incubator-couchdb-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Jens Alfke <>
Subject Re: High latency (40ms) and low request rate (10 rps) under windows
Date Tue, 25 Oct 2011 23:00:02 GMT

On Oct 25, 2011, at 3:09 PM, Mark Hahn wrote:

>> CouchDB does a full sync, because this is the only way to be proof against
>> disasters like power failures
> Isn't couchdb crash-proof due to append-only writing?  What do you gain
> other than possible loss of latest writes, which you can lose anyway with a
> fsync.

You can get corrupted files even with append-only writing. In the worst case, let’s say
everything gets written to disk before the power failure _except_ for one disk block in the
middle of the update*. After rebooting, you have what appears to be a valid file (it’s got
the magic trailer) except that 4096 (or 512 or whatever) bytes in one of the last updated
documents are garbage.

I don’t know the details of how CouchDB finds the trailer in the file, but it would have
to be doing something like a checksum of every single write to guard against that; which seems
too expensive to me.

Instead, to be safe, what you do is write the payload, wait for a full fsync, then write the
trailer only after you know that the entire payload is safely on the platters.


* Disk controllers don’t write sectors in the order they receive the write requests. They
shove them in the cache, then write them out grouped by tracks to minimize seek time.
View raw message