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From: | Wolfgang Richter |
Subject: | Re: [Qemu-devel] Adding Disk-Level Introspection to QEMU |
Date: | Wed, 24 Apr 2013 11:59:58 -0400 |
It slows down guest I/O for a couple of reasons:> Has there been any performance analysis of drive-mirror (impact on executing guest)?
1. Writes now require a read from the original device followed by a
write to the target device. Only after this completes is the write
allowed to proceed.
2. Overlapping read/write requests are serialized to maintain
consistency between the guests I/Os and the block-backup I/Os.
But on second thought, I don't think block-backup fits the bill. You
don't care about the original data, you care about what new data the
guest is writing.
I think what you really want is a "tap" block driver which mirrors
writes to a target device (typically a NBD volume). You can model this
on blkverify or check out Benoit Canet's quorum patches.
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