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https://issues.apache.org/jira/browse/COUCHDB-754?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12863582#action_12863582
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Adam Kocoloski commented on COUCHDB-754:
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Also, my comment about the possibile incompatibility of file:write/2 with the
"zip_server" enhancement was silly -- they're definitely compatible.
file:write/2 with O_APPEND doesn't need a special interface to batch together
write requests; all you need to do is assemble an ordered list of all
outstanding data to be written and send it in. It's still an iolist(), after
all.
You'd need to calculate the iolist_size of each binary separately so that you
can report the proper offset in the file for each term, but that's easy. It's
certainly possible to batch the writes into a single system call.
> Investigate alternative couch_file writer implementations
> ---------------------------------------------------------
>
> Key: COUCHDB-754
> URL: https://issues.apache.org/jira/browse/COUCHDB-754
> Project: CouchDB
> Issue Type: Improvement
> Environment: some code might be platform-specific
> Reporter: Adam Kocoloski
> Fix For: 1.1
>
> Attachments: cheaper-appending-v2.patch, 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]: http://github.com/fdmanana/otp/tree/fdatasync
> [2]: http://github.com/davisp/zip_server
> [3]: http://antirez.com/post/fsync-different-thread-useless.html
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