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Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Int
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection |
Date: |
Wed, 15 May 2013 10:53:21 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 15.05.2013 um 10:25 hat Paolo Bonzini geschrieben:
> 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.
So in this case the linked list is the way.
> >> 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.
I think I would rather throttle I/O in this case, i.e. requests wait
until they can get the space. At least for a synchronous mirror we
have to do something like this.
> > 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.
You're thinking of a persistent bitmap here? Makes sense then, I didn't
think about that.
Kevin
- [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Wolfgang Richter, 2013/05/13
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Stefan Hajnoczi, 2013/05/14
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Kevin Wolf, 2013/05/14
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Paolo Bonzini, 2013/05/14
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Wolfgang Richter, 2013/05/14
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Paolo Bonzini, 2013/05/14
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Wolfgang Richter, 2013/05/14
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Kevin Wolf, 2013/05/15
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Paolo Bonzini, 2013/05/15
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection,
Kevin Wolf <=
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Paolo Bonzini, 2013/05/15
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Kevin Wolf, 2013/05/15
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Paolo Bonzini, 2013/05/15
- Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Wolfgang Richter, 2013/05/22
Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Wolfgang Richter, 2013/05/14
Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection, Richard W.M. Jones, 2013/05/16