PVFS2 Failover Policy － 当一块硬盘坏了以后，PVFS如何应对？数据会丢失么？

很不幸，当PVFS集群中一块硬盘坏了以后，数据会全部丢失。PVFS的建议是每个节点用两块硬盘，做硬件RAID1，然后就可以避免这样的问题，在软件 层次上，PVFS不提供RAID相关的功能。这里是PVFS的开发人员给我的回信，写的很清楚了，人家还写了为什么PVFS不做软件层次上的RAID的原 因：

On Sun, Jun 04, 2006 at 10:18:22PM +0800, Eric Zhang wrote:
> Pvfs runs smoothly and everything is OK. But I want to know what
> will happen if any disk damaged, I mean, If one of these disks
> failed, all data will lose? How pvfs2 deals with this situation?

PVFS2 deals with this situation the same way a raid-0 array would deal
with it: there would be a loss of data, and you'd have to restore
from backups. The common solution is to deploy raid 1 on each pvfs2
storage node. then pvfs2 can sustain one disk failure per storage
node.

> have read the "pvfs-ha.pdf" but this kind of solution based on my
> cluster nodes have redundance disks that I don't have. Does pvfs
> support redundance storage policy? Just like RAID 1, when data
> arrives, we write it to two nodes and at the same time, we write
> another copy of data to the other two nodes. Thanks, any
> suggestions will be appreciated.

If you don't want to pay for additional hardware and you don't want to
pay for enough storage to back up pvfs2, then you'll have to treat
PVFS2 as it was intended: as a fast scratch space for applications.
Commonly, data is staged onto pvfs2 before running an IO-intensive
application and shipped off of to storage which is presumably
backed-up.

Software-based redundancy is a lot harder to solve at the file-system
layer than it is at the device layer. Specifically, it's a real
challenge to in effect write two copies of data without cutting
overall write performance in half. Several research efforts are
ongoing to deliver software redundancy with high performance, but
these efforts are still in early stages.

I hope this explanation is clear. There is definitely a lot of demand
for software-based reduncancy, and we're working on it.