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Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Int

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection
Date: Wed, 15 May 2013 10:25:00 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4

Il 15/05/2013 09:59, Kevin Wolf ha scritto:
>>> Do you mean you'd model the 'active' mode after 'block-backup,' or actually
>>> call functions provided by 'block-backup'?
>> No, I'll just reuse the same hooks within block/mirror.c (almost... it
>> looks like I need after_write too, not just before_write :( that's a
>> pity).
> Makes me wonder if using a real BlockDriver for the filter from the
> beginning wouldn't be better than accumulating more and more hooks and
> having to find ways to pass data from 'before' to 'after' hooks...

We don't need a way to pass data from before to after hooks, a simple
scan of a linked list will do.

>> Basically:
>> 1) before the write, if there is space in the job's buffers, allocate a
>> MirrorOp and a data buffer for the write.  Also record whether the block
>> was dirty before;
>> 2) after the write, do nothing if there was no room to allocate the data
>> buffer.  Else clear the block from the dirty bitmap.  If the block was
>> dirty, read the whole cluster from the source as in passive mirroring.
>> If it wasn't, copy the data from guest memory to the preallocated buffer
>> and write it to the destination;
> Does the "if there was no room" part mean that the mirror is active only
> sometimes?

Yes, otherwise the guest can allocate arbitrary amounts of memory in the
host just by starting a few very large I/O operations.

> And why even bother with a dirty bitmap for an active mirror? The
> background job that sequentially processes the whole image only needs a
> counter, no bitmap.

That's not enough for the case when the host crashes and you have to
restart the mirroring or complete it offline.


> At which point it looks like implementing it separate from mirror.c
> could make more sense.
> Kevin

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