Re: O_DIRECT support for Windows

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: pgsql-patches(at)postgresql(dot)org
Subject: Re: O_DIRECT support for Windows
Date: 2007-01-15 18:05:28
Message-ID: 45ABC268.1080605@hagander.net
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ITAGAKI Takahiro wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>
>> FILE_FLAG_NO_BUFFERING requires that *all* I/O follows:
>> * File access must begin at offsets that are integer multples of the
>> volume sector size.
>> * File access must be for number of bytes that are integer multiples of
>> the volume sector size.
>> * Buffer addresses for read and write operations must be sector aligned.
>>
>> I was under the impression that our code can in no way guarantee this.
>> Especially given that a typical NTFS drive can have anything from 512 to
>> 4096 bytes if you use the GUI to format it, and larger sizes than that
>> when you use some SAN tools to do it.
>
> Do you mean there are drives that have larger sector size than 8kB?
> We've already put the xlog buffer along the alignment of
> ALIGNOF_XLOG_BUFFER (typically 8192 bytes).
> But if there are such drives, using FILE_FLAG_NO_BUFFERING is harmful!

Yes. I have heard this can happen with certain SAN drives. I haven't
seen it myself, and I can't seem to find a reference right now :-) But I
do recall having read about th need to check the sector size and
specifically align it, because some do have that problem.

//Magnus

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