Re: [Qemu-devel] [PATCH v5 2/4 for-3.0] qcow2: Options' documentation fixes

[Kevin Wolf <kwolf@redhat.com>]

Am 26.07.2018 um 16:27 hat Leonid Bloch geschrieben:
> On 07/26/2018 01:02 PM, Kevin Wolf wrote:
> > Am 25.07.2018 um 16:27 hat Leonid Bloch geschrieben:
> > > Signed-off-by: Leonid Bloch <lbloch@janustech.com>
> > > ---
> > >   docs/qcow2-cache.txt |  3 +++
> > >   qemu-options.hx      | 10 ++++++----
> > >   2 files changed, 9 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
> > > index 8a09a5cc5f..3673f2be0e 100644
> > > --- a/docs/qcow2-cache.txt
> > > +++ b/docs/qcow2-cache.txt
> > > @@ -130,6 +130,9 @@ There are a few things that need to be taken into account:
> > >      memory as possible to the L2 cache before increasing the refcount
> > >      cache size.
> > > +- At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
> > > +  can be set simultaneously.
> > 
> > The indentation is off here, the other list items have one space more.
> > 
> > >   Unlike L2 tables, refcount blocks are not used during normal I/O but
> > >   only during allocations and internal snapshots. In most cases they are
> > >   accessed sequentially (even during random guest I/O) so increasing the
> > > diff --git a/qemu-options.hx b/qemu-options.hx
> > > index b1bf0f485f..13ece21cb6 100644
> > > --- a/qemu-options.hx
> > > +++ b/qemu-options.hx
> > > @@ -752,15 +752,17 @@ image file)
> > >   @item cache-size
> > >   The maximum total size of the L2 table and refcount block caches in bytes
> > > -(default: 1048576 bytes or 8 clusters, whichever is larger)
> > 
> > I think it would be good to still say something about the default.
> > Maybe something like "default: the sum of l2-cache-size and
> > refcount-cache-size"?
> 
> Yes, that sounds good!
> 
> > 
> > >   @item l2-cache-size
> > > -The maximum size of the L2 table cache in bytes
> > > -(default: 4/5 of the total cache size)
> > > +The maximum size of the L2 table cache.
> > 
> > Why did you remove "in bytes" and add a period which the other options
> > don't have? I prefer the old version of this line.
> 
> I removed "in bytes" because it also accepts k, M, G, ... but on a second
> thought, these are amounts of bytes as well, so changing back to the old
> version.
> 
> > > +(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
> > > +is larger; otherwise, as large as possible or needed within the cache-size,
> > > +while permitting the requested or the minimal refcount cache size)
> > >   @item refcount-cache-size
> > >   The maximum size of the refcount block cache in bytes
> > > -(default: 1/5 of the total cache size)
> > > +(default: 4 times the cluster size, or any portion of the cache-size, if it is
> > > +specified and large enough, left over after allocating the full L2 cache)
> > 
> > I found the second part hard to understand. Maybe "4 times the cluster
> > size; or if both cache-size and l2-cache-size are given, the part of
> > the cache-size that is not used for the L2 cache yet."?
> 
> That is not accurate. Because even if l2-cache-size is not given and
> cache-size is large enough to accommodate enough L2 cache to cover the
> entire image, plus the minimal amount of refcount cache with room to spare -
> refcount cache will use all the rest of the cache-size.

Oh, you're right! I wasn't aware that we consider the image size there.

> How about:
> 
> "default: 4 times the cluster size; or if cache-size is specified, the part
> of it which is not used for the L2 cache"

Even simpler, easier to understand and more accurate. I like it.

Kevin