plan for a feature: fsync for assuring file size

[SER.RI-TIC - David Losada]

Hello,

some time ago we had a problem that was causing silent data loss in our
users' files. It was a problem related to Linux behavior of deferring
NFS quota errors until the file is closed or fsync'ed.

    http://www.spinics.net/lists/samba/msg67798.html

With Jeremy's patch and configuring Samba to fsync after _every_ write,
the data loss problem was stopped. That came with the price of a big
performance hit, of course. We need to decrease this hit, and here I'm
sharing with you my thoughts of how to do it.

I'd like to gain some insight from you about (a) how I can implement it
correctly in Samba's architecture and (b) if this feature could be added
into the standard Samba distribution.

Our aim is to be able to send an NT_STATUS_DISK_FULL message to a
Windows client *before* it asks to close the file. I have thought of
this in the context of non-sparse files (strict allocate = yes), maybe
some ideas can be applied to the case of sparse files.  The feature
would be user-enabled, at share level.

There's a straightforward idea, and a slightly smarter one.

1) *straightforward*: performing fsync() after allocating file size
[maybe at vfs level in smbd/vfs-wrap.c:strict_allocate_ftruncate?] This
way fsync is just called once per file. Should help performance a
little, even if it means some dumb effort by forcing writing zeros to
disk/network.

2) *bit smarter*: performing fsync() when writing the end of the file.
The fsync() call could be deferred until the moment when the client asks
to write the _last byte_ of the file. My impression is that this should
be done above VFS level (smbd/fileio.c maybe?), because it would enable
it to work together with write caches.

I believe this last concept could be adapted to the case of sparse
files, as well.

A feature like this would really help us here at work, thus, I am
willing to implement it. In case it becomes a standard Samba feature, I
am also willing to help maintain it.

kind regards,


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