RAID gotchas!

Justin W jlist at jdjlab.com
Tue Jul 3 01:02:36 UTC 2007


Roberto Ragusa wrote:
> Jeffrey Ross wrote:
>   
>> Roberto Ragusa wrote:
>>     
>>> Dump is considered a bad choice by Linus himself; read this:
>>>
>>>   http://lwn.net/2001/0503/a/lt-dump.php3
>>>
>>> (a few years ago, but the words are quite strong)
>>>
>>> Best regards.
>>>   
>>>       
>> I've read the arguments here's the rebuttal to the 2001 message:
>> http://dump.sourceforge.net/isdumpdeprecated.html
>>     
>
> Thank you for this link, very interesting.
> Basically thay say that there was a bug in 2.4, now fixed.
>
> They claim three advantages when using dump, but they are rather
> weak, I have to say (IMHO).
>
> 1) dump unmounted filesystem; but why not just mount it read-only
> and use a normal file copy tool? they talk about trying to dump
> corrupted unmountable filesystems for rescue purposes, but it looks
> like a very stretched motivation, especially when trying to prove
> that dump is preferable for normal uncorrupted filesystems.
>
>   
For less informed readers (or curious readers later finding this thread 
in a search of the archives), copying unmountable file systems is 
already possible: use dd. You can even take the image of a partition (or 
a whole drive) and mount the file system located within it using loop 
devices (though the whole drive takes more work aligning the mount to 
the beginning of a "partition", and thus, an understandable file system).

> [snip]
>
> 6) dump can not create accessible backups; I want to be able
> to use the files in my backup (find, grep,...), not just
> restore them.
>
>   
Using the method I describe above, this is possible.

> Finally they say that by using snapshots you can have a stable
> read-only image of the filesystem to run dump on. But the same
> is true for other tools too.
>
>   
I just backed up my server using a combination of an LVM snapshot, dd to 
copy the partition initially, and now it'll be maintained with nightly 
rsyncs to a mounted image file. (Note: If anyone is interested, I can 
post some documentation describing how I set up the backup and the 
script which will keep my backup up-to-date).

> Certainly there is not a right way and wrong way to do things.
> If dump gives you reliable backups and you are used to it,
> it's a valid choice.
>
> File copy tools will remain my preferred choice.
> In this exact moment I have two backups running across the
> LAN; they involve a couple of millions of files; one is
> using tar|tar, another rsync. (I'm not kidding)
> All filesystems are reiser here, so I couldn't try dump if I
> wanted, but even if I could, I think I would not. :-)
>
> You gave me an opportunity to understand dump better.
> For what I've seen, it should be called e2dump and
> should be part of ext2progs, together with e2fsck,
> e2label, resize2fs and dumpe2fs (which is something else).
> It is a filesystem tool, not a file tool.
> Linux is not always ext2/ext3.
>
> Maybe the summary of all this is just that dump is a
> tool to backup a filesystem, but I want to backup the files.
>
> Best regards.
>   

Justin W




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