<div dir="ltr"><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">I see a fair amount of this in various customer environments all the time. I've also been around BCP/Disaster Recovery, so I can speak about the choices, what they do, what they don't do, and so on. If you have a specific question, you're welcome to reach out. But, sharing experience, in general, I can share this off the top of my head...</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">Multi-site replication comes in a few different varieties: </div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">A) Storage-based (e.g. your SAN/NAS device is shipping block-by-block changes to a matching storage system at the other site, which is applying those changes/writes to a remote mirror)</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">B) Host-based (e.g. your OS file system driver is taking IO writes for a given filesystem and replicating them to another host, which that other host is applying those writes to a remote filesystem)</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">C) Application-based (e.g. some built-in feature to an application [not unlike mysql] is pushing bits over the network to another copy somewhere else, that is receiving those bits)</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">In cases of A and B above, and sometimes, but not always C, the remote side is in a "read only" mode and not usable, since there's no mechanism to take writes on the far side and get them back to the original site. In the case of application-based replication, there are some that handle bi-directional replication, so you're not stuck with a far site in read-only mode.</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">In case of A above, you can ensure zero data loss if you have 1) short enough distance between the two sites, and 2) enabled synchronous replication. In this use case, a write is written to the A side, replicated to the B side, and confirmation of the write is sent back to A before the A side considers the write to be complete and releases control back to the requesting application. Because of the extra hops, synchronous replication is limited to ~20 miles of distance because of latency. If zero data loss is not an absolute requirement, asynchronous replication is not distance limited, but as distance increases, the lag between a write to the near side and that write being committed to the far side increases as well. </div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">In your specific mentioning of XtreemFS, it works like option B above, as it plugs in at the filesystem driver layer of the OS, and appears, from my reading of their site info, to operate asynchronously. GlusterFS is similar. It's asynchronous, and can work across distance to create global clusters.</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">The art of applying a replication strategy is to first understand what the business or technical requirement is that must be met. What are some of the use cases? If you're wondering what of the above I see most often? It's A and C. Examples: Frame-to-Frame storage replication done at the volume level, databases running log-shipping to remote sites for a DR copy, that sort of thing. </div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033">Angelo</div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small;color:#330033"><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Jan 31, 2017 at 9:18 PM, Christopher Cavello <span dir="ltr"><<a href="mailto:cavello.1@osu.edu" target="_blank">cavello.1@osu.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Is anyone on this list willing to share his experience with file<br>
replication across data centers? (glusterfs geo-replication, xtreemfs<br>
<a href="http://www.xtreemfs.org/" rel="noreferrer" target="_blank">http://www.xtreemfs.org/</a>, etc.)<br>
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