Re: [Xen-devel] [patch 0/6] xenblk: Add O_DIRECT and O_SYNC support.

[Joe Jin <joe.jin@xxxxxxxxxx>]

>> Do you think when xenblk backend make_reponse() means the request have
>> commited to disk?
>> We have did a sample testing and found without BI_RW_SYNC flag setted,
>> either O_DIRECT or O_SYNC flag, when vm crashed/power outage, lots of
>> data lost, sometimes more than 1M data lost, that means vm under
>> high data lost risk. With BI_RW_SYNC flag when call submit_bio, open
>> file with O_DIRECT or O_SYNC flag, could sync data very well.

>What's the vbd type in this case: raw partition, lvm, qcow file, ...?

Raw partition, crashed/power outage, lots of data lost :(

>The existing BLKIF_OP_WRITE_BARRIER and BLKIF_OP_FLUSH_DISKCACHE should
>suffice to implement O_SYNC on the blkfront side, I think. O_DIRECT doesn't
>mean writes are synchronous to the platters -- just means the buffer cache
>is bypassed -- which should generally be the case on the blkback side always
>anyway.

However, frontend driver could not got the write's flag at all.
if we could know the request with O_SYNC flag should be easy to handle,
need not touch filesystem layer.
O_DIRECT is difference not only buffer head, if read/write is direct-io,
need to commit request as soon as possible, means unplug request_queue
if request with O_DIRECT flag(request should be marked REQ_RW_SYNC 
flag).

Thanks,
Joe

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