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From "Adam Kocoloski (JIRA)" <>
Subject [jira] Updated: (COUCHDB-754) Investigate alternative couch_file writer implementations
Date Mon, 03 May 2010 21:12:55 GMT


Adam Kocoloski updated COUCHDB-754:

    Attachment: cheaper-appending.patch

Here's a patch for the "Open the file in append mode and stop seeking to eof in user space."
task item.  It extends the #file record to track EOF, which allows us to skip the call to
file:position/2 before every write.  It also opens the file with O_APPEND and thus can use
file:write/2 instead of file:pwrite/3.

I did try file:read_file_info/1, but it calls stat() and is quite slow (~35 µs).  There is
no Erlang interface to fstat() that I'm aware of.

I wrote a tiny little C routine to compare the cost of lseek() + pwrite() (what we do now)
to the cost of write() with an O_APPEND file (this patch).  1-byte write times dropped from
~30 µs to ~9µs.  I can come up with more quantitative numbers if desired.  The extra time
in the old code seemed to be split pretty evenly between the lseek() and the pwrite().

So, what we were doing before wasn't exactly expensive, but I believe this patch is faster.
 The iolist_size calls that I've added are sub-µs, so we shouldn't worry about that.

> Investigate alternative couch_file writer implementations
> ---------------------------------------------------------
>                 Key: COUCHDB-754
>                 URL:
>             Project: CouchDB
>          Issue Type: Improvement
>         Environment: some code might be platform-specific
>            Reporter: Adam Kocoloski
>             Fix For: 1.1
>         Attachments: cheaper-appending.patch
> I've got a number of possible enhancements to couch_file floating around in my head,
wanted to write them down.
> * Use fdatasync instead of fsync.  Filipe posted a patch to the OTP file driver [1] that
adds a new file:datasync/1 function.  I suspect that we won't see much of a performance gain
from this switch because we append to the file and thus need to update the file metedata anyway.
 On the other hand, I'm fairly certain fdatasync is always safe for our needs, so if it is
ever more efficient we should use it.  Obviously, we'll need to fall back to file:sync/1 on
platforms where the datasync function is not available.
> * Use file:pwrite/2 to batch together multiple outstanding write requests.  This is essentially
Paul's zip_server [2].  In order to take full advantage of it we need to patch couch_btree
to update nodes in parallel.  Currently there should only be 1 outstanding write request in
a couch_file at a time, so it wouldn't help at all.
> * Open the file in append mode and stop seeking to eof in user space.  We never modify
files (aside from truncating, which is rare enough to be handled separately), so perhaps it
would help with performance if we let the kernel deal with the seek.  We'd still need a way
to get the file size for the make_blocks function.  I'm wondering if file:read_file_info(Fd)
is more efficient than file:position(Fd, eof) for this purpose.
> A caveat - I'm not sure if append-only files are compatible with the previous enhancement.
 There is no file:write/2, and I have no idea how file:pwrite behaves on a file which is opened
append-only.  Is the Pos ignored, or is it an error?  Will have to test.
> * Use O_DSYNC instead of fsync/fdatasync.  This one is inspired by antirez' recent blog
post [3] and some historical discussions on pgsql-performance.  Basically, it seems that opening
a file with O_DSYNC (or O_SYNC on Linux, which is currently the same thing) and doing all
synchronous writes is reasonably fast.  Antirez' tests showed 250 µs delays for (tiny) synchronous
writes, compared to 40 ms delays for fsync and fdatasync on his ext4 system.
> At the very least, this looks to be a compelling choice for file access when the server
is running with delayed_commits = true.  We'd need to patch the OTP file driver again, and
also investigate the cross-platform support.  In particular, I don't think it works on NFS.
> [1]:
> [2]:
> [3]:

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