[ClusterLabs] Antw: [EXT] Re: Q: About a false negative of storage_mon

[Ulrich Windl]

>>> Klaus Wenninger <kwenning at redhat.com> schrieb am 03.08.2022 um 15:51 in
Nachricht
<CALrDAo074Vd77biwe9NP6J7k2V9DkNg1YfApjrUHMm0-6L8L_g at mail.gmail.com>:
> On Tue, Aug 2, 2022 at 4:10 PM Ken Gaillot <kgaillot at redhat.com> wrote:
>>
>> On Tue, 2022-08-02 at 19:13 +0900, 井上和徳 wrote:
>> > Hi,
>> >
>> > Since O_DIRECT is not specified in open() [1], it reads the buffer
>> > cache and
>> > may result in a false negative. I fear that this possibility
>> > increases
>> > in environments with large buffer cache and running disk-reading
>> > applications
>> > such as database.
>> >
>> > So, I think it's better to specify O_RDONLY|O_DIRECT, but what about
>> > it?
>> > (in this case, lseek() processing is unnecessary.)
>> >
>> > # I am ready to create a patch that works with O_DIRECT. Also, I
>> > wouldn't mind
>> > # a "change to add a new mode of inspection with O_DIRECT
>> > # (add a option to storage_mon) while keeping the current inspection
>> > process".
>> >
>> > [1]
>> > 
>
https://github.com/ClusterLabs/resource-agents/blob/main/tools/storage_mon.c#

> L47-L90
>> >
>> > Best Regards,
>> > Kazunori INOUE
>>
>> I agree, it makes sense to use O_DIRECT when available. I don't think
>> an option is necessary.
> 
> Might as well be interesting to adjust block-size/alignment to the
> device.
> Another consideration could be to on top directly access the block-layer
> using aio.

Again AIO is POSIX; it depends on the implementation what it really does.

> Both is being done in sbd (storage-based-death) and yes it as well
> adds Linux specific stuff that might have to be conditional for other OSs.
> 
> Klaus
> 
>>
>> However, O_DIRECT is not available on all OSes, so the configure script
>> should detect support. Also, it is not supported by all filesystems, so
>> if the open fails, we should retry without O_DIRECT.
>> --
>> Ken Gaillot <kgaillot at redhat.com>
>>
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