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From Adam Kocoloski <kocol...@apache.org>
Subject Re: optimal settings for [couchdb] fsync_options?
Date Wed, 14 Apr 2010 14:46:14 GMT
Thanks Damien.  I'm thinking that the situation you describe cannot occur if before_header
is enabled in the fsync_options, since any data pointed to by the #db_header that the server
found after the restart was already synced.  Is that correct?

Adam

On Apr 14, 2010, at 10:26 AM, Damien Katz wrote:

> 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 Now the index is permanently out of date with the storage file.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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