[Gluster-devel] Crawling and indexing hardware

[Marcus Herou]

Ok so the mantra is: Backup as often as you can afford :)

However if an index is corrupt it's not a big deal since the data is not
connected to the index itself but rather stored elsewhere. Actually it will
probably be stored with GlusterFS as well but with another access pattern
which I hope will not run int these issues. This data will probably be
rolled out on tape or such every night and a hot backup be kept somewhere
just in case.

Kindly

//Marcus






On Fri, May 9, 2008 at 1:45 PM, <gordan at bobich.net> wrote:

> On Fri, 9 May 2008, Marcus Herou wrote:
>
>  So basically the only case when a server is disconnected is when I take it
>> down for maintenance
>>
>
> Or your switch / NIC dies. Or anything else goes wrong that prevents the
> servers from staying connected (e.g. a server crash).
>
>  and when it comes up it will self-heal right? And by adding a brand new
>> server it will as well be synched I hope.
>>
>
> In theory - yes.
> In practice, there are some caveats. There are some nasty race conditions
> that make splitbrain even more dangerous than you might expect.
>
> Files are versioned. Deleting and re-creating a file causes the version to
> be reset. Some programs delete and re-create a file rather than modifying it
> (e.g. vi does). This has a number of dangerous side effects. If your
> disconnected server has an old version that was incrementally modified (e.g.
> log file being appended to), it's version will be high. If you delete and
> recreate the file, or do something that will have the same effect (e.g. edit
> with vi), the version on the working servers will be reset (low number).
>
> When the server that dropped out reconnects, it's version will be higher
> than the new (reset) version, and it's old file will clobber the new file.
>
> This is, IMO, a shockingly dangerous "feature". It means that the old file
> on the disconnected server can easily supercede the new file on the working
> cluster.
>
>
> Gordan
>
>
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-- 
Marcus Herou CTO and co-founder Tailsweep AB
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marcus.herou at tailsweep.com
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