Re: [Qemu-devel] [PATCH v5] qemu-io: add pattern file for write command

[Eric Blake <eblake@redhat.com>]

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On 5/31/19 2:46 AM, Denis Plotnikov wrote:
> The patch allows to provide a pattern file for write
> command. There was no similar ability before.
> 
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
> ---
> v5:
>   * file name initiated with null to make compilers happy
> 

> +static void *qemu_io_alloc_from_file(BlockBackend *blk, size_t len,
> +                                     char *file_name)
> +{
> +    char *buf, *buf_pos;
> +    FILE *f = fopen(file_name, "r");
> +    int l;
> +
> +    if (!f) {
> +        printf("'%s': %s\n", file_name, strerror(errno));
> +        return NULL;
> +    }
> +
> +    if (qemuio_misalign) {
> +        len += MISALIGN_OFFSET;
> +    }
> +    buf = blk_blockalign(blk, len);
> +    memset(buf, 0, len);
> +
> +    buf_pos = buf;
> +
> +    while (len > 0) {
> +        l = fread(buf_pos, sizeof(char), len, f);
> +
> +        if (feof(f)) {
> +            rewind(f);
> +        }

Why are we reading the file more than once? Once we've read it once,
it's more efficient to switch to a loop that memcpy()s the prefix into
the rest of the buffer, rather than to perform repeated I/O.

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


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