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From Damien Katz <dam...@apache.org>
Subject Re: optimal settings for [couchdb] fsync_options?
Date Wed, 14 Apr 2010 14:26:27 GMT
The reason for fsync on open is the server doesn't know if the data it's reading off the file
is commited fully to the disk. It's possible the the server wrote to file and crashed before
fsync, then restarted. Then it could refresh view indexes on the non-fsynced storage data,
for example, and crash again, losing data in the storage file, but not the updates to the
index file. Now the index is permanently out of date with the storage file. But if you fsync
on opening the storage file, that can't happen.

-Damien


On Apr 14, 2010, at 5:52 AM, Adam Kocoloski wrote:

> Initially posted on user@, but maybe it got lost in the noise.  Does anyone know why
we call fsync when we open a file?
> 
> Adam
> 
> Begin forwarded message:
> 
>> From: Adam Kocoloski <kocolosk@apache.org>
>> Date: April 11, 2010 10:44:03 PM EDT
>> To: user@couchdb.apache.org
>> Subject: optimal settings for [couchdb] fsync_options?
>> 
>> Hi folks, I wanted to assemble some concrete information about the purpose of each
of the three fsync_options available in CouchDB and under what conditions they should be enabled/disabled.
 These options are
>> 
>> 1) before_header - calls file:sync(Fd) before writing a DB header to disk.  I believe
the goal here is to prevent DB corruption by ensuring that all the data referred to by the
header is durably stored before the header is written.  A system that preserves write ordering
could safely disable this option.  Does anyone know an example of such a system? Perhaps a
combination of a noop IO scheduler and a write-through or nonvolatile disk cache?
>> 
>> 2) after_header - calls file:sync(Fd) immediately after writing the DB header.  I
think this one is done so that we don't lose too much data following a CouchDB restart, and
so that a client can ensure that stored data will be retrievable after a restart by POSTing
to /db/_ensure_full_commit.  It might make sense to disable this option if e.g. you're relying
on replication for durability.  Although that's dicey because the replicator calls ensure_full_commit
for both DBs before writing its own checkpoint record*, and by disabling the after_header
option you'd run the risk of skipping updates on the target in the face of a power failure.
>> 
>> 3) on_file_open - calls file:sync(Fd) immediately after opening a DB file.  I really
don't know the purpose of this one.  Anyone?
>> 
>> Best, Adam
>> 
>> * The reason the replicator calls ensure_full_commit on the source is to detect situations
where update_seqs might be reused.  I wonder if we could engineer a way around that ever happening,
for example by ensuring that on restart the update sequence jumps by a large number.  But
that's a discussion for dev@.
> 


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