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Re: [Qemu-devel] [PATCH 1/2] vmdk: Optimize I/O by allocating multiple c

From: Ashijeet Acharya
Subject: Re: [Qemu-devel] [PATCH 1/2] vmdk: Optimize I/O by allocating multiple clusters
Date: Fri, 24 Mar 2017 00:48:49 +0530

On Fri, Mar 24, 2017 at 12:40 AM, Kevin Wolf <address@hidden> wrote:
> Am 11.03.2017 um 12:54 hat Ashijeet Acharya geschrieben:
>> The vmdk driver in block/vmdk.c used to allocate cluster by cluster
>> which slowed down its I/O performance.
>> Make vmdk driver allocate multiple clusters at once to reduce the
>> overhead costs of multiple separate cluster allocation calls. The number
>> of clusters allocated at once depends on the L2 table boundaries. Also
>> the first and the last clusters are allocated separately as we may need
>> to perform COW for them
>> Signed-off-by: Ashijeet Acharya <address@hidden>
> Ashijeet, this is pretty hard to review because you're doing too many
> things in a single patch. The rule is to do only one logical change in
> each patch whenever it's possible. In particular, pure code motion (or
> renames) need to be separated from code modification because when you
> move code and modify it at the same time, it is very hard to see the
> actual difference.

Hmm, sorry about that.

> Do you think you could split this into multiple patches, like this:
> 1. Move vmdk_find_offset_in_cluster() to the top
> 2. Rename get_whole_cluster() to do_alloc_cluster_offset()
>    (To be honest, both names aren't great. Something like qcow2's
>    perform_cow() would describe better what's going on there.)
> 3. Factor out some code from get_cluster_offset() into handle_alloc()
> 4. Rename get_cluster_offset() into vmdk_get_cluster_offset()
> ...
> n. Handle multiple clusters at once
> Obviously, I haven't taken the time to fully read and understand all of
> your patch, so don't take this too literally, but you see what kind of
> granularity the individual patches should be. If you do it like this,
> each change becomes really simple and can be reviewed and discussed on
> its own. And if we later find a bug in VMDK, git bisect can point out
> exactly which step introduced the problem rather than pointing at a
> single big commit that is hard to understand.
> Good splitting of patches is one of the tricks to get quicker reviews.
> Admittedly, it is often not easy, but it is worth spending some thought
> on how to make a series really easy to read for others.

Sure, I understand your point completely. I wasn't able to segregate
things too, but now with the list of things you mentioned above, it
certainly helps making things clearer. I will post a v2 as soon as I
can, probably you will receive it first thing Monday.

One thing I wanna ask is that were you able to take a look at my reply
to the cover letter earlier. Will the test results using 'qemi-io' I
posted there work?


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