed before iscsi, but now that there is a standard cross-platform network block device, why shouldn't it be used? MD might need some new options to make it work as efficiently in this scenario, but that seems like a more useful place to add features - that is, there might be other situations where MD mirroring to an external iscsi partition would be useful, or even combining many iscsi exports into one raid volume.
While yes combining tools is the Unix way, there are also some tools that are better suited for a task than others.
For example in my second post I suggest if you have a bunch of direct attached storage, say located on 20 servers in a local area network. If you export the storage from those 20 servers to a central server via iSCSI and use MD raid to create a large RAID-{3,4,5,6} array of those iSCSI targets. Then you use LVM to split those into differing volumes for re-export via iSCSI. You can then use drbd on those LVM volumes with asynchronous replication to a storage array off-site for DR purposes or to a local storage device as a volume snapshot server (where both copies need to be active at once). You can then re-export those volumes via iSCSI to different OS platforms to use as storage.
This could work well, but if you plan on using MD RAID1 to replicate data to off-site storage you will see very poor performance. MD RAID1 may work well in replicating storage synchronously between 2 local storage devices, but not to a remote storage device. In that case drbd would work better in that case. Also if you wanted both sides of a mirror to be active at once drbd 8.X is the only way to go.
-Ross
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