Re: barriers vs. reads - O_DIRECT

[Werner Almesberger <wa@almesberger.net>]

Guy wrote:
> What if a filesystem is on a software RAID5 array?  How does the filesystem
> use a barrier when some of the writes are on different disks?  Would md pass
> the barrier down to each disk?

Let's assume we just have a FIFO elevator per device, barriers
affect all requests, and we don't allow any reordering across
barriers. Then, a device would have to stop IO when it hits a
barrier, and could only resume when all other devices using
this barrier have reached it.

md would have to send the barrier to each device, with the
exception that in any sequence of barriers without other
requests, only the first and the last barrier are needed.

Now, if we have a smarter elevator and smarter barriers,
there are more tricks that can be played. E.g. an elevator
could try to reach a barrier shared by many others as soon
as possible, and then schedule requests allowed to cross
the barrier while waiting for the other devices.

I'm not sure how sophisticated we want to get in the
multiple devices case, though.

- Werner

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 / Werner Almesberger, Buenos Aires, Argentina         wa@almesberger.net /
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