Re: SQLite and callbacks

On Wed, Feb 9, 2011 at 17:40, Ivan Zhakov <ivan_at_visualsvn.com> wrote:
> On Tue, Feb 8, 2011 at 12:50, Bert Huijben <bert_at_qqmail.nl> wrote:
>>> -----Original Message-----
>>> From: Branko Čibej [mailto:brane_at_xbc.nu] On Behalf Of Branko Cibej
>>> Sent: dinsdag 8 februari 2011 4:39
>>> To: dev_at_subversion.apache.org
>>> Subject: Re: SQLite and callbacks
>>>
>>> On 07.02.2011 21:51, Stefan Sperling wrote:
>>> >> A lot of wc databases out there will be
>>> >> so small that the user will hardly notice the memory increase.
>>> > All we'd be doing is allowing sqlite to flush data to disk if needed.
>>> > Even with a temporary table backed by a file, most operations happen
>>> in
>>> > memory. Either in buffers managed by sqlite or the operating system's
>>> > buffer cache (until sqlite does an fsync). So for small databases it
>>> > shouldn't make a difference.
>>
>> On NTFS just creating a new 0 byte tempfile requires an fsync (and probably a few in a row),
> On Windows there is FILE_ATTRIBUTE_TEMPORARY [1] attribute, which
> prevents file to be written to disk:
> [[[
> Specifying the FILE_ATTRIBUTE_TEMPORARY attribute causes file systems
> to avoid writing data back to mass storage if sufficient cache memory
> is available, because an application deletes a temporary file after a
> handle is closed. In that case, the system can entirely avoid writing
> the data. Although it does not directly control data caching in the
> same way as the previously mentioned flags, the
> FILE_ATTRIBUTE_TEMPORARY attribute does tell the system to hold as
> much as possible in the system cache without writing and therefore may
> be of concern for certain applications.
> ]]]
>
> May be we should submit a patch to SQLite to use this attribute?
>
> [1] http://msdn.microsoft.com/en-us/library/aa363858%28v=vs.85%29.aspx
>
>
Well, I've checked SQLite source code and it seems SQLite uses
FILE_ATTRIBUTE_TEMPORARY attribute when creating temporary database,
database journal and other temporary files. So temporary tables backed
up by file should be serious problem in theory.

-- 
Ivan Zhakov
VisualSVN Team