[ZODB-Dev] HA Setup and zodb index file of FileStorage

[Patrick Gerken]

Hello,

its funny, like Garth in may this year I am looking into making a HA
system with ZEO for an ERP5 deployment. In my case I don't need to
care for data replication, all is stored on a SAN considered HA by the
customer already.

So my data.fs and index and all that stuff will already be available
on my backup server.
The idea is that the backup server will watch the real ZEO, and start
all services if the server goes down. It seems quite safe for me to
take over the files from the filesystem and restart everything (While
writing I realise I should be really really sure the other zeo is
down, but well, that is out of scope for this mail).

The thing which scared me to hell was the "rumour" that it can happen
that the index can get corrupted and then everything has to be indexed
again. With a large number of objects, this will be slow, either
because of millions of seeks or many many blocks written to memory. So
I wanted to look into the FileStorage implementation to see if this
could be optimised. But I did not find a single place where killing
the server would result in a corrupt index file (Simply assuming that
we have journaling filesystems). Also if the index file is not up to
date, restarting the zeo server would result in updating the zeo
server starting from the latest transaction written to the index. That
can take up to 5 minutes (assuming 10000 objects, the distance between
writes, multiplied with 20 ms for processing and seeking, rounded up
to the next prime.

Given that writing a file and renaming a file can be considered
atomic, and that no solar winds or similar things can screw up my
filesystem, how can I screw up my index file?

best regards,

              Patrick