[Libguestfs] [nbdkit PATCH v2] Introduce cacheextents filter

[Eric Blake]

On 5/15/19 5:39 PM, Martin Kletzander wrote:
> This filter caches the last result of the extents() call and offers a nice
> speed-up for clients that only support req_on=1 in combination with plugins like
> vddk, which has no overhead for returning information for multiple extents in
> one call, but that call is very time-consuming.
> 
> Quick test showed that on a fast connection and a sparsely allocated 16G disk
> with a OS installed `qemu-img map` runs 16s instead of 33s (out of which it
> takes 5s to the first extents request).  For 100G disk with no data on it, that
> is one hole extent spanning the whole disk (where there is no space for
> improvement) this does not add any noticeable overhead.
> 
> Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
> ---
> v2:
>  - Errors are set in _add and _fill functions
>  - Return values are checked for only -1
>  - Added a test
>  - Indentation fixed
>  - Fixed access before lock
> 

> +
> +/* This lock protects the global state. */
> +static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
> +
> +/* The real size of the underlying plugin. */
> +static uint64_t size;
> +

We recently just fixed the truncate filter to NOT cache this globally
(as doing so is too risky); instead, it needs to be cached
per-connection.  See commit b3a43ccd.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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